[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
operator and_then (operand1, operand2: Boolean) = Result: Boolean; |
The `and_then' short-circuit logical operator performs the same operation as the logical operator `and'. But while the ISO standard does not specify anything about the evaluation of the operands of `and' -- they may be evaluated in any order, or not at all -- `and_then' has a well-defined behaviour: It evaluates the first operand. If the result is `False', `and_then' returns `False' without evaluating the second operand. If it is `True', the second operand is evaluated and returned.
Since the behaviour described above is the most efficient way to implement `and', GPC by default treats `and' and `and_then' exactly the same. If you want, for some reason, to have both operands of `and' evaluated completely, you must assign both to temporary variables and then use `and' -- or `and_then', it does not matter.
`and_then' is an ISO-10206 Extended Pascal extension.
Some people think that the ISO standard requires both operands of `and' to be evaluated. This is false. What the ISO standard does say is that you cannot rely on a certain order of evaluation of the operands of `and'; in particular things like the following program can crash according to ISO Pascal, although they cannot crash when compiled with GNU Pascal running in default mode.
program AndBug; var p: ^Integer; begin New (p); ReadLn (p^); if (p <> nil) and (p^ < 42) then { This is NOT safe! } WriteLn ('You''re lucky. But the test could have crashed ...') end. |
program And_ThenDemo; var p: ^Integer; begin New (p); ReadLn (p^); if (p <> nil) and_then (p^ < 42) then { This is safe. } WriteLn (p^, ' is less than 42') end. |
section 9.8 and then, section 9.7 and, section 9.183 or_else.
[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |