C++: Initializing Struct and Setting Function Pointer

Question

I am trying to initialize a struct with a function pointer, however I am having trouble doing so unless it is done with a global function. The following code works:

float tester(float v){
    return 2.0f*v;
}

struct MyClass::Example{
    typedef float(*MyFunc)(float);

    MyFunc NameOfFunc;

    float DoSomething(float a){
        return NameOfFunc(a);
    }
};

struct Example e;
e.MyFunc = tester;

My problem is whenever I try to do this with the tester function as a function in MyClass the code no longer works. In other words, I change the tester function to:

float MyClass::tester(float v){
    return 2.0f*v;
}

Any and all help is appreciated (including links)! I have tried googling around for the problem but am unsure what to search (I've tried things such as "C++ struct function pointer initialization in class" to no avail)

You need "c++ pointer to member function". BTW, member function requires context (object to be this), so it can't be used the same way as plain functions. — Artem Sobolev
– Artem Sobolev, Commented Dec 16, 2013 at 23:35
Since MyClass::tester doesn't seem to depend on any non-static members of MyClass, maybe you want it to be static or just a free function as you had before? (and then this code would work) — Joseph Mansfield
– Joseph Mansfield, Commented Dec 16, 2013 at 23:37

user123 · Accepted Answer · 2013-12-16 23:39:47Z

4

This is because when you put the function as a member of the class, it will no longer be of the type float (*)(float), but rather, float (MyClass::*)(float).

Look into std::function and std::mem_fn to solve these problems.

Example:

struct foo {
    float bar(int x, int y) {}
};

// This works
std::function<float(int, int)> func = std::mem_fn(&foo::bar);

answered Dec 16, 2013 at 23:39

user123

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1 Comment

Daiwik Daarun Over a year ago

I have tried doing what you have suggested to some degree, but have posted a follow up question as this led to more problems

cheezsteak · Accepted Answer · 2013-12-16 23:37:50Z

2

If you want NameOfFunc to point to float MyClass::tester(float v) then the Myfunc declaration must be,

typedef float (MyClass::*MyFunc)(float);

But then it cannot point to float tester(float v) I don't think you can have one type that can point to either.

Also shouldn't e.MyFunc = tester; be e.NameOfFunc = tester;?

answered Dec 16, 2013 at 23:37

cheezsteak

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