Thread: Constant Being Initialized AFTER Variable That Uses It

You should place std::string strTest = "Test"; in global scope and extern std::string strTest; in your cpp. I am not sure if it can be a member of a class.
Why you want to do this?

Place the extern declaration also in MyClass.h, before the class definition.

const variables are visible only to the file in which they are declared. That is why your class is not seeing it.

Originally Posted by mercury529

strTest is defined in a global scope. strPath is the member of the class. strTest is a constant and strPath is a regular variable.

strTest is declared in Test.h, defined in other.cpp as shown in the lines of code above. strPath is declared as a static member variable in MyClass.h, and defined in MyClass.cpp as shown above.

Like this?

Code:

// MyClass.h
#ifndef MYCLASS_H
#define MYCLASS_H

#include <string>

class MyClass
{
public:
      static std::string strPath;
};

extern const std::string strTest;

#endif

Code:

// MyClass.cpp
#include "MyClass.h"

std::string MyClass::strPath = strTest;

Code:

// other.cpp
#include "MyClass.h"

const std::string strTest = "Test";

Code:

// main.cpp
#include <iostream>
#include "MyClass.h"

int main()
{
    MyClass object;
    std::cout << object.strPath << '\n';
    std::cin.get();
    return 0;
}

It would be like this

Code:

// MyClass.h
#ifndef MYCLASS_H
#define MYCLASS_H

#include <string>
#include "Test.h"

class MyClass
{
public:
      static std::string strPath;
};
#endif

Code:

// MyClass.cpp
#include "MyClass.h"

std::string MyClass::strPath = strTest;

Code:

// other.cpp

extern const std::string strTest = "Test";

Code:

//Test.h
extern const std::string strTest;

Code:

// main.cpp
#include <iostream>
#include "MyClass.h"

int main()
{
    MyClass object;
    std::cout << object.strPath << '\n';
    std::cin.get();
    return 0;
}

Edit: As you can tell from the example above Mario, I am including Test.h in MyClass.h, so strTest is known inside of MyClass.cpp where it is being assigned to strPath.

That has to be some problem with VC++ 6.0. Probably is is documented somewhere.

There is nothing wrong with your code. Main should output "test".

Compiler problem? const make the variable filescope, in other.cpp, it is const. So it is filescope. Don't make externs const.
[edit] extern keyword is not needed in other.cpp.

Originally Posted by siavoshkc

Compiler problem? const make the variable filescope, in other.cpp, it is const. So it is filescope. Don't make externs const.
[edit] extern keyword is not needed in other.cpp.

The extern keyword in other.cpp is not needed alright. When a variable is defined as it was in other.cpp, the extern keyword can be ommited. However it is considered good practice. It helps remember this particular const is to be used in other files...

And "don't make extern const"? You suggest the OP to not use a popular language feature?

Originally Posted by mercury529

Const is no longer file scope when you define it as an extern const. If it were file scope, the linker would throw an error that it was undefined.

This is true. However you still have to declare the const in those files you wish to use it. Or the const will not be visible.

Originally Posted by mercury529

What is the alternative to making an extern const string? Defining the string in the header file results in multiple definitions. Prefacing it with static would work but also results in a lot more unnecessary variables floating in memory. Also, I favor any method that has the definition of the constants in the cpp file. I am trying to make it flexible in such a way that I can swap in .cpp files to work with different constants.

Defining the const in a cpp file is what is bringing you problems in the first place. One possible way to override this behavior of VC++ 6.0 is perhaps define the const in the header file. Since this particular const of yours is initialized from a constant expression, it is itself a constant expression. You can, and probably should, define it in the header file.

It is, of course possible do what you are doing already. I just now tested your code to be absolute sure it works. And the obvious result is it does. But since VC++ 6.0 is not recognizing correctly the extern keyword, I can only suggest you move the definition to your header file. I'm sorry.

Originally Posted by mercury529

Can anyone else reproduce this in MSVC++ 6.0? Does this code function correctly in Borland or G++? Could it have to do with an optimization setting? The issue I discovered was the const was getting assigned after the static variable. This just seems to go against what should occur.

With a little addition,

Code:

// other.cpp
#include <string>
extern const std::string strTest = "Test";

MSVC6: blank stare.
g++: expected result "Test".

Originally Posted by mercury529

If you remove extern from the example code, you will get a linker error. Extern const in a cpp file is not a matter of good practice, as much as it is a necessity to be able to define a constant in a global way. The code simply does not work without it.

You are absolutely right about this. My bad. Too sleepy I guess.

Originally Posted by mercury529

If I move the definition to the header file, how do I eliminate the re-defintion issue that occurs (when the header file is included in multiple files)

Well, you define it with internal linkage.

Code:

const std::string strTest = "Test";

And, you do provide header guards.

Originally Posted by mercury529

Is making the const static the only way to get rid of it? I believed the better practice was to only mark the const as extern and then define in it in a cpp file.

It can be done so. As I said before, even with constants initialized from constant expressions. However C++ defaults const to have internal linkage. It feels natural to follow on this behavior. Also the compiler will be able to evaluate your const. With external linkage the compiler is left in the dark as to the actual value of the const.

Originally Posted by mercury529

Unforunately, it appears that the MSVC++ 6 compiler runs into the assignment issue I originally detailed when I handle it this way.

I did some searching for you (as you probably did too by now), but came empty handed. Maybe my search string choices were not the best. But this is an old, unsuported and non standards conformant compiler. Do you really need to use it?

Originally Posted by mercury529

Unforunately I am stuck with MSVC++ 6 because it is the only legal version I own.

Why not "buy" gcc?