Why does "extern const int n;" not work as expected?

c++ constants declaration extern
xmllmx picture xmllmx · Feb 15, 2013 · Viewed 23.3k times · Source

My project consists of only two source files:

a.cpp:

const int n = 8;

b.cpp:

extern const int n;

int main()
{
    // error LNK2001: unresolved external symbol "int const n" (?n@@3HB)
    int m = n; 
}

I know there are several methods to make it work; however, I just wonder WHY it does't work?

Answer

James Kanze picture James Kanze · Feb 15, 2013

It's because const implies internal linkage by default, so your "definition" isn't visible outside of the translation unit where it appears.

In this case, by far the best solution is to put the declaration (extern int const n;) in a header file, and include that in both a.cpp and b.cpp. The linkage is determined by the first declaration the compiler sees, so the later definition in a.cpp will have the correct (external) linkage.

Alternatively, you can force the linkage in the definition:

extern int const n = 8;

Despite the extern, this is still a definition; anything with an initializer outside of a class definition is a definition.

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