Commit a07a686b authored by STEFF Laurent's avatar STEFF Laurent
Browse files

fixed various typos

parent e8ead53a
%% Cell type:markdown id: tags:
# [Getting started in C++](./) - [Procedural programming](./0-main.ipynb) - [Functions](./4-Functions.ipynb)
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<h1>Table of contents<span class="tocSkip"></span></h1>
<div class="toc"><ul class="toc-item"><li><span><a href="#Function-definition" data-toc-modified-id="Function-definition-1">Function definition</a></span><ul class="toc-item"><li><span><a href="#Passing-arguments-by-value" data-toc-modified-id="Passing-arguments-by-value-1.1">Passing arguments by value</a></span></li><li><span><a href="#Passing-arguments-by-reference" data-toc-modified-id="Passing-arguments-by-reference-1.2">Passing arguments by reference</a></span></li><li><span><a href="#A-bit-of-wandering:-using-C-like-error-codes" data-toc-modified-id="A-bit-of-wandering:-using-C-like-error-codes-1.3">A bit of wandering: using C-like error codes</a></span></li><li><span><a href="#Passing-arguments-by-pointers" data-toc-modified-id="Passing-arguments-by-pointers-1.4">Passing arguments by pointers</a></span></li></ul></li><li><span><a href="#Function-with-return-value" data-toc-modified-id="Function-with-return-value-2">Function with return value</a></span></li><li><span><a href="#Alternate-function-syntax" data-toc-modified-id="Alternate-function-syntax-3">Alternate function syntax</a></span></li><li><span><a href="#Function-overload" data-toc-modified-id="Function-overload-4">Function overload</a></span><ul class="toc-item"><li><span><a href="#The-easy-cases:-arguments-without-ambiguity" data-toc-modified-id="The-easy-cases:-arguments-without-ambiguity-4.1">The easy cases: arguments without ambiguity</a></span></li><li><span><a href="#[WARNING]-Return-type-doesn't-count!" data-toc-modified-id="[WARNING]-Return-type-doesn't-count!-4.2">[WARNING] Return type doesn't count!</a></span></li><li><span><a href="#Good-practice:-don't-make-signature-vary-only-by-a-reference-or-a-pointer" data-toc-modified-id="Good-practice:-don't-make-signature-vary-only-by-a-reference-or-a-pointer-4.3">Good practice: don't make signature vary only by a reference or a pointer</a></span></li><li><span><a href="#Best-viable-function" data-toc-modified-id="Best-viable-function-4.4">Best viable function</a></span></li><li><span><a href="#Advice:-use-overload-only-when-there-is-no-ambiguity-whatsoever" data-toc-modified-id="Advice:-use-overload-only-when-there-is-no-ambiguity-whatsoever-4.5">Advice: use overload only when there is no ambiguity whatsoever</a></span></li></ul></li><li><span><a href="#Optional-parameters" data-toc-modified-id="Optional-parameters-5">Optional parameters</a></span></li><li><span><a href="#Lambda-functions" data-toc-modified-id="Lambda-functions-6">Lambda functions</a></span></li><li><span><a href="#Passing-a-function-as-a-an-argument" data-toc-modified-id="Passing-a-function-as-a-an-argument-7">Passing a function as a an argument</a></span></li><li><span><a href="#A-very-special-function:-main" data-toc-modified-id="A-very-special-function:-main-8">A very special function: <strong>main</strong></a></span></li><li><span><a href="#inline-functions" data-toc-modified-id="inline-functions-9"><code>inline</code> functions</a></span></li></ul></div>
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## Function definition
To be usable in a C++ instruction, a function must be __defined__ beforehand. This definition includes the name and type of the arguments, the type of the return value and the instruction block that make up the function.
`void` is a special type to indicate a function doesn't return any value.
__BEWARE__: Functions can't be defined in blocks.
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``` C++17
#include <iostream>
void PrintDivision(int arg1, int arg2)
{
if (arg2 == 0)
std::cerr << "Failure: division by zero!" << std::endl;
else
{
int division ;
division = arg1/arg2 ;
std::cout << arg1 << " / " << arg2 << " = " << division << std::endl ;
}
}
```
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``` C++17
{
PrintDivision(12, 3);
PrintDivision(6, 0);
PrintDivision(15, 6);
}
```
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Functions can't be nested in C++, contrary to some other langages such as Python:
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``` C++17
void function_1() // a function might have no arguments
{
void subfunction() // COMPILATION ERROR!
{
}
}
```
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To reintroduce hierarchy, __namespaces__ can be used (they will be introduced [a bit later](../6-InRealEnvironment/5-Namespace.ipynb)); __lambda functions__ introduced later in this notebook are not limited by the same rule.
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### Passing arguments by value
In the simple example above, we passed the arguments by value, which is to say the values passed by the arguments were copied when given to the function:
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``` C++17
#include <iostream>
void increment_and_print(int i)
{
++i;
std::cout << "Inside the function: i = " << i << std::endl;
}
{
int i = 5; // I could have named it differently - it doesn't matter as the scope is different!
increment_and_print(i);
std::cout << "Outside the function: i = " << i << std::endl;
}
```
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The `i` in the block body and in the function definition is not the same: one or the other could have been named differently and the result would have been the same.
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### Passing arguments by reference
If we intended to modify the value of `i` outside the function, we should have passed it by reference:
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``` C++17
#include <iostream>
void increment_and_print_by_reference(int& i)
{
++i;
std::cout << "Inside the function: i = " << i << std::endl;
}
{
int i = 5; // I could have named it differently - it doesn't matter as the scope is different!
increment_and_print_by_reference(i);
std::cout << "Outside the function: i = " << i << std::endl;
}
```
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As in C++ you can't return several values in the return type, passing by reference is a way to get in output several values (C++ 11 introduced in the standard library a workaround to get several values in return type with a so-called `std::tuple`, but the passing by reference remains the better way to do so in most cases).
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``` C++17
int compute_division(int arg1, int arg2, int& quotient, int& remainder)
{
if (arg2 == 0)
return -1; // error code.
quotient = arg1 / arg2;
remainder = arg1 % arg2;
return 0; // code when everything is alright.
