C++ Function Pointer as Argument

Question

I have tried multiple google searches and help guides, but I'm out of ideas on this one. I have a function pointer that I am using as an argument for another function. Both functions are within the same class. However, I keep getting type conversion errors. I'm sure this is just a syntax problem, but I can't understand what the correct syntax is. Here is a simplified version of my code:

Header File

#ifndef T_H
#define T_H

#include <iostream>
#include <complex>

namespace test
{

class T
{
public:
    T();
    double Sum(std::complex<double> (*arg1)(void), int from, int to);
    int i;
    std::complex<double> func();
    void run();
};

}
#endif // T_H

Source File

#include "t.h"

using namespace test;
using namespace std;

//-----------------------------------------------------------------------
T::T()
{
}

//-----------------------------------------------------------------------
double T::Sum(complex<double>(*arg1)(void), int from, int to)
{
    complex<double> out(0,0);

        for (i = from; i <= to; i++)
        {
            out += arg1();
            cout << "i = " << i << ", out = " << out.real() << endl;
        }

    return out.real();
}

//-----------------------------------------------------------------------
std::complex<double> T::func(){
    complex<double> out(i,0);
    return out;
}

//-----------------------------------------------------------------------
void T::run()
{
    Sum(&test::T::func, 0, 10);
}

Whenever I try to compile, I get the following error:

no matching function for call to 'test::T::Sum(std::complex<double> (test::T::*)(),int,int)'
note:  no known conversion for argument 1 from 'std::complex<double> (test::T::*)()' to 'std::complex<double>(*)()'

Any advice appreciated. Or at least a link to a thorough site on how to use function pointers. I am using Qt Creator 2.6.2, compiling with GCC.

Change complex<double>(*arg1)(void) to complex<double>(T::*arg1)(void) — David G
– David G, Commented Jan 24, 2014 at 1:15

Wojtek Surowka · Accepted Answer · 2014-01-24 01:15:10Z

3

Your Sum function expects pointer to a function. And then you try to call it with a pointer to a member function. Learn about pointers to members.

answered Jan 24, 2014 at 1:15

Wojtek Surowka

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Comments

user534498 · Accepted Answer · 2014-01-24 01:25:33Z

1

The code itself is a bit messy, I'll only correct the grammer to make it work.

firstly, you shall change the function prototype from

double Sum(std::complex<double> (*arg1)(void), int from, int to);

to

double Sum(std::complex<double> (T::*arg1)(void), int from, int to);

Meaning that it is a pointer to class T's member.

Then, when calling the function, you cant just arg1(),

for (i = from; i <= to; i++)
{
    out += arg1();
    cout << "i = " << i << ", out = " << out.real() << endl;
}

you have to use (this->*arg1)();

for (i = from; i <= to; i++)
{
    out += (this->*arg1)();
    cout << "i = " << i << ", out = " << out.real() << endl;
}

answered Jan 24, 2014 at 1:25

user534498

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2 Comments

pmr Over a year ago

Now the code doesn't support the plain use-case of passing a normal function anymore.

Nicholas Barczak Over a year ago

Good point. For the moment, I only need it to work for member functions. At a later revision, I may add an overload to allow normal functions. Possibly as a template function for maximum flexiblity. But I still have several thousand lines of code to debug after this. For now, I plan to keep it simple.

pmr · Accepted Answer · 2014-01-24 02:02:50Z

1

How to pass functions as arguments in C++? In general, use a template, unless you have very compelling reasons not do it.

template<typename Func>
void f(Func func) {
  func(); // call
}

On the call side, you can now throw in a certain amount of objects (not just pointers to functions):

Functors;

struct MyFunc {
  void operator()() const {
    // do stuff
  }
};

// use:
f(MyFunc());

Plain functions:

void foo() {}

// use
f(&foo) {}

Member functions:

struct X {
  void foo() {}
};

// call foo on x
#include <functional>
X x;
func(std::bind(&X::foo, x));

Lambdas:

func([](){});

If you really want a compiled function and not a template, use std::function:

void ff(std::function<void(void)> func) {
  func();
}

answered Jan 24, 2014 at 2:02

pmr

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2 Comments

Nicholas Barczak Over a year ago

Turns out that I needed to learn about using pointers to member functions. I also like the idea of using a template which would allow for lambdas. I tried lambdas earlier, and ran into compiler errors. I think for the moment, I will simply write the function to allow pointers to member functions. In a later revision, I may add an overload that uses Sum() as a template function to allow lambdas. Thanks all for the great advice.

pmr Over a year ago

@NicholasBarczak You might want to check if your compiler supports C++11. gcc upwards of 4.7 does, clang upwards of 3.3 as well. Both require the -std=c++11 argument to be passed. MSVC kind of supports it, but there is the mess of versions and compiler pre-releases and what not.

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