{ A simple and interesting question about #include }

[meili100]

I got an interesting problem:

Lib.h

namespace MySpace{
class Utility{
...omit constructor...
void foo();
};
}

Lib.cc

#include "Lib.h"
...omit constructor...
void MySpace::Utility::foo(){
...
}

Main.h

#include "Lib.h"
namespace MySpace{
class ABC{
void abcFoo();
};
}

Main.cc

#include "Main.h"
void MySpace::ABC::abcFoo(){
Utility uty;
uty.foo();
}

The linker complains:
Main.cc line3: undefined reference to MySpace::Utility::Utility()
Main.cc line4: undefined reference to MySpace::Utility::foo()
collect2: ld returned 1 exit status

But when I put Utility class in Main.h the problem disappears!

Main.h

namespace MySpace{
class ABC{
void abcFoo();
};

class Utility{
void foo();
};
}

Main.cc

#include "Main.h"
void MySpace::ABC::abcFoo(){
Utility uty;
}

void MySpace::Utility::foo(){
...
}

What's going on here?

[brewbuck]

Is this the EXACT code you are using? Simplifications like "constructor omitted" are often where the problem truly lies

[cpjust]

Also as a side note, why are the file extensions .cc instead of .cpp? I've never seen a .cc extension.

[brewbuck]

Also as a side note, why are the file extensions .cc instead of .cpp? I've never seen a .cc extension.

I use .cc for my C++ source files. I understand that I am in the minority, but the minority does consist of a few more people than just me

[Daved]

Did you add Lib.cc to your project/makefile/command line or whatever you use to compile the program?

[bithub]

If Lib.h and Lib.cc are part of a separate library (guessing by the name), then you need to link to that library before you can instantiate a class from it.

[matsp]

gcc/g++ recognizes the following extensions as C++: '.C', '.cc', '.cpp'. Any others are treated as either C or object files.

In linux/unix, where filename case isn't ignored, the extension '.C' is quite often used for C++, but that obviously doesn't work for Windows/DOS based environments, where a.c and A.C are considered the same file.

The .cc isn't a common extension, but it's used in places.

Back to the main programme here:
It sounds like the compiler isn't told about lib.o (or lib.cc) when it's linking the application, which is why you get the errors for "no symbol".

--
Mats

[robwhit]

gcc also supports cxx, c++, cp, CPP, and others for header files. Check the man/info pages for more information.

[manav]

hey, should not you put semicolons at the end of namespace declarations?

[Elysia]

Not on a namespace...

[meili100]

Problem solved! I forgot to put lib.o in Makefile