Duplicate:
C++: undefined reference to static class member
If I have a class/struct like this
// header file class Foo { public: static int bar; int baz; int adder(); }; // implementation int Foo::adder() { return baz + bar; } This doesn't work. I get an "undefined reference to `Foo::bar'" error. How do I access static class variables in C++?
You must add the following line in the implementation file:
int Foo::bar = you_initial_value_here; This is required so the compiler has a place for the static variable.
It's the correct syntax, however, Foo::bar must be defined separately, outside of the header. In one of your .cpp files, say this:
int Foo::bar = 0; // or whatever value you want 
You need add a line:
int Foo::bar; That would define you a storage. Definition of static in class is similar to "extern" -- it provides symbol but does not create it. ie
foo.h
class Foo { static int bar; int adder(); }; foo.cpp
int Foo::bar=0; int Foo::adder() { ... } for use of static variable in class, in first you must give a value generaly (no localy) to your static variable (initialize) then you can accessing a static member in class :
class Foo { public: static int bar; int baz; int adder(); }; int Foo::bar = 0; // implementation int Foo::adder() { return baz + bar; } 