C++ pointers to member functions

问题:

I'd like to use a pointer to member function in C++, but it doesn't work:

pointer declaration:

int (MY_NAMESPACE::Number::*parse_function)(string, int);

pointer assignation:

parse_function = &MY_NAMESPACE::Number::parse_number;

This call works perfectly (itd is an iterator to elements of a map):

printf("%s\t%p\n",itd->first.c_str(),itd->second.parse_function);

But this one doesn't work:

int ret = (itd->second.*parse_function)(str, pts);
$ error: 'parse_function' was not declared in this scope

And this one neither

int ret = (itd->second.*(MY_NAMESPACE::Number::parse_function))(str, pts);
$ [location of declaration]: error: invalid use of non-static data member 'MY_NAMESPACE::Number::parse_function'
$ [location of the call]: error: from this location

I don't understant why ...

Thx in advance !!

回答1:

int (MY_NAMESPACE::Number::*parse_function)(string, int);

This shows, parse_function is a pointer to a member function of class Number.

This call works perfectly (itd is an iterator to elements of a map):

printf("%s\t%p\n",itd->first.c_str(),itd->second.parse_function);

and from this we can see parse_function is a member of itd->second, whatever this is.

For this call

int ret = (itd->second.*parse_function)(str, pts);

or this call

int ret = (itd->second.*(MY_NAMESPACE::Number::parse_function))(str, pts);

to succeed, itd->second must be of type Number, which it presumably isn't. And parse_function must be defined as either a variable in the current or enclosing scope (fist case) or a static variable of class Number (second case).

So you need some Number and apply parse_function to that

Number num;
(num.*(itd->second.parse_function))(str, pts);

or with a pointer

Number *pnum;
(pnum->*(itd->second.parse_function))(str, pts);

Update:

Since itd->second is a Number, you must apply parse_function, which is a member of it, like this

int ret = (itd->second.*(itd->second.parse_function))(str, pts);

回答2:

You can define pointers to functions like so: type(*variable)() = &function; For example:

int(*func_ptr)();
func_ptr = &myFunction;

I might just not realize your code this early morning, but problem could be that parse_function is a pointer, yet you're calling it like itd->second.*parse_function. Pointers are called with the ->*, so try doing itd->second->parse_function.

Might not fix anything tho, I can't really seem to catch onto your code. Posting more information, it's hard to tell from two lines of code.

---

Here's one example on how it's used in actual code, this one calls func() through cb() using pointers and parameters only:

int func()
{
    cout << "Hello" << endl;
    return 0;
}

void cb(int(*f)())
{
    f();
}

int main()
{
    int(*f)() = &func;
    cb(f);
    return 0;
}