C++ Namespace Declaration and Definition - c++

Folks,
I crated a name space in a single file, but how exactly do I separate the definition and declaration?
Can I create a .h for declaration and a .cpp for definition just like in a class set of file? If yes, then, do I need to include the namespace header file in my new program, or is the using namspace mynamespace enough?

Any decent C++ book should cover namespaces and code separation.
Try something like this:
myfile.h
#ifndef myfile_H
#define myfile_H
namespace myns
{
class myclass
{
public:
void doSomething();
};
}
#endif
myfile.cpp:
#include "myfile.h"
void myns::myclass::doSomething()
{
//...
}
And then you can use it like this:
#include "myfile.h"
myns::myclass c;
c.doSomething();
Or:
#include "myfile.h"
using namespace myns;
myclass c;
c.doSomething();

Related

Including header file in header file

I have this code in C++:
#ifndef MYCLASS_H
#define MYCLASS_H
#include "gspace.h"
class myclass {
public:
void update(gspace **);
}
gspace is another class defined in gspace.h. The compiler however is telling me:
include/myclass.h error: ‘gspace’ has not been declared|
Is there anything wrong in what I'm doing?
EDIT:
#ifndef GSPACE_H
#define GSPACE_H
#include <vector>
#include <iostream>
#include <math.h>
using namespace std;
class gspace
{
public:
gspace();
Assuming you have even declared a class named "gspace" in gspace.h I could think of two possible errors.
1) class gspace would not be in global scope i.e could be in some namespace.
2) Please check if header gaurds used in gspace not "MYCLASS_H".

C++: Inclusion inside inclusion

The title might not be very clear, it's a bit more complex than that. I searched the web for something like my problem but I did not find anything that could help me.
This is not about infinite looping inclusions, I already put preprocessor directives to avoid that.
I have two classes Monster and Character, respectively declared in their own header files, monster.hpp and character.hpp, and respectively implemented in their own source files, monster.cpp and character.cpp.
The problem now is that both classes need each other to work.
monster.hpp :
#ifndef INCLUDE_MONSTER_HPP
#define INCLUDE_MONSTER_HPP
#include "character.hpp"
class Monster
{
private: //Atributes
public: //Methods
void attackM(Monster& id);
void attackC(Character& id);
};
#endif //MONSTER_HPP_INCLUDED
character.hpp :
#ifndef INCLUDE_CHARACTER_HPP
#define INCLUDE_CHARACTER_HPP
#include "monster.hpp"
class Character
{
private: //Attributes
public: //Methods
void attackM(Monster& id);
void attackC(Character& id);
};
#endif //CHARACTER_HPP_INCLUDED
and the main.cpp :
#include <iostream>
#include "character.hpp"
#include "monster.hpp"
using namespace std;
int main(int argc, char **argv)
{
//Whatever
return 0;
}
And I get this error from the compiler :
In file included from character.hpp:7:0,
from main.cpp:3:
monster.hpp:24:16: error: 'Character' has not been declared
void attackC(Character& id);
(the line and column numbers may be wrong)
From what I understand, when monster.hpp is included into character.hpp, the compiler sees that the class Monster uses the class Character, which is not declared yet right at the moment when monster.hpp is included into character.hpp.
And I don't know how to fix that.
Any ideas ?
The way this works is that the header files of char and monster do not include each other. Instead you forward declare the classes and include the headers within the CPP files.
So basically replace #include "monster.hpp" in the.h with class Monster; and #include "monster.hpp" in the .cpp - and same for the other class.
See this question for more details:
What are forward declarations in C++?
#ifndef INCLUDE_MONSTER_HPP
#define INCLUDE_MONSTER_HPP
#include "character.hpp"
class Character;
class Monster
{
private: //Atributes
public: //Methods
void attackM(Monster& id);
void attackC(Character& id);
};
#endif //MONSTER_HPP_INCLUDED
You need to use a forward declaration. See: http://en.wikipedia.org/wiki/Forward_declaration
Basically, the compiler doesn't know what a "Character" is. You temporarily indicate that it's something you can point to by adding the following stub to Character.hpp:
class Character;
Use predeclaration in *.h:
class Character;
class Monster;
use in *cpp the includes:
#include "character.h"
#include "monster.h"
Or put everything in one *.hpp with predeclaration.

