Declaration vs. Definition

• A declaration tells the compiler the type of a variable, object or function.

• A definition allocates memory for a variable or object and is the implementation of a function.

• Multiple declarations are allowed, but only one definition.

• Some declarations are not definitions: extern int i ; int add_them (int, int) ;

Scope & Lifetime

• The scope of a declaration is the part of the program for which the declaration is in effect.

• C/C++ use lexical scoping.

• The lifetime of a variable or object is the time period in which the variable/object has valid memory.

• Lifetime is also called "allocation method" or "storage duration."

Lifetime

• Static: A static variable is stored in the data segment of the "object file" of a program. Its lifetime is the entire duration of the program's execution.

• Automatic: An automatic variable has a lifetime that begins when program execution enters the function or statement block or compound and ends when execution leaves the block. Automatic variables are stored in a "function call stack".

• Dynamic: The lifetime of a dynamic object begins when memory is allocated for the object (e.g., by a call to malloc() or using new) and ends when memory is deallocated (e.g., by a call to free() or using delete). Dynamic objects are stored in "the heap".

Scope

• Local scope: "visible" within function or statement block from point of declaration until the end of the block.

• Class scope: "seen" by class members.

• Namespace scope: visible within namespace block.

• File scope: visible within current text file.

• Global scope: visible everywhere unless "hidden".

Need for namespace

• Global namespace "pollution"

• Enclose functions, classes, etc by namespace NS { // inside namespace block int i ; ... }

• Hides identifiers within namespace block from others.

• Nested namespaces allowed.

The :: operator

• Use :: operator to refer to identifiers, (e.g, NS::i).

• The using declaration makes one identifier visible as local declaration: using NS::i ;

• The using namespace directive makes all identifiers in namespace visible, as if namespace NS { did not exist. using namespace NS ;