关键字typeof in c ++ 11

Question

I know:

• typeof is a gcc extension and is not part of the C++ standard.

Questions:

1. Is the word typeof deprecated in C++11? in other words, is it allowed to still use it as a gcc extension when using C++11?
2. Is it correct to say that replacing every typeof with decltype yields the same behaviour of the code?
3. Assume I have template<typename T> class wrapper . What is the best way to declare wrapper_some_field such that it is equivalent to: Wrapper<typeof(some_field)> wrapper_some_field

Answer 1

Is the word typeof deprecated in C++11? in other words, is it allowed to still use it as a gcc extension when using C++11?

It's not deprecated . It never existed as a keyword. gcc suggests that if you compile with -std=c++** that you instead use __typeof__ .

Is it correct to say that replacing every typeof with decltype yields the same behaviour of the code?

No. For example, given:

int& foo();

decltype(foo()) is int& but __typeof__(foo()) is int .

Assume I have template<typename T> class wrapper . [...]

You could write wrapper<std::remove_reference_t<decltype(some_field)>> wrap{some_field} , but it'd be cleaner to write a construction function template:

template <class T> wrapper<T> make_wrapper(T const& val) { return {val}; }
auto wrap = make_wrapper(some_field);

Or, with forwarding:

template <class T>
wrapper<std::decay_t<T>> make_wrapper(T&& val) {
    return {std::forward<T>(val)};
}

Although in C++17 you wouldn't do this at all and would just use class template argument deduction:

template <class T> struct wrapper { T t; };
template <class T> wrapper(T) -> wrapper<T>;
auto wrap = wrapper{42}; // wrap is a wrapper<int>

And in C++20, you won't even need the deduction guide.

Answer 2

#define typeof(...) std::remove_reference_t<decltype(__VA_ARGS__)>;

However, if you want to create storage for a type T , the way to do it in C++11 is to use std::decay_t , or in some situations write your own extension that stores C-style arrays into a std::array .

Wrapper<std::decay_t<T>> wrapper_some_field;

if you want to pass Wrapper a type suitable for storing inside of it.

decay removes references, converts functions to pointers-to-functions, and arrays of T to pointers-to-T, and removes top-level const and volatile after that. These are operations similar to what happens when you pass things to a function as part of the "decay-to-pointer/value" operations.

The result is a type "suitable for storage". As noted, I'd prefer that a int[4] decay to a std::array<int,4> but you cannot have everything .