Question

Consider the following code :

#include <iostream>
#include <vector>
#include <type_traits>

// Version A
template<typename T>
void f(const T& x)
{
    std::cout<<"Version A"<<std::endl;
}

// Version B
template<template<typename> class T, typename T1>
void f(const T<T1>& x)
{
    std::cout<<"Version B"<<std::endl;
}

// Main
int main(int argc, char* argv[])
{
    f(double());
    f(std::vector<double>()); // <- How to force the use of version B ?
    return 0;
}

By default, it will produce :

Version A
Version A

How to force the use of Version B when the passed type is a template template with the good shape (I can add new versions of f, I can add std::enable_if or other C++11 type traits syntax, but if possible I would like to avoid adding an helper class) ?

Answer 1

std::vector does not take a single typename parameter, it takes 2! Don’t forget the allocator.

Thus, use variadic templates:

template<template<typename...> class T, typename T1>
void f(const T<T1>& x)
{
    std::cout<<"Version B"<<std::endl;
}

Now it works as you want.