Parameter elision

When you pass an argument to a function, and the argument is a prvalue expression of the function’s parameter type, and this type is not a reference, then the prvalue’s construction can be elided.

void func(std::string str) { ... }

func(std::string("foo"));

This says to create a temporary string, then move it into the function parameter str. Copy elision permits this expression to directly create the object in str, rather than using a temporary+move.

This is a useful optimization for cases where a constructor is declared explicit. For example, we could have written the above as func("foo"), but only because string has an implicit constructor that converts from a const char* to a string. If that constructor was explicit, we would be forced to use a temporary to call the explicit constructor. Copy elision saves us from having to do a needless copy/move.