C++ function pointer with string

Question

In C++, I have many functions (one, two three...) that look like this:

int MyClass::one(Mynamespace::Data* data)
{
    //do something
    return 0;
}

I also have an action function that I want to use to call one, two, three... functions with a function pointer.

int MyClass::action(Mynamespace::Data* data)
{
    int (*actionFunction)(Mynamespace::Data*);
    actionFunction = data->name; // the name is a string with the function name (one, two, three...)
    return (*actionFunction)(data);

}

The error I get says:

int (MyClass::)(Mynamespace::Data*)’ does not match ‘int (*)(Mynamespace::Data*)

What am I doing wrong? If there is another way to do this fast and easy I would like to learn it too. Thank you

It is clearly saying the types are different, and I know that you know what type name actually is, if you have declared the member yourself (and if you have declared it yourself, then you should be able to figure out how actionFunction should be declared). So could you let us know how exactly have you declared name? — Sarfaraz Nawaz
– Sarfaraz Nawaz, Commented Jul 5, 2013 at 8:03
Thank you for your help. David's and Stewart's answers were very helpful. However, I forgot to mention that MyClass is a base class and one, two, three... functions are overrided in other classes. I've have decided to use an enum and a normal switch case to call them for the moment as pointers gave me problems with that. — Katu
– Katu, Commented Jul 5, 2013 at 9:44
One of the other things that is special about pointers to members is that they handle virtual functions in the way you would expect, so if you have a base class with a virtual method foo, and you take a pointer to member and call it on a derived class which overrides foo, the derived foo will be called even though you took the pointer to member on the base class. — Stewart
– Stewart, Commented Jul 5, 2013 at 13:03

David Schwartz · Accepted Answer · 2013-07-05 08:03:33Z

4

You need a table that maps function names to function addresses. You can use a variable of type
std::map<std::string, int (MyClass::*)(Mynamespace::Data*)> to hold it.

The ParaShift C++ faq has an article on arrays of pointers to member functions that you should probably understand completely before attempting this, since this is more complex.

answered Jul 5, 2013 at 8:03

David Schwartz

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Comments

Stewart · Accepted Answer · 2013-07-05 08:09:33Z

Pointers to non-static member functions in C++ are different to pointers to standalone functions. This is because non-static member functions must have a this pointer passed to them implicitly when they are called.

As a result of this, you cannot assign a pointer to a member function to a regular function pointer. The compiler is telling you this, but in a complicated way. What the compiler is saying is that you are trying to assign a value of type:

int (MyClass::)(Mynamespace::Data*)

which is how you spell the type of a pointer to a non static member of MyClass to a variable of type

int (*)(Mynamespace::Data*)

Which is how you spell the type of a regular function pointer.

To fix it, you should change the definition of actionFunction to the following

int (MyClass::* actionFunction)(Mynamespace::Data*)

Which declares a member function pointer, and then when you call through the pointer on the return line you should use the member-function operator to call it like so:

return (this->*actionFunction)(data);

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