Nested Subprograms and Blocks Quiz

Variable Scope

• A variable is an abstraction of a memory cell, characterized by a sextuple of attributes: name, address, value, type, lifetime, and scope.
• The scope of a variable is the range of statements over which it is visible.
• The local variables of a program unit are those that are declared in that unit.
• The nonlocal variables of a program unit are those that are visible in the unit but not declared there.
• Global variables are a special category of nonlocal variables.

Static Scope (Lexical Scope)

• The scope of a variable can be statically determined prior to execution.
• Used in many imperative and nonimperative languages.
• To connect a name reference to a variable and its attributes, you (or the compiler) must find the declaration.
• Search process: search declarations, first locally, then in increasingly larger enclosing scopes, until one is found for the given name.
• Terminology: Enclosing static scopes (to a specific scope) are called its static ancestors; the nearest static ancestor is called a static parent.

Blocks

• A method of creating static scopes inside program units.
• Syntax: BEGIN … END, {....}
• Example: C program with nested blocks, showing the scope of variables x and y.

Declaration and Order

• In C89, all data declarations in a function must appear in the beginning.
• C99, C++, Java, and C# allow variable declarations to appear anywhere a statement can appear.
• In C99, C++, and Java, the scope of all local variables is from the declaration to the end of the block.
• In C# documentation, the scope of any variable declared in a block is the whole block, regardless of the position of the declaration in the block.

Declaration vs. Definition

• In C and C++, there are both declarations (just attributes) and definitions (attributes and storage).
• A declaration outside a function definition specifies that it is defined in another file.

Global Scope

• C, C++, PHP, and Python support a program structure that consists of a sequence of function definitions in a file.
• These languages allow variable declarations to appear outside function definitions.
• Global variables can be accessed in a function through the $GLOBALS array or by declaring it global.

Dynamic Scope

• Based on calling sequences of program units, not their textual layout (temporal versus spatial).
• Referencing variables to declaration: References to variables are connected to declarations by searching back through the chain of subprogram calls that forced execution to this point.
• Example: Static vs. Dynamic program scopes in Pascal.

Evaluation of Dynamic Scoping

• Advantage: convenience.
• Disadvantages:
  • While a subprogram is executing, its variables are visible to all subprograms it calls.
  • Impossible to statically type check.
  • Poor readability: it is not possible to statically determine the type of a variable.

Summary

• Scope: Static and Dynamic.
• Blocks and their Scope.
• Declaration and their Order.
• Declaration vs. Definition in C/C++.