Question

I am now building a C++ DLL library. Today I have met a confusing problem: in this library I can define class but not functions. To be more specific, I give the following codes to illustrate my problem:

namespace fundamental
{

    class Tree
    {
    public:
        Tree() {};
        ~Tree() {};
        int x;
    };

     /*int anyfunction()
    {
        return 1;
    }*/

}

The above definition is in the header file, and this file will be invoked by other files. My problem is that if I commented the function part (int anyfunction()) everything was fine, but if I added this function, I would get the following errors:

page_analysis.obj : error LNK2005: "int __cdecl fundamental::anyfunction(void)" (?anyfunction@fundamental@@YAHXZ) already defined in geo_box.obj
1>pa_region_properties.obj : error LNK2005: "int __cdecl fundamental::anyfunction(void)" (?anyfunction@fundamental@@YAHXZ) already defined in geo_box.obj

My question is why I will get LNK2005 error only for functions but not for classes. Any ideas?

Answer 1

If you define something in a header file, then that definition will be duplicated in any translation unit (roughly speaking, every source file) that includes that header. Sometimes, multiple definitions are an error.

Classes can be defined in multiple translation units, as long as the definitions are identical; indeed, they must be defined in any translation unit that uses them.

Functions usually can’t, but you can allow it by declaring it inline:

inline int anyfunction() {return 1;}

or you could move the definition to a single source file, and only declare it in the header:

// header
namespace fundamental {
    int anyfunction();
}

// source file
int fundamental::anyfunction() {return 1;}