}
```
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``` C++17
#include <iostream>
{
int quotient, remainder;
if (compute_division(5, 3, quotient, remainder) == 0)
std::cout << "5 / 3 = " << quotient << " with a remainder of " << remainder << '.' << std::endl;
if (compute_division(5, 0, quotient, remainder) == 0)
std::cout << "5 / 0 = " << quotient << " with a remainder of " << remainder << '.' << std::endl;
else
std::cerr << "Can't divide by 0!" << std::endl;
}
```
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### A bit of wandering: using C-like error codes
The function above gets two outputs: the quotient and the remainder of the euclidian division. Moreover, this function returns an error code: by convention this function returns 0 when everything is alright and -1 in case of a zero divider.
Using such an error code is a very common pattern in C, that might as well be used in C++... The issue is that it requires a lot of discipline from the user of the function: there are no actual incentive to use the return value! Just calling `compute_division()` as if it was a void function is perfectly fine (and yet completely ill-advised). We will see [later](../5-UsefulConceptsAndSTL/1-ErrorHandling.ipynb) the `exception` mechanism C++ recommends instead of error codes.
Below is an example where things go awry due to the lack of check:
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``` C++17
#include <iostream>
void print_division(int arg1, int arg2)
{
int quotient, remainder;
compute_division(arg1, arg2, quotient, remainder); // the dev 'forgot' to check the error code.
std::cout << "Euclidian division of " << arg1 << " by " << arg2 << " yields a quotient of "
<< quotient << " and a remainder of " << remainder << std::endl;
}
print_division(8, 5);
print_division(8, 0); // bug!
```
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The developer made two important mistakes:
* The return value of `compute_division` is not checked, so something is printed on screen.
* This is something completely out of control: quotient and remainder don't get a default value that would help to see if something is askew. The behaviour is undefined: you have no guarantee on the values the program will print (currently I see the same values as in the previous function call, but another compiler/architecture/etc... might yield another wrong value.
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**UPDATE:** I still do not advise to use error codes, but the [nodiscard](https://en.cppreference.com/w/cpp/language/attributes/nodiscard) attribute introduced in C++ 17 helps your compiler to warn you when the return value was left unused.
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### Passing arguments by pointers
When the argument of a function is a pointer, each function call
results in the creation of a temporary pointer which is given the address provided as argument. Then using the `*` operator, you can access the
original variable, not a copy.
Except in the case of interaction with a C library or some _very_ specific cases, I wouldn't advise using passing arguments by pointers: by reference does the job as neatly and in fact more efficiently (dereferencing a pointer with `*i` is not completely costless).
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``` C++17
#include <iostream>
void increment_and_print_by_pointer(int* i)
{
*i += 1;
std::cout << "Inside the function: i = " << *i << std::endl;
}
{
int i = 5; // I could have named it differently - it doesn't matter as the scope is different!
increment_and_print_by_pointer(&i);
std::cout << "Outside the function: i = " << i << std::endl;
}
```
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## Function with return value
The value to return should come after the keyword `return`.
A C++ function may include several return values in its implementation:
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``` C++17
#include <iostream>
//! \brief Returns 1 if the value is positive, 0 if it is 0 and -1 if it's negative.
int sign(int a)
{
if (a > 0)
return 1;
if (a == 0)
return 0;
return -1;
}
{
for (int a = -3; a < 4; ++a)
std::cout << "Sign of " << a << " = " << sign(a) << std::endl;
}
```
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## Alternate function syntax
There is now since C++ 11 another way to declare a function; it is not widespread but is advised by some developers (see for instance [this blog post](https://blog.petrzemek.net/2017/01/17/pros-and-cons-of-alternative-function-syntax-in-cpp/) which lists pros and cons of both syntaxes).
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``` C++17
auto sum(int a, int b) -> int // Currently doesn't compile in Xeus-cling environment
{
return a + b;
}
```
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The return value is optional (and was the reason of Xeus-cling failure):
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``` C++17
auto sum(int a, int b) // compiles just fine in Xeus-cling
{
return a + b;
}
```
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``` C++17
#include <iostream>
int a = 8;
int b = -3;
std::cout << a << " + " << b << " = " << sum(a, b) << std::endl;
```
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## Function overload
### The easy cases: arguments without ambiguity
It is possible to define several different functions with the exact same name, provided the type of the argument differ:
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``` C++17
#include <string>
void f();
void f(int); // Ok
double f(int, double); // Ok
auto f(char) -> int; // Ok (alternate function syntax)
std::string f(double, int, char*); // Ok
```
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### [WARNING] Return type doesn't count!
It's not the entire signature of the function that is taken into account when possible ambiguity is sought by the compiler: the return type isn't taken into account. So the following cases won't be accepted:
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``` C++17
void g(int);
int g(int); // COMPILATION ERROR
```
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If we think about it, it is rather logical: in C++ we are not required to use the return type of a function (it's not the case in all languages: Go follows a different path on that topic for instance). The issue then is that the compiler has no way to know which `g(int)` is supposed to be called with `g(5)` for instance.
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### Good practice: don't make signature vary only by a reference or a pointer
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On the other hand, compiler is completely able to accept signatures that differs only by a reference or a pointer on one of the argument:
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``` C++17
#include <iostream>
void h(double a)
{
std::cout << "h(double) is called with a = " << a << '.' << std::endl;
}
```
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``` C++17
#include <iostream>
void h(double* a) // Ok
{
std::cout << "h(double*) is called with a = " << *a << "; a is doubled by the function." << std::endl;
*a *= 2.;
}
```
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``` C++17
#include <iostream>
void h(double& a) // Ok... but not advised! (see below)
{
std::cout << "h(double*) is called with a = " << a << "; a is doubled by the function." << std::endl;
std::cout << "h(double&) is called with a = " << a << "; a is doubled by the function." << std::endl;
a *= 2.;
}
```
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``` C++17
{
h(5); // Ok
double x = 1.;
h(&x); // Ok
}
```
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However, there is a possible ambiguity between the pass-by-copy and pass-by-reference:
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``` C++17
{
double x = 1.;
h(x); // COMPILATION ERROR: should it call h(double) or h(double& )?
