CODEWITHSUNDEEP
c++type-traits
rubix_addict
rubix_addict

Reputation: 1921

How to check if copy constructor is noexcept without using type_traits?

After some experimenting I've managed to write a piece of code that will check whether the move constructor of a class is noexcept (without using type_traits; this is just an exercise):

std::cout << std::boolalpha << noexcept(Widget(std::declval<Widget>())) << std::endl;

The declval function "returns" Widget&&, so the whole expression is a move constructor call. But how to achieve something similar for the copy constructor?

Upvotes: 3

Views: 1338

Answers (2)

David G
David G

Reputation: 96845

Use std::declval<Widget&>(). Reference collapsing will turn T&& & into T&:

std::cout << std::boolalpha << noexcept(Widget(std::declval<Widget&>())) << std::endl;
//                                                          ^^^^^^^^

Upvotes: 9

TartanLlama
TartanLlama

Reputation: 65640

You could declare your own declval-like function:

template< class T >
typename std::add_lvalue_reference<T>::type mydeclval() noexcept;

//outputs true if the copy constructor is declared noexcept
std::cout << std::boolalpha << noexcept(Widget(mydeclval<Widget>())) << std::endl;

Upvotes: 2

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