自C ++ 14起合法使用尾随返回类型语法 [英] Legitimate uses of the trailing return type syntax as of C++14

问题描述

实际上是否有理由再使用以下语法:

Is there actually any reason to use the following syntax anymore :

template<typename T>
auto access(T& t, int i)
  -> decltype(t[i])
{
    return t[i];
}

现在我们可以使用了:

template<typename T>
decltype(auto) access(T& t, int i)
{
    return t[i];
}

尾随返回类型语法现在看起来有点多余吗?

The trailing return type syntax now seems a little redundant?

推荐答案

推论的返回类型不是SFINAE友好的.如果t[i]无效，则此过载将简单地退出过载设置:

Deduced return types are not SFINAE friendly. This overload will simply drop out of the overload set if t[i] is invalid:

template<typename T>
auto access(T& t, int i)
  -> decltype(t[i])
{
    return t[i];
}

此过载不会导致严重错误:

Whereas this overload will not, leading to a hard error:

template<typename T>
decltype(auto) access(T& t, int i)
{
    return t[i];
}

演示

此外，您可能会遇到推论的返回类型冲突的问题.考虑是否要返回std::optional<T>.由于std::nullopt_t与std::optional<T>的类型不同，因此以下代码无法编译:

Also, you can run into issues with conflicting deduced return types. Consider if I wanted to return a std::optional<T>. The following code doesn't compile since std::nullopt_t is not the same type as std::optional<T>:

#include <optional> // C++17 standard library feature

template <typename T>
auto foo(T const& val)
{
    if (val.is_invalid()) return std::nullopt;
    return val.some_function_returning_an_optional();
}

通过跟踪返回类型，您可以准确指定要返回的表达式的类型:

Trailing return types let you specify exactly which expressions' type to return:

template <typename T>
auto foo(T const& val)
    -> decltype(val.some_function_returning_an_optional())
{
    if (val.is_invalid()) return std::nullopt;
    return val.some_function_returning_an_optional();
}

您可以使用前导返回类型，但是这需要使用std::declval，这使得它很难理解:

You could use a leading return type, but it would require the use of std::declval, which makes it harder to understand:

template <typename T>
decltype(std::declval<T const&>().some_function_returning_an_optional())
foo(T const& val)
{
    if (val.is_invalid()) return std::nullopt;
    return val.some_function_returning_an_optional();
}

演示

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