Declaration and initialization of global variable [duplicate]

Question

Maybe it is base knowledge, I am apologize.

I have a header file CTemp.h, with namespace CTemp:

namespace CTemp { bool bFlag; }; #ifndef _CTEMP #define _CTEMP bool CTemp::bFlag = true; #endif

and if I include CTemp.h to my cpp file and try to use CTemp::bFlag

bool bb = CTemp::bFlag;

the compiler throw error "redefinition"

I surely know, that I should to put initialization to a .cpp file, but I find some way, to solve it only with .h file. Because I don't want to add the .cpp file to my project. I thought, I could solve it with preprocessor directives #ifdef....

Thanks for advice.

You can use extern bool bFlag in your H file (declaration), and bool bFlag = true; in the CPP (definition). However - using global variables is usually not a good idea (to say the least). — wohlstad
– wohlstad, Commented Sep 21, 2022 at 9:15
Because both bool bFlag; and bool CTemp::bFlag = true; are definitions. — Richard
– Richard, Commented Sep 21, 2022 at 9:15
Dupe: How can I call a method of a variable, which contains in a namespace? and Global Constants in .h included in multiple c++ project — Richard
– Richard, Commented Sep 21, 2022 at 9:28

Some programmer dude · Accepted Answer · 2022-09-21 09:23:53Z

In the header file you should only declare the variable:

#ifndef CTEMP_H #define CTEMP_H namespace CTemp { extern bool bFlag; // Need extern to only declare the variable } #endif // CTEMP_H

Then in one single source file you define the variable with its initialization:

#include "ctemp.h" bool CTemp::bFlag = true;

Alternatively define the variable as inline:

#ifndef CTEMP_H #define CTEMP_H namespace CTemp { inline bool bFlag = true; } #endif // CTEMP_H

Now you should not define the variable in a source file.

@MichalHadraba Who said that? With C++17, you can use inline and bFlag will still be global as shown in this answer. Also the exact same thing is explained here: How can I call a method of a variable, which contains in a namespace? in the dupe. See method 2 of the above linked answer.

Richard · Accepted Answer · 2022-09-21 09:18:47Z

The problem is that both bool bFlag and bool CTemp::bFlag = true; are definitions. In other words, you're defining bFlag for the second time when you wrote: bool bFlag = true;.

namespace CTemp { bool bFlag; //this is a definition for the first time }; #ifndef _CTEMP #define _CTEMP bool CTemp::bFlag = true; //this is also a definition but for the second time and hence the error

Solution

To solve this you can use the extern keyword:

someheader.h

#ifndef _CTEMP #define _CTEMP namespace CTemp { extern bool bFlag; //this is a declaration and not a definition }; #endif

somesource.cpp

#include "someheader.h" bool CTemp::bFlag = true; //this is a definition

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