How to pass a member function to a function used in another member function?

Question

I found something about my problem, but I don't already understand very well. I need to do something like this:

class T
{
    double a;
public:
    double b;
    void setT(double par)
    {
        a = par;
    }

    double funct(double par1)
    {
        return par1 / a;
    } 

    void exec()
    {
        b = extfunct(funct, 10);
    }
};

double extfunct(double (*f)(double),double par2)
{
    return f(par2)+5;
}

Operation and function are only for example, but the structure is that. The reason of this structure is that I have a pre-built class which finds the minimum of a gived function (it's extfunct in the example). So I have to use it on a function member of a class. I understood the difference between pointer to function and pointer to member function, but I don't understand how to write it. Thanks, and sorry for the poor explanation of the problem.

Edit your question dont add information in the comments

Borgleader
– Borgleader

2013-07-01 16:11:40 +00:00
Commented Jul 1, 2013 at 16:11 — Borgleader
– Borgleader, Commented Jul 1, 2013 at 16:11
Random semicolon lottery day...

Kerrek SB
– Kerrek SB

2013-07-01 16:11:55 +00:00
Commented Jul 1, 2013 at 16:11 — Kerrek SB
– Kerrek SB, Commented Jul 1, 2013 at 16:11

Kerrek SB · Accepted Answer · 2013-07-01 16:13:22Z

1

Use a pointer to member function:

struct Foo
{
    void f(int, int) {}
    void g(int, int) {}

    void execute((Foo::*ptmf)(int, int), int a, int b)
    {
        // invoke
        (this->*ptmf)(a, b);
    }
};

Usage:

Foo x;
x.execute(&Foo::f, 1, 2);   // calls x.f(1, 2)
x.execute(&Foo::g, 2, 1);   // calls x.g(2, 1)

These pointers work as expected with virtual functions.

answered Jul 1, 2013 at 16:13

Kerrek SB

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7 Comments

The Newbie Toad Over a year ago

There is a thing that I don't understand. In Usage you define Foo x; but I have to use it in a member internaly the class, in the function exec(). How I could do that.

Kerrek SB Over a year ago

@TommasoFerrari: It's all in the (this->*ptmf). The ->* is an operator. You could invoke the member function on any instance; I just chose this for this example.

The Newbie Toad Over a year ago

Sorry but I don't understand. My function extfunct as an output double, I can't really find out how could be my function extfunct and the other exec. Sorry again, I'm trying to understand.

Kerrek SB Over a year ago

@TommasoFerrari: double extfunct((T::*ptfm)(double), double par2) { return (this->*ptmf)(par2) + 5; }

The Newbie Toad Over a year ago

I have still some problems, I've done exactly what you write, but reports: "[Error] invalid use of 'this' in non-member function" in extfunct, if I make this a member it works!

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