}
```
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You can lift the ambiguity for the pass-by-value:
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``` C++17
{
double x = 1.;
h(static_cast<double>(x)); // Ok
}
```
%% Cell type:markdown id: tags:
But not to my knowledge for the pass-by-reference... So you should really avoid doing so: if you really need both functions, name them differently to avoid the ambiguity.
I would even avoid the pointer case: granted, there is no ambiguity for a computer standpoint, but if you get a developer who is not 100% clear about the pointer syntax he might end-up calling the wrong function:
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``` C++17
#include <iostream>
void h2(double a)
{
std::cout << "h2(double) is called with a = " << a << '.' << std::endl;
}
```
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``` C++17
#include <iostream>
void h2(double* a)
{
std::cout << "h2(double*) is called with a = " << *a << "; a is doubled by the function." << std::endl;
*a *= 2.;
}
```
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``` C++17
{
double x = 5.;
double* ptr = &x;
h2(x); // call h2(double)
h2(ptr); // call h2(double*)
h2(*ptr); // call h2(double)
h2(&x); // call h2(double*)
}
```
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### Best viable function
In fact, overloading may work even if the match is not perfect: the **best viable function** is chosen if possible... and some ambiguity may appear if none matches!
The complete rules are very extensive and may be found [here](https://en.cppreference.com/w/cpp/language/overload_resolution); as a rule of thumb you should really strive to write overloaded functions with no easy ambiguity... or not using it at all: sometimes naming the function differently avoids loads of issues!
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``` C++17
#include <iostream>
int min(int a, int b)
{
std::cout << "int version called!" << std::endl;
return a < b ? a : b;
}
```
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``` C++17
#include <iostream>
double min(double a, double b)
{
std::cout << "double version called!" << std::endl;
return a < b ? a : b;
}
```
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``` C++17
{
int i1 { 5 }, i2 { -7 };
double d1 { 3.14}, d2 { -1.e24};
float f1 { 3.14f }, f2 { -4.2f};
short s1 { 5 }, s2 { 7 };
min(5, 7); // no ambiguity
min(i1, i2); // no ambiguity
min(f1, f2); // conversion to closest one
min(f1, d2); // conversion to closest one
min(s1, s2); // conversion to closest one
}
```
%% Cell type:markdown id: tags:
However, with some other types it doesn't work as well if implicit conversion is dangerous and may loose data:
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``` C++17
{
unsigned int i1 { 5 }, i2 { 7 };
min(i1, i2); // COMPILATION ERROR: no 'obvious' best candidate!
}
```
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``` C++17
{
long i1 { 5 }, i2 { 7 };
min(i1, i2); // COMPILATION ERROR: no 'obvious' best candidate!
}
```
%% Cell type:markdown id: tags:
Likewise, if best candidate is not the same for each argument:
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``` C++17
{
float f1 { 5.f };
int i1 { 5 };
min(f1, i1); // for i1 the 'int'version is better, but for f1 the 'double' is more appropriate...
}
```
%% Cell type:markdown id: tags:
### Advice: use overload only when there is no ambiguity whatsoever
That is when:
- The number of arguments is different between overloads.
- Or their types do not convert implicitly from one to another. For instance the following overloads are completely safe to use and the interface remains obvious for the end-user:
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``` C++17
#include <string>
#include <iostream>
std::string GenerateString()
{
std::cout << "No argument!";
return "";
}
```
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``` C++17
std::string GenerateString(char one_character)
{
std::cout << "One character: ";
return std::string(1, one_character);
}
```
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``` C++17
std::string GenerateString(char value1, char value2)
{
std::cout << "Two characters: ";
std::string ret(1, value1);
ret += value2;
return ret;
}
```
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``` C++17
std::string GenerateString(const std::string& string)
{
std::cout << "Std::string: ";
return string;
}
```
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``` C++17
std::string GenerateString(const char* string)
{
std::cout << "Char*: ";
return std::string(string);
}
```
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``` C++17
{
std::cout << GenerateString() << std::endl;
std::cout << GenerateString('a') << std::endl;
std::cout << GenerateString('a', 'b') << std::endl;
std::cout << GenerateString("Hello world!") << std::endl;
std::string text("Hello!");
std::cout << GenerateString(text) << std::endl;
}
```
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## Optional parameters
It is possible to provide optional parameters in the **declaration** of a function:
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``` C++17
// Declaration.
void FunctionWithOptional(double x, double y = 0., double z = 0.);
```
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``` C++17
#include <iostream>
// Definition
void FunctionWithOptional(double x, double y, double z) // notice the absence of default value!
{
std::cout << '(' << x << ", " << y << ", " << z << ')' << std::endl;
}
```
%% Cell type:code id: tags:
``` C++17
// WARNING: Xeus-cling issue!
{
FunctionWithOptional(3., 5., 6.); // ok
FunctionWithOptional(3.); // should be ok, but Xeus-cling issue.
}
```
%% Cell type:markdown id: tags:
The reason not to repeat them is rather obvious: if both were accepted you may modify one of them and forget to modify the others, which would be a bad design...
There is a way to put it in the same place, that I do not recommend it (and your compiler should warn you most of the time): if you do not declare the function beforehand, default arguments may be specified at definition:
%% Cell type:code id: tags:
``` C++17
#include <iostream>
// Definition which double acts as declaration
void FunctionWithOptional2(double x, double y = 0., double z = 0.)
{
std::cout << '(' << x << ", " << y << ", " << z << ')' << std::endl;
}
```
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``` C++17
{
FunctionWithOptional2(3., 5., 6.); // ok
FunctionWithOptional2(3.); // ok
}
```
%% Cell type:markdown id: tags:
In C and C++, arguments are only **positional**: you do not have a way to explicitly set an argument with a name for instance.
Therefore:
* Optional arguments must be put together at the end of the function.
* You must think carefully if there are several of them and put the less likely to be set manually by the function user at then end. In our example above, if you want do call the function with a `x` and a `z` you must mandatorily also provide explicitly `y`.
