问题:

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++?

回答1:

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.

回答2:

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

回答3:

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() { ... }

回答4:

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;
}