Why Do I Get a Linker Error When Defining Static Const Integer Members in a C Class?

Linker Error in Defining Static Const Integer Members in Class Definition

Class declarations in C allow for the definition of static const integer members within the class. However, users may encounter linker errors with code similar to the example provided:

class test {
public:
    static const int N = 10;
};

int main() {
    std::cout <p>While the compiler accepts the class definition, the linker reports an undefined reference to test::N. The issue arises because the declaration in the class is not a true definition.</p><p>In C  , static const integral members must be defined outside the class in a namespace scope. This is because they cannot be initialized within the class declaration according to the C   standard (9.4.2/4):</p><blockquote>If a static data member is of const integral or const enumeration type, its declaration in the class definition can specify a constant-initializer [...] In that case, the member can appear in integral constant expressions. The member shall still be defined in a namespace scope if it is used in the program and the namespace scope definition shall not contain an initializer.</blockquote><p>To resolve the linker error, one can define the static const member outside the class, typically in the corresponding source file:</p><pre class="brush:php;toolbar:false">const int test::N = 10;

Alternatively, for C 11 and later, the constexpr keyword can be used to create a true definition within the class declaration:

class test {
public:
    static constexpr int N = 10;
};

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