Справочник Пользователя для AMD 250

25112

Rev. 3.06

September 2005

In expressions that use the logical AND (

&&

) or logical OR (

||

) operator, arrange the operands for 

quick evaluation of the expression:

This optimization applies to:

•

32-bit software

•

64-bit software

C and C++ compilers guarantee short-circuit evaluation of the boolean operators 

&&

 and 

||

. In an 

expression that uses 

&&

, the first operand to evaluate to false terminates the evaluation; subsequent 

operands are not evaluated. In an expression that uses ||, the first operand to evaluate to true terminates 
the evaluation.

When used to control program flow, expressions involving && and || are translated into a series of 
conditional branches. This optimization minimizes the total number of conditions evaluated and 
branches executed.

In the following code, the operands of 

&&

 are not arranged for quick expression evaluation because the 

first operand is not the condition case most likely to be false (it is far less likely for an animal name to 
begin with a ‘y’ than for it to have fewer than four characters):

char animalname[30];

char *p;

p = animalname;

if ((strlen(p) > 4) && (*p == 'y')) { ... }

&&

 (logical AND)

False

||

 (logical OR)

True