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## Lambda functions
C++ 11 introduced a shorthand to define functions called __lambda functions__.
An example is the best way to introduce them:
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``` C++17
#include <iostream>
{
// Locally defined function.
auto square = [](double x) -> double
{
return x * x;
};
std::cout << square(5.) << std::endl;
}
```
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Several notes:
* Use `auto` as its return type; said type is not reproducible (see the _square_ and _cube_ example below).
* The symbol `->` that specifies the type of the returned value is optional.
* Parameters come after the `[]` in parenthesis with the same syntax as ordinary functions.
* This is not the same as the [alternate syntax](../1-ProceduralProgramming/4-Functions.ipynb#Alternate-function-syntax) explained earlier, even if they look similar: a lambda may be defined locally (here within a block) whereas a standard function (with usual or alternate syntax) can't.
%% Cell type:code id: tags:
``` C++17
#include <iostream>
{
// Locally defined function.
auto square = [](double x)
{
return x * x;
};
auto cube = [](double x)
{
return x * x * x;
};
std::cout << "Are the lambda prototypes the same type? "
<< (std::is_same<decltype(square), decltype(cube)>() ? "true" : "false") << std::endl;
}
```
%% Cell type:markdown id: tags:
Inside the `[]` you might specify values that are transmitted to the body of the function; by default nothing is transmitted:
%% Cell type:code id: tags:
``` C++17
#include <iostream>
{
int a = 5;
auto a_plus_b = [](int b)
{
return a + b;
};
std::cout << a_plus_b(3) << std::endl; // COMPILATION ERROR: a is not known inside the lambda body.
}
```
%% Cell type:code id: tags:
``` C++17
#include <iostream>
{
int a = 5;
auto a_plus_b = [a](int b) // Notice the `[a]` here!
{
return a + b;
};
std::cout << a_plus_b(3) << std::endl;
}
```
%% Cell type:markdown id: tags:
The values captured in the lambda might be transmitted by reference:
%% Cell type:code id: tags:
``` C++17
#include <iostream>
{
int a = 5;
auto add_to_a = [&a](int b) // Notice the `[&a]` here!
{
a += b;
};
add_to_a(3);
std::cout << a << std::endl;
}
```
%% Cell type:markdown id: tags:
It is possible to capture everything (in the scope where the lambda is defined) by reference by using `[&]` but it is really ill-advised; don't do this!
Lambda functions really shines when you want to use them in a very special context; see below an example using the sort function provided by the standard library, in which for some reasons we want to sort integers but in two blocks: first the odd numbers properly ordered and then the even numbers:
%% Cell type:code id: tags:
``` C++17
// Nice Jupyter Xeus-cling feature that doesn't work for the Conda version of cling in macOS.
?std::sort
```
%% Cell type:code id: tags:
``` C++17
#include <vector>
#include <iostream>
#include <algorithm> // for sort
{
std::vector<int> list { 3, 5, 2, -4, 8, -17, 99, 15, 125447, 0, -1246 };
std::cout << "Initial list = ";
for (int value : list)
std::cout << value << ' ';
// My very specific sort operation:
// Returns true if lhs is odd and rhs isn't or is lhs < rhs.
// Returns true if lhs is odd and rhs isn't or if lhs < rhs.
auto odd_first = [](auto lhs, auto rhs)
{
const bool is_lhs_odd = !(lhs % 2 == 0);
const bool is_rhs_odd = !(rhs % 2 == 0);
if (is_lhs_odd != is_rhs_odd)
return is_lhs_odd;
return lhs < rhs;
};
std::sort(list.begin(), list.end(), odd_first);
std::cout << std::endl << "Sorted list = ";
for (int value : list)
std::cout << value << ' ';
}
```
%% Cell type:markdown id: tags:
Please notice the use of an intermediate local variable for the lambda is not mandatory; the lambda may be provided on the fly:
%% Cell type:code id: tags:
``` C++17
#include <algorithm>
#include <iostream>
{
std::vector<int> list { 3, 5, 2, -4, 8, -17, 99, 15, 125447, 0, -1246 };
std::vector<int> even_only;
// Don't worry about the syntax of `copy_if` or `back_inserter` here; we will see that later!
std::copy_if(list.cbegin(),
list.cend(),
std::back_inserter(even_only),
[](int value)
{
return value % 2 == 0;
});
for (int value : even_only)
std::cout << value << ' ';
}
```
%% Cell type:markdown id: tags:
## Passing a function as a an argument
In some cases, you might want to pass a function as an argument (and honestly most of the time you should refrain to do so: it may underline your design is not top notch).
The syntax to do so is a bit ugly and stems directly from C; it relies upon using a pointer to a function.
The syntax looks like:
`unsigned int (*f) (int, double)`
where:
* `unsigned int` is the return type.
* `int, double` are the type of the parameters of the function given as argument.
* `f` is the name of the argument.
It will be clearer in an example:
%% Cell type:code id: tags:
``` C++17
#include <iostream>
void PrintFunctionCall(int (*f) (int, int), int m, int n)
{
std::cout << "f(" << m << ", " << n << ") = " << f(m, n) << std::endl;
};
```
%% Cell type:code id: tags:
``` C++17
int multiply(int a, int b)
{
return a * b;
}
```
%% Cell type:code id: tags:
``` C++17
int add(int a, int b)
{
return a + b;
}
```
%% Cell type:code id: tags:
``` C++17
PrintFunctionCall(multiply, 5, 6);
PrintFunctionCall(add, 5, 6);
```
%% Cell type:markdown id: tags:
There are other ways to do this task:
* Using a template parameter. Templates will be reached [later in this tutorial](../4-Templates/0-main.ipynb), but for me it's usually the way to go.
* Using [functors](../3-Operators/5-Functors.ipynb)
* Using `std::function`, introduced in C++ 11. However <a href="https://vittorioromeo.info/index/blog/passing_functions_to_functions.html">this blog</a> explains why it's not a good idea; on top of the arguments given there it doesn't seem to respect the prototype closely (a function with double instead of int is for instance accepted).
