pointer_to_function？ [英] pointer_to_function?
问题描述
有人可以帮我理解K& R的第119页吗？
这是qsort的一个版本，取决于命令行可选
参数（-n）按字母顺序对行进行排序和字典顺序排序。
我认为相关的行（好吧，我遇到问题
）是如下：
#include .....
等
void qsort（void * lineptr []，int left，int right，int （* comp）（void *，
void *））; / **声明** /
主要...... {
.....
if（ qsort（（void **）lineptr，0，nlines-1，（int（*）（void *，void *））
（数字？numcmp：strcmp））;
........
}
我看到qsort的decalartion说qsort是
函数，其参数是指针数组（类型为void），2
整数，以及指向函数的指针（接收2 ptrs为
返回一个整数的void类型的参数。
我想我理解void ptrs的相关性（来自我所拥有的
读取）。但是，在主要电话中调用qsort让我感到困惑。我已经看了很多日子，一直在查看常见问题解答等等，所以它是时间到了请求帮助。
mdh写道：
可能有人请帮助我理解K& R的第119页？
T.他是qsort的一个版本，取决于命令行可选
参数（-n）按字母顺序对行进行排序与字典顺序排序。
我认为相关行（好吧，我遇到问题
）如下：
#include .....
等
void qsort（void * lineptr []，int left，int right，int（* comp）（void *，
void *））; / **声明** /
主要...... {
.....
if（ qsort（（void **）lineptr，0，nlines-1，（int（*）（void *，void *））
（数字？numcmp：strcmp））;
........
}
int（*）（void *，void *））（数字？numcmp：strcmp）
将（numeric？numcmp：strcmp）的结果转换为所需的
函数指针类型。我假设numcmp使用int *而strcmp使用const
char *作为参数，因此需要演员。
-
Ian柯林斯。
4月30日下午5:46，Ian Collins< ian-n ... @ hotmail.comwrote：
> int（*）（void *，void *））（数字？numcmp：strcmp）
正在铸造（numeric？numcmp：strcmp）的结果为所需的
函数指针类型。我假设numcmp使用int *而strcmp使用const
char *作为参数，因此需要进行强制转换。
正确。
此外，您可能猜到的数字是一个整数变量。
所以，一旦条件表达式得到解决，你最终会得到
> int（* numcmp）（void *，void *））
或
> int（* strcmp）（void *，void *）？
这对我来说有点令人费解......好吧，其中一件事是
困惑!!
mdh写道：
有人可以帮我理解K& R的第119页？
这是qsort的一个版本，取决于命令行可选
参数（-n）对数字排序vs按字典顺序排列。
我认为相关的行（好吧，我遇到问题
）如下：
#include .....
等
void qsort（void * lineptr []，int left，int right，int（* comp）（ void *，
void *））; / **声明** /
主要...... {
....
if（qsort （（void **）lineptr，0，nlines-1，（int（*）（void *，void *））
（数字？numcmp：strcmp））;
.......
}
我看到的是qsort的decalartion说qsort是一个
函数，其参数是指针数组（void类型），2
整数，以及指向函数的指针（接收2 ptrs作为
参数类型void）返回一个整数。
我想我理解void ptrs的相关性（来自我所拥有的
读取）。但是，调用在主要的qsort，让我感到困惑。我已经看了很多一天的大部分时间，一直在阅读常见问题解答等等，所以它是时候要求b / b
帮助。
首先，要预防一些可能的混淆：本练习中的qsort（）
函数与函数不同
C标准库中的名称不要理会别人
谁告诉你strcmp（）可能不是正确的比较
函数与qsort（）一起使用 - 它可能不是，但是他们总是想着一个不同的qsort（）函数。
现在问题：你显然对最后一个感到困惑
电话中的参数
qsort（...，（数字？ numcmp：strcmp））;
这是通常称为三元运算符
或？：的示例;接线员：
条件？ true_result：false_result
这个表达式首先计算`condition''（在这种情况下是
`cigue''）。如果条件非零（即为真），则表达式的值为true_result（或者在这种情况下，
为numcmp） '，指向名为numcmp的函数的指针。如果
条件为零（false），则值为false_condition，
或者在这种情况下为strcmp。其他计算机语言有类似的构造，但拼写它不那么令人困惑：
如果数字那么numcmp其他strcmp
....是Algol所说的方式。 C'对简洁的热情得到了完全消除'if''，然后将'then'和'else'拼写为
`？''和''：''分别。
结果是这个对qsort（）的调用传递为
第四个参数，指向numcmp的指针（ ）函数或
a指向strcmp（）函数的指针，取决于变量`numeric''的值
。围绕
整个第四个参数的括号是不必要的，并且可能只是为了强调而提供的
。
我希望这个帮助。
并记住：当你毕业时使用真实的qsort（），
几乎可以肯定*不*使用strcmp（）作为最终的
参数。我相信这在FAQ中讨论过。
-
Eric Sosman
es ***** @ acm-dot-org.inva 盖子
Could someone please help me understand p 119 of K&R?
