C++ restricts the use of the extern storage
class specifier to the names of objects or functions. Using the extern specifier
with type declarations is illegal. An extern declaration
cannot appear in class scope.The extern storage class specifier lets you declare objects that several source files can use. An extern declaration makes the described variable usable by the succeeding part of the current source file. This declaration does not replace the definition. The declaration is used to describe the variable that is externally defined.
An extern declaration can appear outside a function or at the beginning of a block. If the declaration describes a function or appears outside a function and describes an object with external linkage, the keyword extern is optional.
If a declaration for an identifier already exists at file scope, any extern declaration of the same identifier found within a block refers to that same object. If no other declaration for the identifier exists at file scope, the identifier has external linkage.
C++ restricts the use of the extern storage
class specifier to the names of objects or functions. Using the extern specifier
with type declarations is illegal. An extern declaration
cannot appear in class scope.