宏和副作用 [英] macros and side effects

问题描述

C宏是否与侧面效果相结合是不安全的，即使它们是干净的，也是干净的。例如可以


foo_macro（++ c）;


执行++ c多次，即使foo_macro是干净的？我总是认为干净是干净的。宏（即宏中所有参数都是定义中括号括起来的
）没有这个问题...


我想知道因为我有在参考文献中查找了putc（），并且它表示如果通过的话，它是不安全的。值有副作用，我假设

标准宏是干净的......

解决方案

>>>>" C" == copx< co ** @ gazeta.plwrites：


c如果C与宏结合使用是不安全的，那么即使它们是b
"清洁"例如可以


cfoo_macro（++ c）;


#define foo_macro（x）（x）*（x）
< br $> b $ b哎呀。


Charlton


-

Charlton Wilbur
cw*****@chromatico.net

copx写道：

如果C宏与侧面效果相结合是不安全的，即使

它们是干净的例如可以


foo_macro（++ c）;


执行++ c多次，即使foo_macro是干净的？我总是认为干净是干净的。宏（即宏中所有参数都是定义中括号括起来的
）没有这个问题...


我想知道因为我有在参考文献中查找了putc（），并且它表示如果通过的话，它是不安全的。值有副作用，我假设

标准宏是干净的...

你的引用是正确的，并且 ;清洁"宏可以多次评估他们的

参数。举个简单的例子，如果你有
#define foo_macro（x）（（x）*（x））


那么foo_macro'的参数是完全括号的，但是foo_macro（++ c）

仍将扩展为（（++ c）*（++ c））。


对于大多数标准库函数，如果它们也是作为一个宏实现的，那么它必须是宏，那个宏只计算每个参数

一次。 putc是一个例外。

copx说：

C宏是不是真的不安全

如果写得正确并且使用不正确，则不会。

与副作用结合使用即使它们是"清洁"例如可以


foo_macro（++ c）;


执行++ c多次，即使foo_macro是干净的？

这不是一个广泛用于描述宏的术语。 （翻译：这是我第一次看到它时的
。）

我有

总是

认为干净宏（即宏中所有参数都是定义中用圆括号括起来的
）没有这个

问题...

你认为错了。

我想知道因为我刚刚在参考文献中查找了putc（），

和它说，如果通过它是不安全的。值有副作用，

你的引用说错了。 putc函数，如果实现为宏，

可以多次评估流，但*不是*字符。并且

你很难把它称为putc（''\ n''，fp ++），我相信吗？

和我假设标准宏是干净的...

为什么？


-

Richard Heathfield

Usenet是一个奇怪的地方 - dmr 29/7/1999
http://www.cpax.org.uk

电子邮件：rjh在上述域名中， - www。

Is it true that C macros are unsafe when combined with side effects even if
they are "clean" e.g. could

foo_macro(++c);

execute "++c" multiple times even if foo_macro is "clean"? I have always
thought that "clean" macros (i.e. macros where all parameters are
surrounded by parentheses in the definition) do not have this problem...

I am wondering because I have just looked up putc() in a reference, and it
says that it is unsafe if the "passed" value has a side effect, and I assume
standard macros are "clean"...

解决方案

>>>>"c" == copx <co**@gazeta.plwrites:

cIs it true that C macros are unsafe when combined with side
ceffects even if they are "clean" e.g. could

cfoo_macro(++c);

#define foo_macro(x) (x)*(x)

Whoops.

Charlton

--
Charlton Wilbur
cw*****@chromatico.net

copx wrote:

Is it true that C macros are unsafe when combined with side effects even if
they are "clean" e.g. could

foo_macro(++c);

execute "++c" multiple times even if foo_macro is "clean"? I have always
thought that "clean" macros (i.e. macros where all parameters are
surrounded by parentheses in the definition) do not have this problem...

I am wondering because I have just looked up putc() in a reference, and it
says that it is unsafe if the "passed" value has a side effect, and I assume
standard macros are "clean"...

Your reference is correct, and "clean" macros can evaluate their
arguments multiple times. As a simple example, if you have

#define foo_macro(x) ((x) * (x))

then foo_macro''s arguments are fully parenthesised, but foo_macro(++c)
will still expand to ((++c) * (++c)).

For most standard library functions, it is required that if they are
also implemented as a macro, that that macro evaluates each argument
exactly once. putc is an exception.

copx said:

Is it true that C macros are unsafe

Not if written correctly and used correctly.

when combined with side effects even if they are "clean" e.g. could

foo_macro(++c);

execute "++c" multiple times even if foo_macro is "clean"?

That is not a widely-used term for describing macros. (Translation: it''s
the first time I have ever seen it.)

I have
always
thought that "clean" macros (i.e. macros where all parameters are
surrounded by parentheses in the definition) do not have this
problem...

You thought wrong.

I am wondering because I have just looked up putc() in a reference,
and it says that it is unsafe if the "passed" value has a side effect,

Your reference says wrong. The putc function, if implemented as a macro,
may evaluate the stream more than once, but *not* the character. And
you''re hardly likely to call it as putc(''\n'', fp++), I trust?

and I assume standard macros are "clean"...

Why?

--
Richard Heathfield
"Usenet is a strange place" - dmr 29/7/1999
http://www.cpax.org.uk
email: rjh at the above domain, - www.

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