Priority when choosing overloaded template functions in C++

Question

I have the following problem:

class Base
{
};

class Derived : public Base
{
};

class Different
{
};

class X
{
public:
  template 
  stat         


        

Answer 1

I was looking to setup priorities on overlapping enable_if's, specifically for falling back on calling STL container methods, where my traits were things such as is_assignable is_insterable etc.. with which there is overlap on a number of containers.

I wanted to prioritise assign, if it existed, else use an insert iterator. This is a generic example of what I came up with (modified with infinite levels of priority by some handy folk in the #boost irc channel). It works as the implicit conversion of the priority level ranks the overload below another that is otherwise an equally valid option - removing the ambiguity.

#include 
#include 

template 
struct priority : priority {};

template <>
struct priority<0> {};

using priority_tag = priority<2>;

template  
void somefunc(T x, priority<0>)
{
    std::cout << "Any" << std::endl;
}

template  
std::enable_if_t::value >
somefunc(T x, priority<2>)
{
    std::cout << "is_pod" << std::endl;
}

template 
std::enable_if_t::value >
somefunc(T x, priority<1>)
{
    std::cout << "is_float" << std::endl;
}

int main()
{
    float x = 1;
    somefunc(x, priority_tag{});
    int y = 1;
    somefunc(y, priority_tag{}); 
    std::string z;
    somefunc(z, priority_tag{});
    return 0;
}

It was also suggested that in C++ 14 I could just use constexpr if statements to achieve the same thing, which was far cleaner if Visual Studio 2015 supported them. Hopefully this will help someone else.

#include 
#include 

template 
void somefunc(T x)
{
    if constexpr(std::is_floating_point::value) {
      static_assert(std::is_floating_point::value);
      std::cout << "is_float" << std::endl;
    } else if constexpr(std::is_pod::value) {
      static_assert(std::is_pod::value);
      std::cout << "is_pod" << std::endl;
    } else {
      static_assert(!std::is_floating_point::value);
      static_assert(!std::is_pod::value);
      std::cout << "Any" << std::endl;
    }
}

int main()
{
    float x = 1;
    somefunc(x);
    int y = 1;
    somefunc(y); 
    std::string z;
    somefunc(z);
    return 0;
}

// thanks to k-ballo @ #boost!