How do I store a function to a variable?

Question

I think they are called functors? (it\'s been a while)

Basically, I want to store a pointer to a function in a variable, so I can specify what function I want to use

Answer 1

IF you have Boost installed, you can also check out Boost Function.

Answer 2

You could also use function either from the c++0x or from boost. That would be

boost::function<int(int)>

and then use bind to bind your function to this type.

Have a look here and here

Ok here would be a example. I hope that helps.

int MyFunc1(int i)
{
    std::cout << "MyFunc1: " << i << std::endl;
    return i;
}

int MyFunc2(int i)
{
    std::cout << "MyFunc2: " << i << std::endl;
    return i;
}

int main(int /*argc*/, char** /*argv*/)
{
    typedef boost::function<int(int)> Function_t;

    Function_t myFunc1 = boost::bind(&MyFunc1, _1);
    Function_t myFunc2 = boost::bind(&MyFunc2, _1);

    myFunc1(5);
    myFunc2(6);
}

Answer 3

No, these are called function pointers.

unsigned int (*fp)(unsigned int) = func_1;

Answer 4

From C++11 you can use std::function to store functions. To store function you use it as follsonig:

std::function<return type(parameter type(s))>

as an example here it is:

#include <functional>
#include <iostream>

int fact (int a) {
    return a > 1 ? fact (a - 1) * n : 1;
}

int pow (int b, int p) {
    return p > 1 ? pow (b, p - 1) * b : b;
}

int main (void) {
    std::function<int(int)> factorial = fact;
    std::function<int(int, int)> power = pow;

    // usage
    factorial (5);
    power (2, 5);
}

Answer 5

unsigned int (* myFuncPointer)(unsigned int) = &func_1;

However, the syntax for function pointers is awful, so it's common to typedef them:

typedef unsigned int (* myFuncPointerType)(unsigned int);
myFuncPointerType fp = &func_1;

Answer 6

The simplest you can do is

unsigned int (*pFunc)(unsigned int) = func_1;

This is a bare function pointer, which cannot be used to point to anything other than a free function.

You can make it less painful if your compiler supports the C++0x auto keyword:

auto pFunc = func_1;

In any case, you can call the function with

unsigned int result = pFunc(100);

There are many other options that provide generality, for example:

• You can use boost::function with any C++ compiler
• With a compiler implementing features of C++0x you can use std::function

These can be used to point to any entity that can be invoked with the appropriate signature (it's actually objects that implement an operator() that are called functors).

Update (to address updated question)

Your immediate problem is that you attempt to use Config::current_hash_function (which you declare just fine) but fail to define it.

This defines a global static pointer to a function, unrelated to anything in class Config:

unsigned static int (*current_hash_function)(unsigned int) = kennys_hash_16;

This is what you need instead:

unsigned int (*Config::current_hash_function)(unsigned int) = kennys_hash_16;