Thread: Why do G++ and MSVS handle s1 += " world!" differently?

This thread is related to this thread: Memory leak in Soln8_05.cpp from Ivor Horton's Beginning Visual C++ 2008 book?.

I basically stuck debug output statements all through a modified copy of the author's solution for one of the chapter exercises in an effort to dissect what constructors, destructors and overloaded functions were being used.

I compiled Soln8_05.cpp (r11) and got two different results that I did not expect.

1. CSimpleString(const char* p) was called and I could not find where
2. I saw that Microsoft Visual Studio 2010 called a copy constructor when I wasn't expecting it

So, I fired up another VM where I have Ubuntu 10.04 installed and used g++ to compile the code.

I saved the output from both and this is what I have (first two lines in both files were added manually, as was the first line in the diff file):

• Microsoft Visual Studio 2010 executable output
• G++ executable output
• Diff between the two

So, that brings me to my questions:

1. Why do G++ and MSVS handle
   Code:

   s1 += " world!"

   differently?
2. Why is
   Code:

   CSimpleString(const char* p)

   called as part of resolving
   Code:

   s1 += " world!"

   ?

Thanks in advance!

1) MSVC is doing it by the book. I am guessing that g++ is applying optimizations here. Try running the MSVC build in Release mode.
2) Because there is no operator += that takes a const char*. However, there is one operator that takes a const CSimpleString&. So the compiler checks the constructors to see if there is CSimpleString(const char*), and there is. So it invokes that to create a temp object, and then calls operator +=. This is called an implicit conversion.

Am I right to assume that you speak german?

g++ scheint die Konstruktion/Destruktion des temporären Rückgabeobjektes des Op+ an den Op+= wegzuoptimieren.
Wegen solcher Dinge (Der Standard erlaubt solche Optimierungen explizit) sollte man beim Debuggen alle Optimierungen abschalten.
Am Rande: Warum hast Du den Op+= mit Hilfe des Op+ gebaut und nicht andersrum, wie empfohlen?

Edit: Too late again...

Google Translate says:

g + + seems to be the construction / destruction of the temporary return of the object on the Op Op wegzuoptimieren + + =.
Because of such things (The standard explicitly allows such optimizations) you should turn off all optimizations when debugging.
By the way: Why did you build the Op + = with the help of the Op + and not the other way, as recommended?

Please keep to English on the boards. This is supposed to be an international forum, and you are basically destroying the ability to read what you write for 99% of the people. So unless you have a very special reason for using german, do please refrain from doing so.

Originally Posted by Elysia

unless you have a very special reason for using german, do please refrain from doing so.

I did have a reason.
Looking at deoren's web site I had reason to assume that he might speak german.
I used german to say something I couldn't have said nearly as clearly in english (since english is not my mother tongue).

Don't you think that you are overreacting a little?

PS: Is this site about helping each other or about using the right language?
You said it yourself: "This is supposed to be an international forum".

No, international as in "english."
I posted that as both as a reminder (there are people who don't know) and as a message to others who read it should they wish to use a non-english language.
I'm not overreacting as in "stop doing this or else." It was simply a little reminder. Now that that's done, you can do what you want.

Compilers are allowed to perform optimisation to avoid copy-construction in certain cases.
As such, the standard says that you're not allowed to rely on side-effects that occur during copy-construction. Outputting any kind of debug message is a side-effect. In both cases, any temporary object that is created is of course destructed.
Just remove any logging you have and the problem is gone. Yes the compilers optimised differently, but the net result is code that has the same effect. Of course if your code is broken, then no matter what the compiler does, your code might not work.

There are a lot of differences between compilers and this is very much not one of the differences between compilers that you should worry about. There are far bigger issues that you'll run into eventually if using multiple compilers.

Edit: Please keep it in English. It's not fair to exclude a large portion of the posters who cannot read other languages. Posting in a different language could mean that you are giving totally wrong advice and without anyone else to understand and refute your post, the originaly poster could be very much sent down the wrong path.
You might say that it is unfair that this site be for English speakers, but I'm sure you could find a German programming site if you wanted to, and like it or not, C and C++ are English programming languages.

Originally Posted by iMalc

[...] side-effects [...] Just remove any logging you have and the problem is gone.

Thanks iMalc. I didn't test it but what you say makes (a.) perfect sense and (b.) widened(broadened?) my horizons on the overall matter.

Am I starting a major upheaval here?

Originally Posted by Shakti

there is little doubt that content does not further the community.

Does that mean that deoren is not part of the community?

As I tried to explain earlier: Trying to help him I told him something in the only language I saw myself able to express myself in without introducing more confusion.

Given that choice(speaking german or remaining silent): you mean I should stay quiet rather than try and help out?
Think that is constructive?


It really feels like people are offended by having to overlook three foreign sentences...

Originally Posted by Elysia

This is supposed to be an international forum

Originally Posted by whiteflags

even though this person writes fluently.

But I couldn't in that particular situation.

(the fact that I obviously didn't manage to make myself clear yet ultimately supports my point here)

If this is supposed to be an english only forum it should be stated clearly. And I would gladly stay (far) away.

I use english in forums like this almost always, because it is the lingua franca of our time. But when I hit a point when my english fails my communication needs I use what enables me to express myself. Believe it or not, I even use latin phrases sometimes.

If that's inacceptable, only native english speakers should be granted full membership...

I really don't understand this uproar. Not a bit!