%% Cell type:markdown id: tags:
## A very special function: __main__
Any C++ program must include one and only one `main` function. Its prototype is __int main(int argc, char** argv)__ where:
* __argc__ is the number of arguments given on the command line. This is at least 1: the name of the program is one argument. For instance, if your program creates a _isPrime_ executable that takes an integer as argument, `argc` will return 2.
* __argv__ is the list of arguments read on the command line, given as an array of C-strings. In our _isPrime_ example, __argv[0]__ is _isPrime_ and __argv[1]__ is the integer given.
Please notice the internal mechanics of C/C++ compiler returns these values; if a user type `isPrime qwerty 20`, the main functions will return argc = 3. It is up to the writer of the main to ensure the arguments are correct.
If some of these values should be interpreted as numbers, it is also up to the developer to foresee the conversion from the C-string to a numerical value.
In the very specific of our Jupyter notebook, a unique main might be defined or not in the file: _cling_ performs some magic to generate one under the hood.
The __main__ function may also be defined as __int main()__ without arguments if the program doesn't actually need any.
Sometimes, in old programs you may see __void main()__; this is not correct and is now refused by most modern compilers.
The return value of the main function is an integer, __EXIT_SUCCESS__ should be returned when the program succeeds and __EXIT_FAILURE__ if it fails. You will often see a numerical value instead of these: __EXIT_SUCCESS__ is just a macro which value is 0. I recommend its use as you should strive to avoid any magic number in your codes.
We will deal with main functions later when we will work in a true C++ environment.
%% Cell type:markdown id: tags:
## `inline` functions
You may also in a function declaration and definition function prepend the prototype by an `inline`. This indicates the compiler this function might be **inlined**: this means the content of the function may be copied directly, thus avoiding a function call and potentially making your code a tiny bit faster. So for instance if you have a function:
%% Cell type:code id: tags:
``` C++17
inline double square(double x)
{
return x * x;
}
```
%% Cell type:markdown id: tags:
when this function is called somewhere, the compiler may replace directly the function by the code inside the definition:
%% Cell type:code id: tags:
``` C++17
{
square(5.); // The compiler might substitute 5. * 5. to the actual function call here
}
```
%% Cell type:markdown id: tags:
This behaviour is pretty similar to the often frowned-upon **macros** from C, but the use of `inline` is absolutely not considered a bad practice... provided you have in mind the way it works:
* You have probably notice the conditional in my statements regarding `inline`: the keyword is an _hint_ given to the compiler... that might be followed or not.
* On the syntactic side, `inline` must be provided both in the declaration `and` the definition.
* `inline` definitions must be provided in header file (see the [upcoming notebook](../6-InRealEnvironment/2-FileStructure.ipynb) that will deal extensively with the file structure to follow in a C++ program). You therefore pay the price in compilation time whenever you change its implementation (as we'll see more in detail in aforementioned notebook, modifying a header file yields more re-compilation).
* Don't bother inlining functions with any complexity whatsoever, so if your function includes a loop or is more than few lines long, write a normal function instead.
The `square` example was sound: this is typically the kind of functions that might be inlined.
Just to finish, my comparison with a macro was not fair; one of the known drawback of macros is perfectly handled:
%% Cell type:code id: tags:
``` C++17
#define SQUARE(x) ((x) * (x))
```
%% Cell type:code id: tags:
``` C++17
#include <iostream>
{
double x = 5.;
std::cout << square(++x) << std::endl;
}
{
double x = 5.;
std::cout << SQUARE(++x) << std::endl;
}
```
%% Cell type:markdown id: tags:
© _CNRS 2016_ - _Inria 2018-2021_
_This notebook is an adaptation of a lecture prepared by David Chamont (CNRS) under the terms of the licence [Attribution-NonCommercial-ShareAlike 4.0 International (CC BY-NC-SA 4.0)](http://creativecommons.org/licenses/by-nc-sa/4.0/)_
_The present version has been written by Sébastien Gilles and Vincent Rouvreau (Inria)_
......
%% Cell type:markdown id: tags:
# [Getting started in C++](./) - [Procedural programming](./0-main.ipynb) - [Dynamic allocations](./5-DynamicAllocation.ipynb)
%% Cell type:markdown id: tags:
<h1>Table of contents<span class="tocSkip"></span></h1>
<div class="toc"><ul class="toc-item"><li><span><a href="#Introduction" data-toc-modified-id="Introduction-1">Introduction</a></span></li><li><span><a href="#Stack" data-toc-modified-id="Stack-2">Stack</a></span></li><li><span><a href="#Heap-and-free-store" data-toc-modified-id="Heap-and-free-store-3">Heap and free store</a></span><ul class="toc-item"><li><span><a href="#Free-store?" data-toc-modified-id="Free-store?-3.1">Free store?</a></span></li></ul></li><li><span><a href="#Arrays-on-heap" data-toc-modified-id="Arrays-on-heap-4">Arrays on heap</a></span></li></ul></div>
%% Cell type:markdown id: tags:
## Introduction
In C++, we can finely control the life cycle of objects and manage the memory allocated to them. This is what makes it possible to create more powerful applications than with many other languages, but it is also the main source of errors in the language. Pointers and dynamic memory management: watch out for danger!
## Stack
The ordinary variables of C++ have a lifetime limited to the current instruction block, whether it is the current function, or an instruction block attached to an `if`, `for` or just independant.
The memory allocated to them is located in an area called a **stack**, and is automatically relieved when exiting the current block using the **last in, first out** principle.
%% Cell type:code id: tags:
``` C++17
{
{
int a { 5 };
double b { 7.4 };
} // at the end of this block, b is released first and then a - but 99.99 % of the time you shouldn't care
// about that order!
// a and b are not available here
}
```
%% Cell type:markdown id: tags:
There are few limitations with the stack:
* The number of memory you can allocate on the stack is rather limited. On a current POSIX OS the order of magnitude is ~ 8 Mo (on Unix type `ulimit -a` in a terminal to get this information). If you allocate more you will get a **stack overflow** (and now you know why the [most popular developers forum](https://stackoverflow.com/) is named this way!)
