C ++中的类对象创建

Question

I have a basic C++ question which I really should know the answer to.

Say we have some class A with constructor A(int a) . What is the difference between:

A test_obj(4);

and

A test_obj = A(4);

?

I generally use the latter syntax, but after looking up something unrelated in my trusty C++ primer I realized that they generally use the former. The difference between these two is often discussed in the context of built-in types (eg int a(6) vs int a = 6 ), and my understanding is that in this case they are equivalent.

However, in the case of user-defined classes, are the two approaches to defining an object equivalent? Or is the latter option first default constructing test_obj , and then using the copy constructor of A to assign the return value of A(4) to test_obj ? If it's this second possibility, I imagine there could be some performance differences between the two approaches for large classes.

I'm sure this question is answered somewhere on the internet, even here, but I couldn't search for it effectively without finding questions asking the difference between the first option and using new , which is unrelated.

Answer 1

A test_obj = A(4); conceptually does indeed construct a temporary A object, then copy/move-construct test_obj from the temporary, and then destruct the temporary.

However this process is a candidate for copy elision which means the compiler is allowed to treat it as A test_obj(4); after verifying that the copy/move-constructor exists and is accessible.

From C++17 it will be mandatory for compilers to do this; prior to that it was optional but typically compilers did do it.

Answer 2

Performance-wise these are equivalent, even if you have a non-standard copy constructor, as mandated by copy elision . This is guaranteed since C++17 but permitted and widely present even in compilers conforming to earlier standards.

Try for yourself, with all optimizations turned off and the standard forced into C++11 (or C++03, change the command line in the top right): https://godbolt.org/g/GAq7fi