The standard signature of std::forward is:
template<typename T>
constexpr T&& forward(std::remove_reference_t<T>&) noexcept;
template<typename T>
constexpr T&& forward(std::remove_reference_t<T>&&) noexcept;
Because the parameter type isn't T directly, we should specify the template argument when using std::forward:
template<typename... Args>
void foo(Args&&... args)
{
bar(std::forward<Args>(args)...);
}
However sometimes the template argument is not as simple as Args. auto&& is a case:
auto&& vec = foo();
bar(std::forward<decltype(vec)>(vec));
You can also imagine more complicated template argument types for std::forward. Anyway, intuitively std::forward should know what T is but it actually don't.
So my idea is to omit <Args> and <decltype(vec)> no matter how simple they are. Here is my implementation:
#include <type_traits>
template<typename T>
std::add_rvalue_reference_t<std::enable_if_t<!std::is_lvalue_reference<T>::value, T>>
my_forward(T&& obj)
{
return std::move(obj);
}
template<typename T>
T& my_forward(T& obj)
{
return obj;
}
int main()
{
my_forward(1); // my_forward(int&&)
int i = 2;
my_forward(i); // my_forward(int&)
const int j = 3;
my_forward(j); // my_forward(const int&)
}
When obj is rvalue reference, for example int&&, the first overload is selected because T is int, whose is_lvalue_reference is false;
When obj is lvalue reference, for example const int&, the second overload is selected because T is const int& and the first is SFINAE-ed out.
If my implementation is feasible, why is std::forward still requiring <T>? (So mine must be infeasible.)
If not, what's wrong? And still the question, is it possible to omit template parameter in std::forward?
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