Defining static const integer members in class definition

Question

My understanding is that C++ allows static const members to be defined inside a class so long as it's an integer type.

Why, then, does the following code give me a linker error?

#include <algorithm> #include <iostream>  class test { public:     static const int N = 10; };  int main() {     std::cout << test::N << "\n";     std::min(9, test::N); } 

The error I get is:

test.cpp:(.text+0x130): undefined reference to `test::N' collect2: ld returned 1 exit status 

Interestingly, if I comment out the call to std::min, the code compiles and links just fine (even though test::N is also referenced on the previous line).

Any idea as to what's going on?

My compiler is gcc 4.4 on Linux.

Answer 1

My understanding is that C++ allows static const members to be defined inside a class so long as it's an integer type.

You are sort of correct. You are allowed to initialize static const integrals in the class declaration but that is not a definition.

Interestingly, if I comment out the call to std::min, the code compiles and links just fine (even though test::N is also referenced on the previous line).

Any idea as to what's going on?

std::min takes its parameters by const reference. If it took them by value you'd not have this problem but since you need a reference you also need a definition.

Here's chapter/verse:

9.4.2/4 - If a static data member is of const integral or const enumeration type, its declaration in the class definition can specify a constant-initializer which shall be an integral constant expression (5.19). 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.

See Chu's answer for a possible workaround.

Answer 2

Bjarne Stroustrup's example in his C++ FAQ suggests you are correct, and only need a definition if you take the address.

class AE {     // ... public:     static const int c6 = 7;     static const int c7 = 31; };  const int AE::c7;   // definition  int f() {     const int* p1 = &AE::c6;    // error: c6 not an lvalue     const int* p2 = &AE::c7;    // ok     // ... } 

He says "You can take the address of a static member if (and only if) it has an out-of-class definition". Which suggests it would work otherwise. Maybe your min function invokes addresses somehow behind the scenes.