Re: std::vector and object copying

From:

"Tom1s" <NULL@NULL.NULL>

Newsgroups:

comp.lang.c++

Date:

Tue, 09 May 2006 08:14:03 GMT

Message-ID:

<fbY7g.8891$j7.304933@news.indigo.ie>

bb posted:

Hi,

...
std::vector<MyClass> v1;
v1.push_back(MyClass("abc"));
...

The above results in a single call to the 'copy constructor' of MyClass
when I use gcc. However, the same code results in 2 calls to the 'copy
constructor' when I use Visual C++ (VStudio 2005).

There's an optimization going on there. You can see where the second one is
coming from though:

std::vector<MyClass> v1;

MyClass object("abc");

v1.push_back(object); /* <- Results in copy-construction */

Here's some sample code that can create anywhere from 1 to 3 objects (or
maybe even 4?). First though, here's a handy class for testing:

class AnyClass {
public:

    static unsigned times_constructor_called;
    static unsigned times_copy_constructor_called;
    static unsigned times_destructor_called;
    static unsigned times_assignment_performed;

    AnyClass()
    {
        ++times_constructor_called;
    }

    AnyClass( const AnyClass & )
    {
        ++times_copy_constructor_called;
    }

    AnyClass& operator=( const AnyClass & )
    {
        ++times_assignment_performed;

        return *this;
    }

    ~AnyClass()
    {
        ++times_destructor_called;
    }
};

unsigned AnyClass::times_constructor_called = 0;
unsigned AnyClass::times_copy_constructor_called = 0;
unsigned AnyClass::times_destructor_called = 0;
unsigned AnyClass::times_assignment_performed = 0;

Now here's some code to try it out on:

AnyClass FuncReturnByValue()
{
    AnyClass local_object;

    return local_object;
}

int main()
{
    AnyClass any_class = FuncReturnByValue();
}

-Tom1s