What's the difference between "auto x = vector<int>()" and "vector<int> x"?

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.