Question

I was reading intro on gtest and found this part confusing:

The compiler complains about “undefined references” to some static
const member variables, but I did define them in the class body.
What’s wrong?

If your class has a static data member:

// foo.h
class Foo {
  ...
  static const int kBar = 100;
};

You also need to define it outside of the class body in foo.cc:

const int Foo::kBar;  // No initializer here.

Otherwise your code is invalid C++, and may break in unexpected
ways. In particular, using it in Google Test comparison assertions
(EXPECT_EQ, etc) will generate an “undefined reference” linker error.

Can somebody explain why defining a static const in in a class without defining it outside class body is illegal C++?

Answer 1

First things first, inside a class body is not a definition, it’s a declaration. The declaration specifies the type and value of the constant, the definition reserves storage space. You might not need the storage space, for instance if you only use the value as a compile time constant. In this case your code is perfectly legal C++. But if you do something like pass the constant by reference, or make a pointer point to the constant then you are going to need the storage as well. In these cases you would get an ‘undefined reference’ error.