Pointer to deque <int> :: push_back

Question

#include <iostream> #include <deque> using namespace std; main() { typedef void (deque<int>::*func_ptr)(int); func_ptr fptr = &deque<int>::push_back; } 

Im trying to get a pointer to this function, but I get a compilation error

 error: cannot convert 'void (std::deque<int>::*)(const value_type&) {aka void (std::deque<int>::*)(const int&)}' to 'func_ptr {aka void (std::deque<int>::*)(int)}' in initialization func_ptr fptr = &deque<int>::push_back; 

I want to do this so that I can get a pointer to different member functions based on different conditions.

I named this link .

Answer 1

push_back() takes a const T & parameter as indicated in the error message:

cannot convert 'void (std :: deque :: *) (const value_type &) ...

Change the type alias:

 typedef void (deque<int>::*func_ptr)(const int &); 

Answer 2

As the accepted answer says, the problem is that the type signature is different - for std::deque<T> , push_back only has an overload that accepts T const& , and not T directly. typedef void (deque<int>::*func_ptr)(const int &) is a very concise and complicated way to write this.

I wanted to turn to C ++ 11 to do this - enter aliases. First you might wonder, “Why not use auto ?” This is not possible because the available function is an overloaded function, and auto does not know which of the overloads to access. Since the original question introduces knowledge of the type of the void(int) function, the &deque<int>::push_back operator selects the correct overload. The problem with the simpler statement is that it bakes knowledge of the container and the type contained in it. If you want to switch to std::vector or from int to short , you will have to create all new types. Using type aliases, we can template everything to avoid entering knowledge types:

 using func_ptr = void(Cont::*)(typename Cont::const_reference); 

We can do something simple as before:

 func_ptr<deque<int>> fptr = &deque<int>::push_back; 

... or we can reference the container somewhere:

 vector<short> container; 

... search for its type at compile time and save a pointer to its push_back function, regardless of what the container is:

 using container_type = decltype(container); func_ptr<container_type> fptr2 = &container_type::push_back; 

Answer 3

There is a big problem with what you are trying to do. Implementations are allowed to add default parameters for any member functions, so the actual signature for push_back() may vary. It may even add additional features with the same name (but a different signature).

I advise against accepting pointers to container functions. You are usually better off without it.