Passing a Function as a Parameter in C++
A function is a set of statements that take inputs, perform some specific computation, and produce output. The idea to use functions is to perform some commonly or repeatedly done tasks together and make a function so that instead of writing the same code again and again for different inputs.
The general form of a function is in the below format:
return_type function_name([ arg1_type arg1_name, ... ])
{
// Perform Operations
}
Passing a function as an argument is a useful concept in C++. This concept has already been used while passing a custom comparator function as an argument in std::sort() to sort a sequence of objects as per the need. In this article, we will discuss different ways to design functions that accept another function as an argument.
A function can be passed as a parameter with 3 approaches i.e.
- Passing as Pointer
- Using std::function<>
- Using Lambdas
1. Passing Pointer to a Function
A function can also be passed to another function by passing its address to that function; In simple terms, it could be achieved via pointers.
Example:
C++
#include <iostream>
using namespace std;
int add( int x, int y) { return x + y; }
int multiply( int x, int y) { return x * y; }
int invoke( int x, int y, int (*func)( int , int ))
{
return func(x, y);
}
int main()
{
cout << "Addition of 20 and 10 is " ;
cout << invoke(20, 10, &add) << '\n' ;
cout << "Multiplication of 20"
<< " and 10 is " ;
cout << invoke(20, 10, &multiply) << '\n' ;
return 0;
}
|
Output:
Addition of 20 and 10 is 30
Multiplication of 20 and 10 is 200
2. Using std::function<>
In C++ 11, there is a std::function<> template class that allows to pass functions as objects. An object of std::function<> can be created as follows.
std::function<return_type(arg1_type, arg2-type...)> obj_name
// This object can be use to call the function as below
return_type catch_variable = obj_name(arg1, arg2);
Example:
C++
#include <functional>
#include <iostream>
using namespace std;
int add( int x, int y) { return x + y; }
int multiply( int x, int y) { return x * y; }
int invoke( int x, int y, function< int ( int , int )> func)
{
return func(x, y);
}
int main()
{
cout << "Addition of 20 and 10 is " ;
cout << invoke(20, 10, &add) << '\n' ;
cout << "Multiplication of 20"
<< " and 10 is " ;
cout << invoke(20, 10, &multiply) << '\n' ;
return 0;
}
|
Output:
Addition of 20 and 10 is 30
Multiplication of 20 and 10 is 200
3. Using Lambdas
Lambdas in C++ provide a way to define inline, one-time, anonymous function objects. These lambdas can be defined in a place where it is required to pass a function as an argument.
Example:
C++
#include <functional>
#include <iostream>
using namespace std;
int invoke( int x, int y,
function< int ( int , int )> func)
{
return func(x, y);
}
int main()
{
cout << "Addition of 20 and 10 is " ;
int k = invoke(20, 10,
[]( int x,
int y) -> int {
return x + y;
});
cout << k << '\n' ;
cout << "Multiplication of 20"
<< " and 10 is " ;
int l = invoke(20, 10,
[]( int x,
int y) -> int {
return x * y;
});
cout << l << '\n' ;
return 0;
}
|
Output:
Addition of 20 and 10 is 30
Multiplication of 20 and 10 is 200
Last Updated :
10 Oct, 2022
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