Declarations vs. definitions
It’s important to understand the
difference between declarations and
definitions because these terms will be used
precisely throughout the book. Essentially all C and C++ programs require
declarations. Before you can write your first program, you need to understand
the proper way to write a declaration.
A declaration introduces a name
– an identifier – to the compiler. It tells the compiler “This
function or this variable exists somewhere, and here is what it should look
like.” A definition, on the other hand, says: “Make this
variable here” or “Make this function here.” It allocates
storage for the name. This meaning works whether you’re talking about a
variable or a function; in either case, at the point of definition the compiler
allocates storage. For a variable, the compiler determines how big that variable
is and causes space to be generated in memory to hold the data for that
variable. For a function, the compiler generates code, which ends up occupying
storage in memory.
You can declare a variable or a function
in many different places, but there must be only one definition in C and C++
(this is sometimes called the ODR: one-definition
rule). When the linker is uniting all the object modules, it will usually
complain if it finds more than one definition for the same function or
A definition can also be a declaration.
If the compiler hasn’t seen the name x before and you define int
x;, the compiler sees the name as a declaration and allocates storage for it
all at once.