Friday, November 28, 2008

Move constructor

Gaetano, the owner of cpp-today blog (see the link on the right) read my previous post and explain me that a constructor like T(T&) is the base of the "Move Constructor Idiom"

The programmer that wrote the (horrible) code of the previous post hasn't thinking about that, he just don't know the correct prototype of a copy constructor, but I didn't know anything about that move constructor so I read something about

This idiom comes out when there-s the need of "move semantic". In practice we want to "copy" an object modifying the one passed as parameter. The main example of a class that need a move constructor is std::auto_ptr.
You can read more here

Then, it a good practice to pay attention on constructor declarations, to avoid undesirable behaviors

Thursday, November 27, 2008

Why following the standard is the right thing..

Today I saw an interesting example of bad C++ coding:

Let's say we have a class, named A, that have a strange copy constructor prototype:

class A {
A(A& an_a) {...}

the standard says that the copy constructor for a type T has to have this prototype:

T(const T&);

so, the instance passed as argument has to be const!

My compiler (g++ 4.2.4), doesn't complain even if I specify -Wall and -pedantic, and the code is generated...

Let's go ahead, let's say we have a class MyClass that has an attribute of type A, and this method:

A MyClass::getA() const {return this->the_a;}

We don't return a reference to the attribute, we want to return a copy, no matter why...

We aren't using this method yet, but we are careful programmers so we try to compile our code... surprise! The compile complains because it's missing a A::A(A) constructor... damn, why it isn't using the copy constructor we wrote??? Oh yes!, the getA() method is const! Let's erase that swaerword!!! Oh, that's fine, the compilers doesn't complain anymore!

Now, we want to use our nice method so somewhere we have something like this:

A my_a = my_class.getA();

What? The compiler complains again? The same error? What does he wants? Nothing here is const!!!

And that's the truth: we didn't follow the standard and the standard punished ourselves! The compiler need to do a copy of a temporary variable and temporary variables in C++ are all const! If you try to add a constructor like this:


the compilers will complain for a wrong constructor too! That's the end, we have to bow our heads and follow the standard

The moral of the story is: you have to follow the standard, otherwise the standard will have its revenge!

PS Is it possible to read code like that written by a senior C++ developer?????

Thursday, October 16, 2008

look the path!

Today I fix a stupid, trivial issue on building a win32 executable from python code... the problem was a missing library, but the library was in the correct place!!! Al last, I fund there are two python interpreter on my machine, the one I installed and another... installed by cygwin!!! Clearly, when I write "python.exe" on my prompt the first interpreter the system find is the cygwin one, that hasn't the libraries I need... just add the correct path and the issue disappeared!

Monday, October 13, 2008

developer life

Hi to all!

I'm a developer, a C++ developer... not a very good one, but I like to improve my developing abilities...

so, I'll use this space to fix what I'll discover :-)