C++ string initialization

Question

I've seen a statement according to which

string noun("ants");
string noun = "ants";

are exactly equivalent.

This is contrary to my intuition: I thought in the second case a conversion occurs (via a constructor of the string class), then the assignment operator gets called with an argument of class string. What is there actually happening ?

Answer 1

For initialization of a variable definition using assignment, like

std::string a = "foo";

Here two objects are created: The variable you define (a in my example) and a temporary object (for the string "foo"). Then the copy-constructor of your variable (a) is called with the temporary object, and then the temporary object is destructed. The copy-assignment operator is not called.

However, the copying can be avoided if the compiler uses copy elision, which is an optimization technique to avoid unnecessary copies and copying. The copy-constructor needs to be there anyway, even if it's not called.

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For the definition

std::string a("foo");

there is a better constructor which takes a pointer to a constant string, which the literal "foo" can be seen as (string literals are actually constant arrays of char, but like all arrays they decay to pointers).