class definition and class declaration

Question

I am reading programming principles and practice using c++ (Bjarne Stroustrup),

9.4.4 Defining member functions ,page 312, it says

1.

writing the definition of a member function within the class definition has two effects...

2.

Don't put member function bodies in the class declaration unless you know that ...

does the author write wrong? it talks about the same thing, why first sentence is "class definition" and second is "class declaration" ?

thanks

Answer 1

does the author write wrong?

Not wrong, but imprecise. A class declaration can be a forward declaration, such as:

class X;

This just makes the compiler aware of the existence of that class, but does not specify what members the class has, what base classes, and so on (that's what a class definition does).

However, a declaration can also be a definition, such as:

class X
{
    // ...
};

So in a sense, since a definition is also a declaration, the sentence is not wrong.

The sentence would be wrong under the common-sense assumption that by declaration we mean a declaration which is not a definition, but when dealing with such terms it is probably better to keep in mind their formal definition.