* The number of memory you can allocate on the stack is rather limited. On a current POSIX OS the order of magnitude is ~ 8 MB (on Unix type `ulimit -s` in a terminal to get this information). If you allocate more you will get a **stack overflow** (and now you know why the [most popular developers forum](https://stackoverflow.com/) is named this way!)
* The information is very local; you can't use it elsewhere. If you pass the variable as argument in a function for instance a copy is made (or if you're using a reference or a pointer you have to be sure all is done when the block is exited!)
* Stack information must be known at compile time: if you're allocating an array on the stack you must know its size beforehand.
%% Cell type:markdown id: tags:
## Heap and free store
You can in fact also explicitly place a variable in another memory area called **heap** or **free store**; doing so overcomes the stack limitations mentioned above.
This is done by calling the `new` operator, which reserves the memory and returns its address, so that the user can store it _with a pointer_.
The **heap** is independent of the **stack** and the variable thus created exists as long as the `delete` operator is not explicitly called. The creation and destruction of this type of variable is the responsibility of the programmer.
%% Cell type:code id: tags:
``` C++17
#include <iostream>
{
int* n = new int(5); // variable created on the heap and initialized with value 5.
std::cout << *n << std::endl;
delete n; // deletion must be explicitly called; if not there is a memory leak!
}
```
%% Cell type:markdown id: tags:
What is especially tricky is that:
* Creating and destroying can be done in places very disconnected in your program.
* You must ensure that whatever the runtime path used in your program each variable allocated on the heap:
- is destroyed (otherwise you get a **memory leak**)
- is only destroyed once (or your program will likely crash with a message about **double deletion**).
In sophisticated programs, this could lead in serious and tedious bookkeeping to ensure all variables are properly handled, even if tools such as [Valgrind](http://www.valgrind.org/) or [Address sanitizer](https://github.com/google/sanitizers/wiki/AddressSanitizer) may help to find out those you will probably have forgotten somewhere along the way.
To be honest, C++ gets quite a bad name due to this tedious memory handling; fortunately the RAII idiom provides a neat way to automatize nicely memory management (which we'll study [later](../5-UsefulConceptsAndSTL/2-RAII.ipynb)) and some vocal critics on forums that regret the lack of [garbage collection](https://en.wikipedia.org/wiki/Garbage_collection_(computer_science)) might actually not be aware of this fundamental (from my point of view at least) idiom.
%% Cell type:markdown id: tags:
### Free store?
**Free store** is very similar in functionality to the **heap** (to the point I had to [check the difference](https://stackoverflow.com/questions/1350819/c-free-store-vs-heap) before writing this...) , and more often than not one word might be used as the other. If you want to be pedantic:
* When memory is handled by `new`/`delete`, you should talk about **free store**.
* When memory is handled by `malloc`/`free` (the C functions), you should talk about **heap**.
Pedantry aside, the important thing to know is to never mix both syntax: if you allocate memory by `new` don't use `free` to relieve it.
%% Cell type:markdown id: tags:
## Arrays on heap
If you want to init an array which size you do not know at compile time or that might overflow the stack, you may to do with `new` syntax mixed with `[]`:
%% Cell type:code id: tags:
``` C++17
{
const unsigned int Ndigit = 5; // not known at compile-time!
int* pi_first_five_digits = new int[Ndigit];
pi_first_five_digits[0] = 3;
pi_first_five_digits[1] = 1;
pi_first_five_digits[2] = 4;
pi_first_five_digits[3] = 1;
pi_first_five_digits[4] = 5;
delete[] pi_first_five_digits;
}
```
%% Cell type:markdown id: tags:
Please notice that:
* No value can be assigned in construction: you must first allocate the memory for the array and only in a second time fill it.
* A `[]` **must** be added to the **delete** instruction to indicate to the compiler this is actually an array that is destroyed.
In fact, my advice would be to avoid entirely to deal directly with such arrays and use containers from the standard library such as `std::vector`:
%% Cell type:code id: tags:
``` C++17
#include <vector>
{
std::vector<int> pi_first_five_digits { 3, 1, 4, 1, 5 };
}
```
%% Cell type:markdown id: tags:
that does the exact same job in a shorter way and is much more secure to use (spoiler: `std::vector` is built upon the RAII idiom mentioned briefly in this notebook).
We shall see `std::vector` more deeply [later](../5-UsefulConceptsAndSTL/3-Containers.ipynb) but will nonetheless use it before this as it is a rather elementary brick in most C++ codes.
%% Cell type:markdown id: tags:
© _CNRS 2016_ - _Inria 2018-2021_
_This notebook is an adaptation of a lecture prepared by David Chamont (CNRS) under the terms of the licence [Attribution-NonCommercial-ShareAlike 4.0 International (CC BY-NC-SA 4.0)](http://creativecommons.org/licenses/by-nc-sa/4.0/)_
_The present version has been written by Sébastien Gilles and Vincent Rouvreau (Inria)_
......
%% Cell type:markdown id: tags:
# [Getting started in C++](./) - [Templates](./0-main.ipynb) - [Introduction](./1-Intro.ipynb)
%% Cell type:markdown id: tags:
<h1>Table of contents<span class="tocSkip"></span></h1>
<div class="toc"><ul class="toc-item"><li><span><a href="#Motivation" data-toc-modified-id="Motivation-1">Motivation</a></span></li><li><span><a href="#Function-templates-(or-methods)" data-toc-modified-id="Function-templates-(or-methods)-2">Function templates (or methods)</a></span><ul class="toc-item"><li><span><a href="#static_assert" data-toc-modified-id="static_assert-2.1"><code>static_assert</code></a></span></li></ul></li><li><span><a href="#Class-templates" data-toc-modified-id="Class-templates-3">Class templates</a></span><ul class="toc-item"><li><span><a href="#Template-method-of-a-template-class" data-toc-modified-id="Template-method-of-a-template-class-3.1">Template method of a template class</a></span></li><li><span><a href="#Friendship-syntax" data-toc-modified-id="Friendship-syntax-3.2">Friendship syntax</a></span></li></ul></li><li><span><a href="#Type-or-non-type-template-parameter" data-toc-modified-id="Type-or-non-type-template-parameter-4">Type or non-type template parameter</a></span></li><li><span><a href="#Few-precisions-about-templates" data-toc-modified-id="Few-precisions-about-templates-5">Few precisions about templates</a></span></li></ul></div>
%% Cell type:markdown id: tags:
## Motivation
The typical introduction to the need of generic programming is implementing a function that determines the minimum between two quantities (by the way don't do that: STL provides it already...)
