“auto x = vector <int>（）”和“vector <int> x”之间有什么区别？

Question

What is the difference between:

auto x = vector<int>();

and

vector<int> x;

Are both of these declarations equivalent, or is there some difference with the run-time complexity?

Answer 1

They have the same effect since C++17. Both construct an object named x with type std::vector<int> , which is initialized by the default constructor of std::vector .

Precisely the 1st one is copy initialization , x is copy-initialized from a value-initialized temporary. From C++17 this kind of copy elision is guaranteed, as the result x is initialized by the default constructor of std::vector directly. Before C++17, copy elision is an optimization:

even when it takes place and the copy/move (since C++11) constructor is not called, it still must be present and accessible (as if no optimization happened at all), otherwise the program is ill-formed:

The 2nd one is default initialization , as a class type x is initialized by the default constructor of std::vector .

Note that the behaviors might be different for other types, depending on the type's behavior and x 's storage duration.