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c++classexternallinkage

Do classes have external linkage?

I have 2 files A.cpp and B.cpp which look something like

A.cpp
----------
class w
{
public:
    w();
};


B.cpp
-----------
class w
{
public:
    w();
};

Now I read somewhere (https://en.cppreference.com/w/cpp/language/static) that classes have external linkage. So while building I was expecting a multiple definition error but on the contrary it worked like charm. However when I defined class w in A.cpp, I got the redefinition error which makes me believe that classes have internal linkage.

Am I missing something here?

Solution

  • External linkage means the symbol (function or global variable) is accessible throughout your program and Internal linkage means that it's only accessible in one translation unit. you explicitly control the linkage of a symbol by using the extern and static keywords and the default linkage is extern for non-const symbols and static (internal) for const symbols.

    A name with external linkage denotes an entity that can be referenced via names declared in the same scope or in other scopes of the same translation unit (just as with internal linkage), or additionally in other translation units.

    The program actually violates the One Definition Rule but it is hard for the compiler to detect the error, because they are in different compilation units. And even the linker seems cannot detect it as an error.

    C++ allows a workaround to bypass the One Definition Rule by making use of namespace.

    [UPDATE] From C++03 Standard
    § 3.2 One definition rule, section 5 states:

    There can be more than one definition of a class type ... in a program provided that each definition appears in a different translation unit, and provided the definitions satisfy the following requirements. Given such an entity named D defined in more than one translation unit, then each definition of D shall consist of the same sequence of tokens.