Copy constructor elision?

Question

Don\'t quite understand why this copy constructor is not invoked when I build with debug mode using VC2010.

class SomeClass
{
public:
    SomeClass(int meani         


        

Answer 1

It is an optimization done by the compiler. According to the language specification, the compiler is allowed to omit the call to the copy-constructor whenever it can.

An accessible copy-constructor is needed for semantic check only, even though it is not actually called. Semantic check is done much before the optimization.

However, if you compile it with -fno-elide-constructors option with GCC, then the copy-elision will not be performed, and the copy-constructor will be called. The GCC doc says,

-fno-elide-constructors

The C++ standard allows an implementation to omit creating a temporary which is only used to initialize another object of the same type. Specifying this option disables that optimization, and forces G++ to call the copy constructor in all cases.

With MSVC10, you can use /Od which according to the MSDN turns off all optimizations in the program.

Note : Wikipedia has an article on copy elision

Answer 2

According to the C++11 standard, §12.8.31:

…This elision of copy/move operations, called copy elision, is permitted in the following circumstances (which may be combined to eliminate multiple copies):

…

• when a temporary class object that has not been bound to a reference would be copied/moved to a class object with the same cv-unqualified type, the copy/move operation can be omitted by constructing the temporary object directly into the target of the omitted copy/move

Temporary objects get a lot of leeway in C++, and compilers will be pretty aggressive when removing them. If your object had a lifetime of its own in any way, then the copy constructor would end up being called.

However, I would definitely expect it to check the copy constructor's access modifier, though I can see an argument that you shouldn't (after all, you just aren't calling the copy constructor). But that probably wouldn't be very good practice, since copy elision is optional.