Changing value_type of a given STL container

Question

Suppose, I\'ve a STL container type (not object), say vector. Now it\'s value_type is A, so I want to change it to <

Answer 1

Just stumbled over this as I was trying to essentially solve the same problem. It can even be made to work without relying on the rebind type that is specific to std::allocator – the only requirement is that the rebound value type is the first template parameter of the respective classes. This is the case for all relevant STL classes (std::vector, std::set, std::list etc. as well as for example std::less and std::allocator).

A pre-C++11 solution would look like this:

template 
struct rebind;

template  class Container, class NewType>
struct rebind, NewType>
{
  typedef Container type;
};

template  class Container, class NewType>
struct rebind, NewType>
{
  typedef Container::type> type;
};

template  class Container, class NewType>
struct rebind, NewType>
{
  typedef Container::type, typename rebind::type> type;
};

// Continue for more parameters (A, B, C, ...)

C++11 makes it a bit easier:

template 
struct rebind;

template  class Container, class NewType>
struct rebind, NewType>
{
  typedef Container::type...> type;
};

In order to support std::array, the following can be added:

template  class Container, class NewType>
struct rebind, NewType>
{
  typedef Container type;
};

The result can be used with just about any STL type:

#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 

#include "rebind.h"

// Make it all a bit more compact
#define REBIND_DEMO(container, new_type)                \
  do {                                                  \
    container test;                                     \
    rebind::type test2;       \
    std::cout << typeid(test).name() << "\n";           \
    std::cout << typeid(test2).name() << "\n";          \
  } while (0)

int main()
{
  REBIND_DEMO(std::set, double);
  REBIND_DEMO(std::list, double);
  REBIND_DEMO(std::deque, double);
  REBIND_DEMO(std::queue, double);
  typedef std::array TestArray;
  REBIND_DEMO(TestArray, double);
  REBIND_DEMO(std::unordered_set, double);

  return 0;
}

Running this and piping the output through c++filt -t on a Linux system gives you

std::set, std::allocator >
std::set, std::allocator >
std::list >
std::list >
std::deque >
std::deque >
std::queue > >
std::queue > >
std::array
std::array
std::unordered_set, std::equal_to, std::allocator >
std::unordered_set, std::equal_to, std::allocator >