Undefined refrence to static variable Foo::a?

I am getting this error and I have no idea what it means:

$ mingw32-g++ Test.cpp -o Test.exe开发者_如何学C
C:\Documents and Settings\BDL\ccksiYhI.o:Test.cpp:(.text+0x11): undefined reference to 'Foo::a'
collect2: ld returned 1 exit status

This is my code.

Test.cpp

#include <vector>
#include "Test.h"

int main() {
    Foo::a.clear();
    return 0;
}

Test.h

#include <vector>
class Foo {
public:
    static std::vector<int> a;
};

This isn't my original code but I have boiled it down to this problem. I am new to c++ and if anyone can explain why this is wrong and how I can work around it I would appreciate it.

You still need to define the member variable, even if it's static. Change your Test.cpp to this:

#include <vector>
#include "Test.h"

std::vector<int> Foo::a; // <-- definition

int
main() {
  Foo::a.clear();
  return 0;
}

You need to define static members as well in one translation unit, e.g. in Test.cpp:

std::vector<int> Foo::a;

Static members must be defined outside the class. Inside you have a declaration.

There are many sites online with examples. Search for "define C++ static members."

Good luck with it and welcome to SO.