Multiple includes

[Cedric Moonen]

Shy Agam wrote:

However, this somehow causes a problem if classes from the same .h file should be used by different .cpp files. The first .cpp file including the .h file gets compiled, but the others throw an "undefined reference" compile error. It's as if I never included the .h file on the other files.

That's not how include guards work. They prevent including the same header file in one compilation unit (thus, one cpp file) but doesn't prevent multiple cpp files to include the same header file.

Cédric Moonen Software developer
Charting control [v3.0] OpenGL game tutorial in C++

[CPallini]

Did you use the guards properly, e.g.

// myclasses.h
#ifndef _MYCLASSES_H_
#define _MYCLASSES_H_

// class definitions here...

#endif // _MYCLASSES_H_

// mysource1.cpp
//...
#include "myclasses.h"

// use the classes here

// mysource2.cpp
//...
#include "myclasses.h"

// use the classes here

? :)

If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler. -- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong. -- Iain Clarke
[My articles]

[Shy Agam]

Indeed. However, I neglected to mention that the headers and their .cpps are in one shared-library project, and there's another project using this library. The other project is the one which fails to compile. It seems that this is some kind of linkage problem of some sort, but I can't figure it out. I copied the main file form my executable project into the library project, just to see if it compiles, and it compiled perfectly. Might I be missing on a specific linkage configuration?

[Cedric Moonen]

Shy Agam wrote:

It seems that this is some kind of linkage problem of some sort, but I can't figure it out.

Please, post the full error message. This confirms that the problem has nothing to do with include guards, otherwise you would get a compilation error, not a linker error. Are you sure you linked correctly to the library ?

Cédric Moonen Software developer
Charting control [v3.0] OpenGL game tutorial in C++

[Shy Agam]

#include <HeaderFileFromLibrary.h>

using namespace NamespaceFromHeaderFile;

int main(int argc, char **args)
{
ClassFromHeaderFile *pClass = ClassFromHeaderFile::GetInstance();

.
.
.

return 0;

}

The first row in main throws the following compile error: undefined reference to `NamespaceFromHeaderFile::ClassFromHeaderFile::GetInstance()'

Cedric Moonen wrote:

Are you sure you linked correctly to the library ?

AAMOF I'm not experienced with C++ coding. As I am an experienced developer in other languages, I can write proper code (Or so I would like to think ;) ), but I don't have the experience for configuring my projects properly. As far as I can tell I did link the library correctly. Any way to make sure of it?

modified on Monday, August 30, 2010 7:40 AM

[Cedric Moonen]

Shy Agam wrote:

As far as I can tell I did link the library correctly.

How did you do it ? I am not experienced with g++ so I don't know exactly how to specify library to link. From the error you get, chances are that you didn't link to the library properly.

Cédric Moonen Software developer
Charting control [v3.0] OpenGL game tutorial in C++

[Lost User]

Are you sure you are using the ClassFromHeaderFile definitions correctly. Is GetInstance() a static member with public visibility?

It's time for a new signature.

[Paul Michalik]

No, what he's getting is an linker error. You either do not link the library which contains the definition, or you're linking in wrong order, or you haven't configured your linker properly. Please, post the makefile (or the command line) you are using to build and link the program.

[Emilio Garavaglia]

This is not a COMPILER, but a LINKER ERROR. It has nothing to deal with guards, but to the fact that your main project (not source files, that way it is defined in the IDE environment) doesn't know about the existence of the library project. Since you're using eclipse, you should go to the main project settings and configure it to link also the artifact produced by the library project and configure the projects dependencies so that the library is always built BEFORE the main project. This is one of the most convoluted and confused things that the eclipse CDT has. I've have to admit that -besides a good syntax parser and analyzer- for these relatively trivial things, really sucks!

2 bugs found. > recompile ... 65534 bugs found. :doh:

[Shy Agam]

Richard MacCutchan wrote:

Is GetInstance() a static member with public visibility?

Yes.

[Lost User]

paul_71 wrote:

No, what he's getting is an linker error.

His messages are not clear as to whether this is a compiler or linker error. BTW this message should have been in reply to the OP not to me.

It's time for a new signature.

