Namespaces and includes generate link error

问题

I was playing around with namespaces when I encountered a lnk2005 error. I can't figure out how to get around the error. Here's the error:

1>Source.obj : error LNK2005: "int Chart::Bars::d" (?d@Bars@Chart@@3HA) already defined in Chart.obj
1>Source.obj : error LNK2005: "class foo Chart::l" (?l@Chart@@3Vfoo@@A) already defined in Chart.obj
1>Source.obj : error LNK2005: "int Chart::t" (?t@Chart@@3HA) already defined in Chart.obj
1>C:\Users\bnm\dev\examples\play\nmspca\Debug\nmspca.exe : fatal error LNK1169: one or more multiply defined symbols found
1>
1>Build FAILED.
1>
1>Time Elapsed 00:00:00.49
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

Here's the code...

Chart.h

#pragma once

#include "foo.h"

namespace Chart
{
    int t;

    foo l;

    namespace Bars
    {

        int d;
    }

}

Foo.h

#pragma once

class foo
{
public:
    int ss;
    char* h;

};

Chart.cpp

#include "Chart.h"

using namespace Chart;

int main ()
{
    l.h = "g";
}

Source.cpp

#include "Chart.h"

using namespace Chart;

int test()
{
    l.ss = 0;

    return l.ss;
}

When the #include "Chart.h" from Source.cpp is removed the problems goes away. However, Source.cpp needs #include "Chart.h" for the namespace definition.

What's the correct way to express that "namespace Chart" is needed in both Chart.cpp and Source.cpp so that everything compiles?

回答1:

If you define any objects in a header file and include that header file in multiple translation units, those objects are now defined multiple times. This is the problem you're having. The declarations of t, l, and d introduce objects and you have done so in a header file.

The proper method for supporting namespace scope variables is to declare them as extern in the header file. This makes them declarations only and not definitions. Then, in a single implementation file, define them.

Change Chart.h to:

#pragma once

#include "foo.h"

namespace Chart
{
    extern int t;

    extern foo l;

    namespace Bars
    {

        extern int d;
    }

}

Then in an implementation file, perhaps Chart.cpp, do:

int Chart::t;
foo Chart::t;
int Chart::Bars::d;

回答2:

Everywhere you're including Chart.h, you're effectively dropping variables t, l, and d into those objects. Declare them as extern, then define them in Chart.cpp

回答3:

I've had the exact same problem and I found a workarroud.

chart.h becomes:

#pragma once

#include "foo.h"

class Chart{
    static int t;
    static foo l;
    class Bars
    {
        static int d;
    };
};

and define the variables in chart.cpp

int Chart::t;
foo Chart::l;
int Chart::Bars::d;

Ugly, I know, but at least the syntax is the same wherever you need to use the vars.

This way only ONE .obj file is created containing the variables which prevents the multiple definition erros.
I haven't tried if you can declare a static variable on a namespace which could solve the problem.

Edit:

P.S. this solution gives some weird errors on msvc, as if every time it is included, sections of the code reference differente variables

Tested on my own program the static-inside-namespace and it seems to work.
Example.h:

namespace sdl{
    static SDL_Surface* screen;

    static int xres = 640;
    static int yres = 480;
    static int bpp  = 32;
    static int flags = SDL_ASYNCBLIT;

};

来源：https://stackoverflow.com/questions/15090800/namespaces-and-includes-generate-link-error

标签

c++

header-files

lnk2005