decltypeとSFINAEによるオーバーロード条件分岐 ref: http://qiita.com/_EnumHack/items/e831a359338e5f134325

file0.cpp

#include <iostream>
#include <string>
#include <algorithm>
#include <vector>
#include <map>

template < typename Container >
auto my_find(Container& c, typename Container::key_type key)
    ->decltype((c.find(key)))
{
    std::cout << "member" << std::endl;
    return (c.find(key));
}
template < typename Container >
decltype(auto) my_find(Container& c, typename Container::value_type v)
{
    std::cout << "free" << std::endl;
    return (std::find(c.begin(),c.end(), v)) ;
}

int main(){
    std::vector<int> a{1,2,3};
    std::map<int,int> b;
    b[0] = 1;
    b[2] = 3;
    b[4] = 5;

    auto&& iter1 = my_find(a,2);    
    std::cout << ( iter1 == a.end() ? "not found" : "found" ) << std::endl;
    std::cout << std::endl;
    auto&& iter2 = my_find(b,2);
    std::cout << ( iter2 == b.end() ? "not found" : "found" ) << std::endl;

    return 0;
}

file1.cpp

template < typename T >
auto f(T&& arg)->decltype(arg.f())
{
    std::cout << "Has f()." << std::endl;
}

template < typename T >
auto f(T&& arg)->void
{
    std::cout << "Hasn't f()." << std::endl;
}

file2.cpp

struct hoge{};

f( hoge{} ) ; // OK! Hasn't f()

file3.cpp

struct piyo{
   void f(){}
};

f( piyo{} ) ; // オーバーロードの解決が曖昧！

file4.cpp

extern std::nullptr_t enabler;

template < typename T >
struct has_func
{
private :
    template < typename U >
    static auto check(U x) -> decltype( x.f(), std::true_type{} ) ;
    static std::false_type check(...) ;

public :
    static bool const value = decltype( check( std::declval<T>() ) )::value ;
} ;

template < typename T >
auto f( T&& arg )->decltype(arg.f())
{
    std::cout << "The class has f()." << std::endl;
}

template < typename T, std::enable_if_t<!has_func<T>::value>*& = enabler >
auto f( T&& )->void
{
    std::cout << "The class hasn't f()." << std::endl;
}