Q:

Declaration and Definition about integral types

I am a little bit confused about this two notions. 
I am reading the "C++ Programming Language", Bjarne Stroustrup. 
According to this book, the following are declarations and definitions : 
char ch; //(dec. and def.)
string s; //(dec. and def.)
int count=1; //(dec. and def.)

But there is an exercise (4.11) that says "
For each declaration in §4.9, do the following: If the declaration is not a definition, write a definition for it. If the declaration is a definition, write a declaration for it that is not also a definition."
So I am confused how can we write the declaration without the definition for the above three things  ??

A:

I suspect the information to answer this is in other questions, but it's explicitly asking how to change from the definition to just a declaration, so here's one way:
char ch;

defines a char, without initialising it (except for some contexts that take us to another question).
extern char ch;

declares there is a charcalled ch somewhere. The linker will have to find it.
You can do likewise with the other lines of code.

