Clause 33: decltype is used for auto & type formal parameters, with std:: forward

Pantomorphism lambda The formula is C++14 One of the characteristics of ：lambda Can be used in formal parameters auto:

auto f = [](auto x) {
     return func(normalize(x));};

Above lambda Type of x The only action taken is to forward to normalize. If normalize Treat left and right values differently , Then we can say lambda Style writing is problematic . The correct way to write is to put x Perfect forward to normalize, Two changes are required in the code ：1：x Change to universal quotation ,2： Use std::forward

auto f = [](auto && x)
			{
    return func(normalize(std::forward<???>(x)));};

The above says ??? How to implement it ？ That is, pass it on to std::forward What type of formal parameter should be of .

General situation , When using perfect forwarding , You are in a accept type parameter T In the template function of , So you write std::forward<T> Just go . And in generics lambda in , No type parameters available T.

The solution is that we are lambda Can be detected by x The type of , To determine whether the passed in argument is Left value or right value .decltype It can be done .

auto f = [](auto && x)
			{
    return func(normalize(std::forward<decltype<x>>(x)));};