知道为什么它没有实施。

Well, I'm not aware of why it wasn't implemented. Perhaps it wasn't proposed, or perhaps there were some subtle gotchas.

也就是说，我想你可以很容易地写。

That said, I think you can write it pretty easily

template<typename T>
struct protect_wrapper : T
{
    protect_wrapper(const T& t) : T(t)
    {

    }

    protect_wrapper(T&& t) : T(std::move(t))
    {

    }
};

template<typename T>
typename std::enable_if< !std::is_bind_expression< typename std::decay<T>::type >::value,
                T&& >::type
protect(T&& t)
{
    return std::forward<T>(t);
}

template<typename T>
typename std::enable_if< std::is_bind_expression< typename std::decay<T>::type >::value,
                protect_wrapper<typename std::decay<T>::type > >::type
protect(T&& t)
{
    return protect_wrapper<typename std::decay<T>::type >(std::forward<T>(t));
}

这两个版本的保护使得非绑定表达式不被包装他们只是通过）。一切都通过move / copy传递给 protect_wrapper ，它只是从类型继承。

The two versions of protect are so that non-bind expressions are not wrapped (they just pass through). Everything else is passed by move/copy to the protect_wrapper, which simply inherits from the type. This allows the type's functions to pass through, or for it to convert to the type.

它可以复制/移动，所以它可以安全地使用rvals。这个函数允许类型的函数通过，或者转换为类型。因为它只保护bind_expressions的类型，所以最小化必须发生的复制量。

It makes a copy/move however, so it can be safely used with rvals. And since it only protects types that are bind_expressions, it minimizes the amount of copying that has to occur.

int main()
{

    //// Ok, with protect
    auto bind_expr =
        std::bind<int>(invoke_with_42{}
          , protect(std::bind(&add, 1, std::placeholders::_1)));


    std:: cout << bind_expr() << std::endl;
    return 0;

}