Question

Just started with K & R and on the 2nd chapter, there is the line:

Declarations list the variables to be
used and state what type they have and
perhaps what their initial values are.

So:

int x = 42 is a definition.

and int x is a declaration but also a definition since every definition is a declaration.

But when we assign an intial value like K & R say, doesn’t that make the declaration a definition?

Answer 1

You confuse two things:

1. A declaration states (declares) what an object’s* type, name and scope is
2. A definition defines what a object’s content is

* object as in: variable, function etc., not an OOP object.

A definition is hence very often also a declaration, since you cannot define what is in an object when you do not state what the type of the object is. Easiest to remember is just: “Every definition is a declaration, but not every declaration is a definition”

For variables

There is only 1 way to declare without defining the variable:

extern typeX variable_name

This tells the compiler that there is a variable called variable_name with type typeX, but not where to get it.
Every other way to declare a variable is also a definition, since it tells the compiler to reserve space for it and perhaps give it an initial value.

The difference is much clearer in structs and functions:

For Structs

A declaration:

struct some_struct{
    int a;
    int b;
}

This declares some_struct to the compiler with a and b as struct variables both with type int.

Only when you define them space is reserved and you can use them:

void foo(){
    struct some_struct s;
    s.a = 1; // For this to work s needs to be defined
}

For functions:

The difference is much more clear

declaration:

// This tells the compiler that there is a function called "foo" that returns void and takes void arguments
void foo();

A definition could be like the one above (in the struct part)