评估if表达式的行为不同于评估BOOL表达式 [英] Evaluating an if expression behaves differently then evaluating a BOOL expression
问题描述
我假设这两个 if
语句的结果始终是相同的
i assumed that the outcome of these two if
statements is ALWAYS the same
BOOL tmp = (expression);
if (expression) {
}
if (tmp) {
}
像检查 0xF00
指针时,可能并不总是这样...
like when checking against a 0xF00
pointer, this might not be always the case...
c>
推荐答案
code> if 语句定义在 spec 。相关部分是:
The if
statement is defined in section 6.8.4.1 of the spec. The relevant section is:
2在两种形式中,如果表达式不等于0,则执行第一个子语句。
2 In both forms, the first substatement is executed if the expression compares unequal to 0.
你会注意到在该语句中任何地方缺少 BOOL
。 BOOL
在C中不存在;它只是Cocoa提供的 signed char
的typedef。 ( bool
存在于C的现代版本中,但它是一个不同的类型,并且仍然与 if
语句无关。 )
You will note the lack of BOOL
anywhere in that statement. BOOL
doesn't exist in C; it's just a typedef to signed char
provided by Cocoa. (bool
exists in modern versions of C, but it's a different type, and still not related to the if
statement.)
所以如果 expression
比较不等于0,但 tmp
比较等于0,则它们的行为会不同。这是汤姆·哈灵顿在评论中提出的观点。
So if expression
compares unequal to 0, but tmp
compares equal to 0, then they will behave differently. This is the point Tom Harrington was making in the comments.
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