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c++syntaxargumentsfunction-pointers
vanna
vanna

Reputation: 1572

How to pass a C++ function pointer as argument with some fixed values?

I would like to use function pointers in a specific case. I am using a function foo with the following prototype

foo(double (*func)(double,double));

I can call foothe normal way :

double bar(double x, double y) {
    //stuff
};

int main(void) {
    foo(bar);
    return 0; 
};

but I would like to freeze the value of x in order to get a function equivalent to double (*func)(double) like this :

foo(bar(x,double))

Does a syntax similar to this exists in C++ ?

Upvotes: 2

Views: 1564

Answers (2)

Timo
Timo

Reputation: 5176

Here's two alternatives if you don't wanna use std::bind/std::function.

Assuming your compiler supports converting stateless lambdas to function pointers you could use a lambda to bind x:

void foo(double (*f)(double, double)) { (*f)(3.14159, 2.71828); }

double bar(double x, double y) { return x * y; };

int main()
{
    foo([](double x, double y) -> double { return bar(1.0, y); });
    return 0;
}

Or you could even change foo into a template that accepts arbitrary function objects. That way you can use lambdas that have captures:

template<typename TFunc>
void foo(TFunc f) { f(3.14159, 2.71828); }

double bar(double x, double y) { return x * y; };

int main()
{
    double fixedprm = 1.0;
    foo([fixedprm](double x, double y) -> double { return bar(fixedprm, y); });
    return 0;
}

Upvotes: 2

Qaz
Qaz

Reputation: 61970

You can use std::bind if you have C++11. Consider this example that transforms a vector by adding 5 to each element in one swift movement:

#include <iostream>
using std::cout;

#include <functional>
using std::plus;
using std::bind;
using std::placeholders::_1;

#include <vector>
using std::vector;

#include <algorithm>
using std::transform;

int main()
{
    vector<int> v {1, 3, 6};

    //here we bind the value 5 to the first argument of std::plus<int>()
    transform (v.begin(), v.end(), v.begin(), bind (plus<int>(), _1, 5));

    for (int i : v)
        cout << i << ' '; //outputs "6 8 11"
}

As for your example, I was able to write something close to it like this:

#include <iostream>
using std::cout;

#include <functional>
using std::bind;
using std::function;
using std::placeholders::_1;

void foo (function<double (double, double)> func) //take function object
{
    //try to multiply by 3, but will do 2 instead
    for (double i = 1.1; i < 5.6; i += 1.1)
        cout << func (i, 3) << ' '; 
}

double bar (double x, double y)
{
    return x * y;
}

int main()
{
    foo (bind (bar, _1, 2));
}

Output:

2.2 4.4 6.6 8.8 11

I might have overcomplicated something though. It was actually my first time using both std::bind and std::function.

Upvotes: 1

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