Initializing a static class member [duplicate]

Question

This question already has answers here:

What is an undefined reference/unresolved external symbol error and how do I fix it? (38 answers)

C++ static member variable and its initialization (5 answers)

Closed 8 years ago.

I'm trying to initialize a static class member and having no luck. Here's a test:

file Test.h

#include <string>

class Test {

public:
    static void init(char*);

private:
    static std::string  *sp;

};

file Test.cpp

#include "Test.h"

// Initialize the class
void
Test::init(char *foo) {
    Test::sp = new std::string(foo);
}

int main(int argc, char** argv) {
    Test::init(argv[1]);  // call the class initializer
}

The linker fails with:

Undefined symbols for architecture x86_64:
  "Test::sp", referenced from:
      Test::init(char*) in Test-OK13Ld.o
ld: symbol(s) not found for architecture x86_64

In the real world, init() is going to do some real work to set the static member. Can someone point out the error?

Answer 1

This is a bit of an embarrassing "feature" of C++: you need to do some hand holding to make sure the linker can generate the symbols. You need to choose some cpp file, and make sure that no such handholding occurs for the same symbols in any other (otherwise the linker will fail when it encounters duplicate symbols). So you have to do another declaration of the static member variables for your class in the cpp file like this:

std::string * Test::sp; // or sp = NULL;

Answer 2

As the error message says, static std::string *sp; has to be defined somewhere since it's not associated with any instance of class Test.

Adding it to Test.cpp at global scope will fix the issue:

std::string *Test::sp = NULL;