%% Cell type:code id: tags:
``` C++17
int min(int lhs, int rhs)
{
return lhs < rhs ? lhs : rhs;
}
```
%% Cell type:code id: tags:
``` C++17
double min(double lhs, double rhs)
{
return lhs < rhs ? lhs : rhs;
}
```
%% Cell type:code id: tags:
``` C++17
float min(float lhs, float rhs)
{
return lhs < rhs ? lhs : rhs;
}
```
%% Cell type:code id: tags:
``` C++17
#include <iostream>
{
std::cout << min(5, -8) << std::endl;
std::cout << min(5., -8.) << std::endl;
std::cout << min(5.f, -8.f) << std::endl;
}
```
%% Cell type:markdown id: tags:
Already tired? Yet we have still to define the same for unsigned versions, other types such as `short` ou `long`... And it's not very [**DRY** (Don't Repeat Yourself)](https://en.wikipedia.org/wiki/Don%27t_repeat_yourself): you may have noticed the implementation is exactly the same!
## Function templates (or methods)
A mechanism was therefore introduced in C++ to provide only one implementation: the **templates**:
%% Cell type:code id: tags:
``` C++17
template<class T>
T min2(T lhs, T rhs)
{
return lhs < rhs ? lhs : rhs;
}
```
%% Cell type:code id: tags:
``` C++17
#include <iostream>
{
std::cout << min2<int>(5, -8) << std::endl;
std::cout << min2(5, -8) << std::endl;
std::cout << min2(5., -8.) << std::endl;
std::cout << min2(5.f, -8.f) << std::endl;
}
```
%% Cell type:markdown id: tags:
As you can see:
* The type is replaced by a parameter (here called `T`)
* In the function call, you might specify the type explicitly between brackets (`<>`). If not specified, the compiler may figure it out if said type is used for one of the parameters. In other words, for the following case it won't work:
%% Cell type:code id: tags:
``` C++17
template<class T>
T Convert(int value)
{
return static_cast<T>(value);
}
```
%% Cell type:code id: tags:
``` C++17
{
double x = Convert(5); // Error: can't figure out which type `T` to use!
}
```
%% Cell type:code id: tags:
``` C++17
{
float x = Convert<double>(5); // Ok
}
```
%% Cell type:markdown id: tags:
You may have see noticed there are no constraints on `T` whatsoever. If you invoke a template and the implementation doesn't make sense for the chosen type the compiler will yell (no doubt you have already seen compilers are extremely talented to do so and it is even truer regarding templates...)
You may have noticed there are no constraints on `T` whatsoever. If you invoke a template and the implementation doesn't make sense for the chosen type the compiler will yell (no doubt you have already seen compilers are extremely talented to do so and it is even truer regarding templates...)
%% Cell type:code id: tags:
``` C++17
#include <string>
{
Convert<std::string>(5); // Doesn't make sense so compiler will yell!
}
```
%% Cell type:markdown id: tags:
### `static_assert`
Of course, it would be nicer to get a clearer error message when an impossible type is provided... C++ 20 will introduce the [concept](https://en.cppreference.com/w/cpp/language/constraints) to constraint properly which type is acceptable, but C++ 11 already introduced a way slightly better than C++ 03 with `static_assert`:
%% Cell type:code id: tags:
``` C++17
#include <type_traits> // for std::is_arithmetic
template<class T>
T Convert2(int value)
{
static_assert(std::is_arithmetic<T>(), "T must be an integer or a floating point!");
return static_cast<T>(value);
}
```
%% Cell type:code id: tags:
``` C++17
#include <string>
{
Convert2<std::string>(5); // Doesn't make sense so compiler will yell!
// But first line is much clearer than previously...
}
```
%% Cell type:markdown id: tags:
`static_assert` evolved in C++ 17:
* In C++ 11, it takes two arguments: the test and a string which features an explanation on the topic at hand.
* In C++ 17 and above, you might just give the test; it is actually handy when the test is already crystal clear:
````static_assert(std::is_same<T, int>(), "Check T is an integer");```` is a tad overkill!
That being said, if the test is not that trivial you should really use the possibility to add an explanation.
## Class templates
We have seen templates in the case of functions, but classes can be templated as well:
%% Cell type:code id: tags:
``` C++17
template<class T>
class HoldAValue
{
public:
HoldAValue(T value);
T GetValue() const;
private:
T value_;
};
```
%% Cell type:code id: tags:
``` C++17
template<class T>
HoldAValue<T>::HoldAValue(T value) // the <T> is mandatory to identify properly the class
: value_(value)
{ }
```
%% Cell type:code id: tags:
``` C++17
template<class T>
T HoldAValue<T>::GetValue() const
{
return value_;
}
```
%% Cell type:code id: tags:
``` C++17
#include <iostream>
#include <string>
{
HoldAValue integer(5);
std::cout << "Integer hold: " << integer.GetValue() << std::endl;
HoldAValue<std::string> string("Hello world!"); // If type not specified explicitly it would have been char*...
std::cout << "String hold: " << string.GetValue() << std::endl;
}
```
%% Cell type:markdown id: tags:
The template must be reminded in the definition as well; please notice before the `::` the brackets with the parameters *without their type*.
