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TransformationTraits

阅读cppreference Type support (basic types, RTTI, type traits)中的一些trait，就会发现，它们添加了很多_t type alias，比如std::remove_reference

template< class T >
using remove_reference_t = typename remove_reference<T>::type;

这是C++14引入的新特性，这就是本章要介绍的内容。

isocpp faq: Type transformation _t aliases Δ

As part of the movement of C++ away from “traits” types, C++14 added type aliases to avoid having to spell out typename and ::type, and actually make the traits more technically usable because they now work in deduced contexts.

// C++11
... typename remove_reference<T>::type ...
... typename make_unsigned<T>::type ...
// new in C++14 
... remove_reference_t<T> ...
... make_unsigned_t<T> ...

NOTE: 简化了编程，强化了TMP。

See also:

abseil The Meta Library#C++14 _t type aliases

The Abseil meta library provides C++11 versions of <type_traits> aliases added to C++14. These aliases allow you to more easily (and intuitively) get the type of a type_traits class template.

For example:

// decay_t is the type of std::decay<T>
template <typename T>
using decay_t = typename std::decay<T>::type;

The Abseil meta library provides aliases for the following type traits that yield a type:

  • absl::remove_cv_t
  • absl::remove_const_t
  • absl::remove_volatile_t
  • absl::add_cv_t
  • absl::add_const_t
  • absl::add_volatile_t
  • absl::remove_reference_t
  • absl::add_lvalue_reference_t
  • absl::add_rvalue_reference_t

Before C++14

在C++14之前，可以自定义的方式来实现:

nlohmann/json/cpp_future.hpp

#pragma once

#include <cstddef> // size_t
#include <type_traits> // conditional, enable_if, false_type, integral_constant, is_constructible, is_integral, is_same, remove_cv, remove_reference, true_type

namespace nlohmann
{
namespace detail
{
// alias templates to reduce boilerplate
template<bool B, typename T = void>
using enable_if_t = typename std::enable_if<B, T>::type;

template<typename T>
using uncvref_t = typename std::remove_cv<typename std::remove_reference<T>::type>::type;
}
}