Move Constructor doesn't get called

[gedamial]

Hello guys.

I've my own class String

// Move Constructor
GString(GString&& move)
{
    std::cout << "Move" << std::endl;
    
    // ... code ...
}

// Should invoke the Move Constructor
GString ToUpper()
{
	GString result{ *this };

	for (int i{ 0 }; i < size; ++i)
	{
		result[i] = toupper(mainString[i]);
	}

	return result;
}

int main()
{
 GString my{ "hello" };
 GString myupper{ my.ToUpper() };

 cout << myupper;

 return 0;
}

When I create myupper, I want the ToUpper() function to MOVE the value of my into myupper

I notice that my Move Constructor doesn't get called in this case, but the function just creates a COPY (exactly what I don't want) of the whole GString object.

I've heard about creating the Move Assignment also.
But it's strange because this I've written with the GString class works fine with my GList class:

GList(GList&& move)
{
	std::cout << "Move constructor" << std::endl;

	// ... code ...
}

GList<T> Reverse()
{
        GList<T> result;

	// ... code ...

	return result;
}

In the main, if I do

GList<int> my(9);
my.push_head(8);

GList<int> my2(my.Reverse());

The output shows "Move Constructor", just like the cout I wrote in the GList Move Constructor.

I've done the SAME thing with the GString class... but it won't work!

Any tip?

[gedamial]

Note:

The copy constructor gets called instead.

Why is it covering the move constructor?

[cire]

http://en.cppreference.com/w/cpp/language/copy_elision

Your copy constructor isn't being called either.

[gedamial]

Oh yeah I already read that documentation.

Still can't find out the issue...

Your copy constructor isn't being called either.

What do you mean by that?
The copy constructor is called, I put a cout

Screenshot: http://prntscr.com/a527an

[cire]

cire wrote:

Your copy constructor isn't being called either.

gedamial wrote:

What do you mean by that?
The copy constructor is called, I put a cout

Perhaps I should be more specific. Neither your copy constructor nor move constructor are called for the return.

Yes, the copy constructor is used inside the function when you make a copy of *this. Line 12 in the first code snippet.

[gedamial]

So yeah I'm using the copy constructor inside the function.

But when I return, how do I move instead of copying?
How do I call the copy constructor?

[cire]

But when I return, how do I move instead of copying?

You just return. You're getting a more efficient strategy than moving or copying with copy elision. However, if for some reason you feel like you want to force a less efficient strategy, then use std::move in your return statement.

[gedamial]

You just return

Why then in the second example (GList), the move constructor gets invoked?

What's the rule here? When does it / doesn't it get called?

[cire]

What's the rule here? When does it / doesn't it get called?

There isn't a hard and fast rule (except as to when it can't happen.) I suppose the rule is to return and let the compiler sort out the most efficient way to do things. ;)

My guess would be that the difference you observed is in the compiler optimization settings. If you're using VC++, it typically forgoes copy elision in the default debug configuration.

[gedamial]

I read from Bjarne Stroustrup book that when returning a large class object I should use a Move operation instead of a COPY (for obvious reasons)
So yeah, let's pretend GString is a very huge class.
I would NEVER want my computer to make a COPY of that object, so I define a Move Constructor... that doesnt get invoked lol

I dont understand the reason of this. It should perform a Move instead of a copy since I explicitly defined a Move Constructor

[cire]

It should perform a Move instead of a copy since I explicitly defined a Move Constructor

A copy is not performed. You say you've visited the link I posted earlier, but you speak as if you haven't.

[gedamial]

WHY does the GList move constructor get called then?
Is it racism? Lol