Create Source file for Header file

I have a file GetL.hxx
#ifndef GetL_included
#define GetL_included
#include <iostream>
using namespace std;
class GetL
{
public:
virtual int getWidth();
};
#endif //GetL_include
Here the class GetL contains only one virtual function. what should i put in source file i.e. in GetL.cxx
#include "GetL.hxx"
int GetL::getWidth() {
// your code goes here
}
By the way, having using namespace std; in a header file is not a good practice.

linux ubuntu c++ .h and .cpp file

I have a my.h file:
#ifndef __MY__
#define __MY__
#include <string>
#include <time.h>
class S
{
public: S();
std::string myname;
};
#endif
my.cpp
#include "my.h";
#include<string>
#include<iostream>
using namespace std;
S::S()
{
// .. code
}
I want to create an so file. There is no error when creating it. But when I compile the .h file it says: string:No such file or directory. If I pus string.h instead of string I have the error: expected '=',',',';','asm', before S (at class S) in my.h.
In the .cpp file (if i change the string with string.h) i have after i compile error: string in namespace std does not name a type. WHERE AM I WRONG?
Well, first, it seems that you come from java because when you typed:
class S
{
public: S();
std::string myname;
};
I guess you actually meant:
class S
{
public:
S();
private:
std::string myname;
};
In the .cpp file, you typed s instead of S: note that C++ is case-sensitive regarding classes names.
Also, regarding your problem, I suspect you are currently using a C compiler and not a C++ compiler. Without knowing the used command-line, I can't say much more on that.
Try this
#ifndef MY_H
#define MY_H
#include <string>
#include <time.h>
class S
{
public: S();
std::string myname;
};
#endif
#include "my.h"
#include<string>
#include<iostream>
using namespace std;
S::S()
{
//code
}

Compiling Error with C++ and namespace

Here's the whole code getting the errors:
Engine.h
#ifndef ENGINE_H
#define ENGINE_H
#include "DXManager.h"
namespace XEngine
{
class Engine
{
};
}
#endif
DXManager.h
#ifndef DX_MANAGER_H
#define DX_MANAGER_H
namespace XEngine
{
class Engine; // forward declaration
class DXManager
{
public:
void run(Engine *engine);
};
}
#endif
DXManager.cpp
#include <iostream>
#include "Engine.h"
#include "DXManager.h"
using namespace XEngine;
void DXManager::run(Engine *engine)
{
std::cout<<"DXManager::run"<<std::endl;
}
With these 30 lines of code, I'm getting 20 errors like:
'XEngine' : a namespace with this name does not exist
'XEngine' : a symbol with this name already exists and therefore this name cannot be used as a namespace name
syntax error : identifier 'Engine'
Clearly, I'm missing something important here. What am I doing wrong?
note: I am aware that circular dependency is a bad thing, but in my particular case I believe that it is relevant.
In DXManager.cpp you are not just using some names from namespace XEngine. You define the function in that namespace.
So must be:
DXManager.cpp
#include <iostream>
#include "Engine.h"
#include "DXManager.h"
namespace XEngine {
void DXManager::run(Engine *engine)
{
std::cout<<"DXManager::run"<<std::endl;
}
}
AFAIK some of the compilers (like MSVC) process using variant too.
But it is not correct because your syntax tries to define function ::DXManager::run - not ::XEngine::DXManager::run you intend to define.
In the forward-declaration of class Engine the namespace XEngine doesn't exist at this point.
A workaround would be moving the declaration inside the namespace block.
When Engine.h includes DXManager.h, the latter defines a class XEngine::Engine without declaring the namespace first.