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How do I write a template function that accepts && and const & both?

for example

template<typename T> void f(T&& t) {}
template<typename T> void f(T const& t) {}

When i call

int i;
f(i); // call f(T&&) which I expect to call f(T const&), how to solve it?
f(10); // call f(T&&), that is fine
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2 answers


This will be one of the ways:

#include <type_traits>

template<typename T>
typename std::enable_if< !std::is_lvalue_reference<T>::value >::type
f(T&& t) {}

template<typename T> void f(T const& t) {}


Another possibility is to send tags:

template<typename T>
void f_(const T&, std::true_type) { std::cout << "const T&\n"; }
template<typename T>
void f_(T&&, std::false_type) { std::cout << "T&&\n"; }

template<typename T>
void f(T&& t)
{
    f_(std::forward<T>(t), std::is_lvalue_reference<T>{} );
}
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Another variant:

template<typename T>
struct f_caller
{
    void operator () (T&& ) { std::cout << "T&&" << std::endl; }
    void operator () (T const& ) { std::cout << "const T&" << std::endl; }
};


template <typename T>
void f(T&& t)
{
    f_caller<typename std::decay<T>::type>()(std::forward<T>(t));
}


Live example

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