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C++ Tutorial - Lesson 28: Scope and Lifespan

Introduction, Local Scope

by John Kopp

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Welcome to EasyCPlusPlus.com's tutorial on C++ programming. This lesson covers the topics of variable scope and lifetime. Every object in a program has an identifier or name. Many complex questions arise from this seemingly simple statement. Does each identifier need to be unique? Where is an identifier valid? Where can it be used? Where can it not? The answers to these questions are found by understanding the concept of scope.

A variable's scope is the region of code in which the compiler can uniquely resolve its identifier. Within a particular scope, an identifier must be unique. In C++, objects can have local, global (namespace) or class scope. A variable with local scope is visible only within the function or code block in which it is defined. Variables with global scope are visible and can be used through out a program. Variables have global scope (or namespace scope) if they are defined in the part of the code that is outside of any class or function definition. A class definition also delimits a block of code and a scope.

Closely related to scope is the topic of lifespan or lifetime. How long does a variable exist? Will it exists for the entire duration of a program or only for the duration of a function or method call? As we will see, an object's lifetime depends largely on its scope.

Local Scope
Functions or class methods define a local scope. Code blocks delimited by brackets, {}, and certain statements also define local scopes. Variables within a particular local scope can only be accessed by code within that particular code block. Outside of the block, they do not exist. Each identifier must be unique within a scope. The compiler must resolve each identifier within a scope to a particular memory location. The definition of an object allocates storage space for the object in memory. During compilation, each name (identifier) must be resolved or linked to a particular physical location in memory. This means that every definition in a local scope must use a distinct identifier.

To understand all of this, let's look at a few examples.

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