[Shy Agam]

It appears you guys were right. This is a linker issue. Eclipse lets you specify a reference to your other project. Being a devoted Visual Studio for .NET developer, I guess I assumed adding the reference was suffice. However, I've found the specific settings needed for linking the library. For future references to this post: 1. Open the project's settings. 2. Expand "C/C++ Build" and select "Settings". 3. Under "Tool Settings" expand "GCC C++ Linker" and select "Libraries". 4. Add the name of the library on the top box, and the path to its files on the bottom one. (Note that the path to the library should be the project's Debug directory) So now the project gets compiled, and the linker no longer whines about not finding the library. However I get some new linking errors. The new console output is as follows: make all Building target: Maple Invoking: GCC C++ Linker g++ -L"/LibraryPath/Debug" -o"Maple" ./Source/Maple.o -lCursesPlus /LibraryPath/Debug/libCursesPlus.so: undefined reference to `CursesPlus::CursesPlusEngine::initialized' . . . collect2: ld returned 1 exit status make: *** [Maple] Error 1 The vertical three dots hold place for a series of "undefined reference to..." errors, which are similar to the first error, but specify a different member of CursesPlusEngine. And to make it clear: Maple is my main executable project, and libCursesPlus is my library project. --Edit-- All of the above members are defined as private static in their corresponding header files.

modified on Tuesday, August 31, 2010 4:56 AM

[Paul Michalik]

Sorry, these hierarchical"replies" suck!

[Paul Michalik]

Since you've only got 1 library and 1 runner, and I assume you're not trying to use code from runner in the library, there is only one possibility left: you forgot to add the proper source file(s) to your project(s)...

[Shy Agam]

It appears I had to declare the static member variables in both the header files, and the source files. The project now compiles successfully. What's the idea behind this repetitive declaration? It's not like a function which is declared in the header, and defined in the source. It's simply a variable... Oo *Confused*

[Aescleal]

When you declare a static data member in a class you're telling the compiler: "This class has a static member called ." When you define it outside of a class you're telling the compiler: "You know that variable ? This is where you reserve memory for it and initialise it with ." So it really is identically like a function which is declared in a header and defined in the source. Cheers, Ash PS: It's also identical to the behaviour of global variables - declare in a header, define in source. Which is a big hint why statics are bad, bad, bad...

[Lost User]

paul_71 wrote:

these hierarchical"replies" suck!

Well, they take a bit of getting used to, but sometimes they work better than the other style.

It's time for a new signature.

[Paul Michalik]

No! You may declare your static class members only once. What you have missed, is the definition (which also has to be unique), and it needs to be provided outside of the class, for non-const and non-trivial objects.

// in AStuff.h
class AStuff {
public:
void Hello() {
// ...
}
};

// in A.h
class A {
public:
static AStuff sStaticAMember; // <-declaration
};

// in A.cpp
AStuff A::sStaticAMember; //<-definition (required!)

The constructor for A::sStaticAMember will be called by the runtime (dynamic initialization) if the instance is used by the program. You do not (really) have the control over when this is going to happen, neither when the destructor is going to be called. This is one of the reason why the poster above damned "statics" ... and basically I have to agree with him. Are you using C# otherwise? If yes, then forget about the comfort of static class objects and their well defined initialization there - and enter one of the many dark zones of c++. I assume you will probably run into the problem of undefined order of initializations of your non-local objects, but we stand ready to help :)

modified on Tuesday, August 31, 2010 12:56 PM

[Tim Craig]

Aescleal wrote:

PS: It's also identical to the behaviour of global variables - declare in a header, define in source. Which is a big hint why statics are bad, bad, bad...

That's a rather sweeping statement. Parts of the language are there for a reason. There are cases where you absolutely need them. Can you misuse them? Certainly. But just yelling bad, bad, bad, ignores the valid cases.

Once you agree to clans, tribes, governments...you've opted for socialism. The rest is just details.

[Aescleal]

So where would you use a class static instead of just declaring a variable in an anonymous namespace? i.e. where would you use:

// A.h

class A
{
private:
static B b;
};

// A.cpp

A::B b( b_init );

over:

// A.cpp

namespace
{
B b;
}

'Cause I'm not sure I can see any case where that would be of any use. And if you're talking about using public class static data member why not use a global? They've got exactly the same visibility, thread safety issues and initialisation order problems.