Scope 作用域

pre-knowledge -- declarations and definitions

1. Definition

Scope determines the region of the program where a variable or function is visible and can be accessed. If you try to access a variable outside its scope, the compiler will generate an error.

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2. Types of Scope

A. Block Scope (Local Scope)

• Definition: Variables declared inside a block { ... }.

• Visibility: Only accessible within that specific block (functions, loops, if-statements).

• Lifetime: Created when execution enters the block(the point of declaration), destroyed when it exits (unless static).

• Example:

  void func() {
      int x = 10; // Block scope (visible only in func)
      if (x > 5) {
          int y = 20; // Block scope (visible only in this 'if')
      }
      // y is not accessible here
  }
  

B. File Scope (Global Scope)

• Definition: Variables declared outside of all functions (usually at the top of the file).

• Visibility: Accessible from the point of declaration to the end of the file. Can be accessed by other files if not marked static.

• Lifetime: Exists for the entire duration of the program.

C. Function Scope

• Specific Case: Applies only to Labels (used with goto).

• Rule: A label is visible throughout the entire function, regardless of which block it is in.

D. Function Prototype Scope

• Definition: Parameter names in a function declaration.

• Rule: The names (a, b) only matter within that specific prototype line.

  void myFunc(int a, int b); // 'a' and 'b' have prototype scope here
  

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3. Impact of Keywords (static & extern)

A. static (Internal Linkage) Learn more about static

• On Global Variables: Limits the scope to the current file only. Other files cannot access it using extern.

  • Use: To create "private" global variables.

• On Local Variables: Does not change scope (still only visible in the block), but extends lifetime (persists between function calls).

B. extern (External Linkage)

• Purpose: Declares a variable that is defined in another file.

• Effect: Extends the scope of a global variable from File A to File B.

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4. Variable Shadowing

• Concept: If an inner block defines a variable with the same name as an outer block (or global), the inner variable "shadows" (hides) the outer one.

• Example:

  int x = 100; // Global
  
  void test() {
      int x = 5; // Local 'x' shadows Global 'x'
      printf("%d", x); // Prints 5
  }
  

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5. Summary Table

Scope Type	Declaration Place	Visibility	Lifetime (Default)
Block	Inside { }	Inside that block only	Ends when block ends
File	Outside functions	Entire file (downwards)	Entire Program
Function	Labels (label:)	Entire Function	Function execution
Static Global	Outside { } + static	Current File Only	Entire Program