Changing value_type of a given STL container

Question

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

Basically, I want a class template of this form, or a variant of it:

template<typename container, typename new_value_type>
struct change_value_type
{
    typedef /*....*/  new_container;
};

So that I can use it in the following way:

typename change_value_type<vector<A>, B>::new_container  vectorOfB; 
vectorOfB.push_back(B());
vectorOfB.push_back(B());
vectorOfB.push_back(B());
//etc

Means, new_container is vector<B>

Is it possible?

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 <class Container, class NewType>
struct rebind;

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

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

template <class ValueType, class A, class B, template <class, class, class> class Container, class NewType>
struct rebind<Container<ValueType, A, B>, NewType>
{
  typedef Container<NewType, typename rebind<A, ValueType>::type, typename rebind<B, ValueType>::type> type;
};

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

C++11 makes it a bit easier:

template <class Container, class NewType>
struct rebind;

template <class ValueType, class... Args, template <class...> class Container, class NewType>
struct rebind<Container<ValueType, Args...>, NewType>
{
  typedef Container<NewType, typename rebind<Args, NewType>::type...> type;
};

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

template <class ValueType, std::size_t N, template <class, std::size_t> class Container, class NewType>
struct rebind<Container<ValueType, N>, NewType>
{
  typedef Container<NewType, N> type;
};

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

#include <iostream>
#include <typeinfo>
#include <vector>
#include <set>
#include <deque>
#include <queue>
#include <list>
#include <array>

#include "rebind.h"

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

int main()
{
  REBIND_DEMO(std::set<float>, double);
  REBIND_DEMO(std::list<float>, double);
  REBIND_DEMO(std::deque<float>, double);
  REBIND_DEMO(std::queue<float>, double);
  typedef std::array<float, 4> TestArray;
  REBIND_DEMO(TestArray, double);
  REBIND_DEMO(std::unordered_set<float>, double);

  return 0;
}

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

std::set<float, std::less<float>, std::allocator<float> >
std::set<double, std::less<double>, std::allocator<double> >
std::list<float, std::allocator<float> >
std::list<double, std::allocator<double> >
std::deque<float, std::allocator<float> >
std::deque<double, std::allocator<double> >
std::queue<float, std::deque<float, std::allocator<float> > >
std::queue<double, std::deque<double, std::allocator<double> > >
std::array<float, 4ul>
std::array<double, 4ul>
std::unordered_set<float, std::hash<float>, std::equal_to<float>, std::allocator<float> >
std::unordered_set<double, std::hash<double>, std::equal_to<double>, std::allocator<double> >

Answer 2

You might try specializing with template template parameters.

#include <vector>
#include <list>
#include <deque>
#include <string>

template <class T, class NewType>
struct rebind_sequence_container;

template <class ValueT, class Alloc, template <class, class> class Container, class NewType>
struct rebind_sequence_container<Container<ValueT, Alloc>, NewType >
{
     typedef Container<NewType, typename Alloc::template rebind<NewType>::other > type;
};

template <class Container, class NewType>
void test(const NewType& n)
{
    typename rebind_sequence_container<Container, NewType>::type c;
    c.push_back(n);
}

int main()
{
    std::string s;
    test<std::vector<int> >(s);
    test<std::list<int> >(s);
    test<std::deque<int> >(s);
}

However, containers might not have those two template parameters.

Also, in container adapters and associative containers, not just the allocator would need replacing (underlying container in adapters, predicate in std::set). OTOH, their usage is so different from sequence containers that it is hard to imagine a template that works with any container type.

Answer 3

You're referring (I believe) to the Policy Clone idiom, using rebind