Cannot declare member function ‘static int Foo::bar()’ to have static linkage

You can get the error

cannot declare member function ‘static int Foo::bar()’ to have static linkage

if you declare a method to be static in your .cc file.
The reason is that static means something different inside .cc files than in class declarations It is really stupid, but the keyword static has three different meanings. In the .cc file, the static keyword means that the function isn't visible to any code outside of that particular file.
This means that you shouldn't use static in a .cc file to define one-per-class methods and variables. Fortunately, you don't need it. In C++, you are not allowed to have static variables or static methods with the same name(s) as instance variables or instance methods. Therefore if you declare a variable or method as static in the class declaration, you don't need the static keyword in the definition. The compiler still knows that the variable/method is part of the class and not the instance.
WRONG

Foo.h:
class Foo
{
public:
static int bar();
};
Foo.cc:
static int Foo::bar()
{
// stuff
}

WORKS

Foo.h:
class Foo
{
public:
static int bar();
};
Foo.cc:
int Foo::bar()
{
// stuff
}

A way to bypass this problem is to embed the defintion in the .h file, but this causes the function to be inline by default.
ALSO WORKS

Foo.h:
class Foo
{
public:
static int bar()
{
// stuff
};
};

转自 http://cplusplus.syntaxerrors.info/index.php?title=Cannot_declare_member_function_%E2%80%98static_int_Foo::bar%28%29%E2%80%99_to_have_static_linkage