北京大学计算概论课件第十一讲 变量的生存期作用域.ppt

Lesson 11 Functions Ⅱ —— Variable Scope, Storage Class and Recursion 2007.10.31 Objectives Variable Scope Variable Storage Class Pass by Reference Pointer Recursion Variable Scope Variable Scope (continue) Scope is the section of the program where the variable is valid or “known” Variables created inside a function are local variables Variables created outside and in front of all the functions are Globe variables Global variables ——sharing data among functions Can be access from all the functions Exist till the end of program running Coding a function prototype as a global makes sense when the function is used by a number of other functions in a source code file Except for symbolic constants and prototypes, global variables should almost never be used Formal parameters: int drawOut (int, int); … trans = drawOut ( i , j ); if (trans = 0) i = trans; else printf(“ Overdraw\n”); … Variable Storage Class In addition to the space dimension represented by its scope, variables also have a time dimension Called the variable’s “lifetime” variable’s life duration is determined by the storage class of the variable auto, static, extern, and register Variable Storage Classes Variable scope: local global Storage class: auto static Automatic storage —— Object created and destroyed within its block Stack: auto vs static Static storage Variables exist for entire program execution static: local variables defined in functions. Keep value after function ends Only known in their own function extern: default for global variables and functions Known in any functions Control scope of a global variables the extern storage class is to extend the scope of a global variable declared in one source code file into another source code file The static global class is used to prevent the extension of a global variable into a second file The “extern” specifier: register: Tries to put variable into high-speed registers Can only be