Question

I am new to C and I experience some confusion between the declaration and definition of a variable. Another thing I would like to know is if the following is true:

“Declaration appears many times and definition comes once.”

Also:

int x; 

Is this a declaration only? Since memory is allocated for x then why isn’t this a definition instead of a declaration?

Answer 1

This isn’t something you see too much in C, but it works like this:

In a header file, you can have a line like this:

extern int x; //declaration

Because of the extern modifier, this tells the compiler that there is an int named x somewhere. The compiler doesn’t allocate space for it – it just adds int x to the list of variables you can use. It’ll only allocate space for x when it sees a line like this:

int x; //definition

You can see that because only the int x; line changes your executable, you can have as many extern int x; lines as you feel like. As long as there’s only int x; line, everything will work like you want it to – having multiple declarations doesn’t change a thing.

A better example comes from C++ (sorry if this is a C-only question – this applies to structs as well, but I don’t know the syntax off the top of my head):

class Pineapple; //declaration

Pineapple* ptr;  //this works
Pineapple pine;  //this DOES NOT work

This declaration tells the compiler that there’s a class called “Pineapple”. It doesn’t tell us anything about the class (how big it is, what its members are). We can use pointers to Pineapples now, but we can’t yet have instances – we don’t know what makes up a Pineapple, so we don’t know how much space an instance takes up.

class Pineapple
{
public:
    int ounces;
    char* name;
}; //definition

Pineapple* ptr;   //still works
Pineapple pine;   //this works now too!
//we can even get at member variables, 'cause we know what they are now:
pine.ounces = 17;

After a definition, we know everything about the class, so we can have instances, too. And like the C example, you can have multiple declarations, but only one definition.

Hope this helps!