### Template method of a template class
Notice a template class may provide template methods:
%% Cell type:code id: tags:
``` C++17
template<class T>
class HoldAValue2
{
public:
HoldAValue2(T value);
T GetValue() const;
template<class U>
U Convert() const;
private:
T value_;
};
```
%% Cell type:code id: tags:
``` C++17
template<class T>
HoldAValue2<T>::HoldAValue2(T value)
: value_(value)
{ }
```
%% Cell type:code id: tags:
``` C++17
template<class T>
T HoldAValue2<T>::GetValue() const
{
return value_;
}
```
%% Cell type:markdown id: tags:
In this case there are two `template` keyword for the definition: one for the class and the other for the method:
%% Cell type:code id: tags:
``` C++17
template<class T> // template type for the class first
template<class U> // then template type for the method
U HoldAValue2<T>::Convert() const
{
return static_cast<U>(value_);
}
```
%% Cell type:code id: tags:
``` C++17
{
HoldAValue2 hold(9);
hold.Convert<double>();
}
```
%% Cell type:markdown id: tags:
### Friendship syntax
There is a weird and specific syntax that is expected if you want to declare a friendship to a function. Quite naturally you would probably write something like:
%% Cell type:code id: tags:
``` C++17
template<class T>
class HoldAValue3
{
public:
HoldAValue3(T value);
friend void print(const HoldAValue3<T>& obj);
private:
T value_;
};
```
%% Cell type:code id: tags:
``` C++17
template<class T>
HoldAValue3<T>::HoldAValue3(T value)
: value_(value)
{ }
```
%% Cell type:code id: tags:
``` C++17
#include <iostream>
template<class T>
void print(const HoldAValue3<T>& obj)
{
// Friendship required to access private data.
// I wouldn't recommend friendship where an accessor would do the same task easily!
std::cout << "Underlying value is " << obj.value_ << std::endl;
}
```
%% Cell type:code id: tags:
``` C++17
{
HoldAValue3<int> hold(5);
print(hold); // LINK ERROR, and that is not something amiss in Xeus-cling!
}
```
%% Cell type:markdown id: tags:
To make the friendship work, you have to use in the friendship declaration another label for the template parameter:
%% Cell type:code id: tags:
``` C++17
template<class T>
class HoldAValue4
{
public:
HoldAValue4(T value);
// 'Repeating' the list of template arguments and not using the ones from the class will fix the issue...
// T wouldn't have work here; the label MUST differ.
template<class U>
friend void print(const HoldAValue4<U>& obj);
private:
T value_;
};
```
%% Cell type:code id: tags:
``` C++17
template<class T>
HoldAValue4<T>::HoldAValue4(T value)
: value_(value)
{ }
```
%% Cell type:code id: tags:
``` C++17
#include <iostream>
// Notice it is only a label: in the definition I'm free to use the same label as for the class definitions!
template<class T>
void print(const HoldAValue4<T>& obj)
{
std::cout << "Underlying value is " << obj.value_ << std::endl;
}
```
%% Cell type:code id: tags:
``` C++17
{
HoldAValue4<int> hold(5);
print(hold); // Ok!
}
```
%% Cell type:markdown id: tags:
This way of declaring friendship works but is not entirely fullproof: `print<int>` is hence a friend of `HoldAValue4<double>`, which was not what was sought. Most of the time it's ok but there are 2 other ways to declare friendship; have a look at [this link](https://web.mst.edu/~nmjxv3/articles/templates.html) if you want to learn more about it.
%% Cell type:markdown id: tags:
## Type or non-type template parameter
The examples so far are using **type** parameters: the `T` in the example stands for a type and is deemed to be substituted by a type. Templates may also use **non type** parameters, which are in most cases `enum` or `integral constant` types (beware: floating point types parameters or `std::string` **can't** be used as template parameters!)
Both can be mixed in a given template declaration:
%% Cell type:code id: tags:
``` C++17
template<class TypeT, std::size_t Nelts>
class MyArray
{
public:
explicit MyArray(TypeT initial_value);
private:
TypeT content_[Nelts];
};
```
%% Cell type:code id: tags:
``` C++17
template<class TypeT, std::size_t Nelts>
MyArray<TypeT, Nelts>::MyArray(TypeT initial_value)
{
for (std::size_t i = 0; i < Nelts; ++i)
content_[i] = initial_value;
}
```
%% Cell type:code id: tags:
``` C++17
{
MyArray<int, 5ul> array1(2);
MyArray<double, 2ul> array2(3.3);
}
```
%% Cell type:markdown id: tags:
However, you can't provide a type parameter where a non-type is expected (and vice-versa):
%% Cell type:code id: tags:
``` C++17
{
MyArray<5ul, 5ul> array1(2); // COMPILATION ERROR!
}
```
%% Cell type:code id: tags:
``` C++17
{
MyArray<int, int> array1(2); // COMPILATION ERROR!
}
```
%% Cell type:markdown id: tags:
## Few precisions about templates
* Template parameters must be known or computed at **compile time**. You can't therefore instantiate a template from a value that was entered by the user of the program or computed at runtime.
* In the template syntax, `class` might be replaced by `typename`:
%% Cell type:code id: tags:
``` C++17
template<typename T>
T Convert3(int value)
{
return static_cast<T>(value);
}
```
%% Cell type:markdown id: tags:
There are exactly zero differences between both keywords; some authors suggest conventions (e.g. use `typename` for POD types and `class` for the more complicated types) but they are just that: conventions!
* The definition of the templates must be provided in header files, not in compiled files. The reason is that the compiler can't know all the types for which the template might be used: you may use `std::min` for your own defined types (provided they define a `<` operator...) and obviously STL writers can't foresee which type you will come up with!
* The compiler will generate the code for each type given to the template; for instance if `Convert<double>`, `Convert<unsigned int>` and `Convert<short>` are used in your code, the content of this template function will be instantiated thrice! This can lead to **code bloat**: lots of assembler code is generated, and compilation time may increase significatively.
* So templates are a _very_ nice tool, but make sure to use them only when they're relevant, and you may employ techniques to limit the amount of code actually defined within the template definitions - the more straightforward being defining in a non template function the parts of the implementation that do not depend on the template parameter type. This way, this part of the code is not duplicated for each different type.
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© _CNRS 2016_ - _Inria 2018-2021_
_This notebook is an adaptation of a lecture prepared by David Chamont (CNRS) under the terms of the licence [Attribution-NonCommercial-ShareAlike 4.0 International (CC BY-NC-SA 4.0)](http://creativecommons.org/licenses/by-nc-sa/4.0/)_
_The present version has been written by Sébastien Gilles and Vincent Rouvreau (Inria)_
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