This is a version of qsort that, depending on a command line optional
argument ( -n) sorts lines numerically vs lexicographically.
I think the relevant lines ( well, the ones I am having a problem
with) are as follows.
# include.....
etc
void qsort(void *lineptr[], int left, int right, int (*comp)(void *,
void *)); /** declaration**/
main......{
.....
if(qsort((void**) lineptr, 0, nlines-1, (int (*)(void*, void*))
(numeric ? numcmp:strcmp));
........
}
What I see is that the decalartion of qsort says that qsort is a
function, whose arguments are an array of pointers ( of type void), 2
integers, and a pointer to a function( that recieives 2 ptrs as
arguments of type void) that returns an integer.
I think I understand the relevance of the void ptrs( from what I have
read). However, the call to qsort in main, leaves me perplexed. I have
been looking at this much of the day, been reading FAQs etc, so it''s
time to ask for help.
mdh wrote:Could someone please help me understand p 119 of K&R?
This is a version of qsort that, depending on a command line optional
argument ( -n) sorts lines numerically vs lexicographically.
I think the relevant lines ( well, the ones I am having a problem
with) are as follows.
# include.....
etc
void qsort(void *lineptr[], int left, int right, int (*comp)(void *,
void *)); /** declaration**/
main......{
.....
if(qsort((void**) lineptr, 0, nlines-1, (int (*)(void*, void*))
(numeric ? numcmp:strcmp));
........
}
int (*)(void*, void*))(numeric ? numcmp:strcmp)
Is casting the result of (numeric ? numcmp:strcmp) to the required
function pointer type. I assume numcmp uses int* and strcmp uses const
char* for their parameters, hence the need for a cast.
--
Ian Collins.
On Apr 30, 5:46 pm, Ian Collins <ian-n...@hotmail.comwrote:
>int (*)(void*, void*))(numeric ? numcmp:strcmp)
Is casting the result of (numeric ? numcmp:strcmp) to the required
function pointer type. I assume numcmp uses int* and strcmp uses const
char* for their parameters, hence the need for a cast.
correct.
Also, numeric, as you probably guessed, is an integer variable.
So, once the conditional expression is resolved, do you end up with
>int (* numcmp) (void*, void*))or
>int (*strcmp)(void*, void*) ?This is somewhat puzzling to me...well, one of the things that is
puzzing!!
mdh wrote:Could someone please help me understand p 119 of K&R?
This is a version of qsort that, depending on a command line optional
argument ( -n) sorts lines numerically vs lexicographically.
I think the relevant lines ( well, the ones I am having a problem
with) are as follows.
# include.....
etc
void qsort(void *lineptr[], int left, int right, int (*comp)(void *,
void *)); /** declaration**/
main......{
....
if(qsort((void**) lineptr, 0, nlines-1, (int (*)(void*, void*))
(numeric ? numcmp:strcmp));
.......
}
What I see is that the decalartion of qsort says that qsort is a
function, whose arguments are an array of pointers ( of type void), 2
integers, and a pointer to a function( that recieives 2 ptrs as
arguments of type void) that returns an integer.
I think I understand the relevance of the void ptrs( from what I have
read). However, the call to qsort in main, leaves me perplexed. I have
been looking at this much of the day, been reading FAQs etc, so it''s
time to ask for help.First, to forestall some possible confusion: The qsort()
function in this exercise is not the same as the function of
that name in the C Standard library. Pay no heed to people
who tell you that strcmp() is probably not the right comparison
function to use with qsort() -- it probably isn''t, but they''re
thinking of a different qsort() function altogether.
Now to the question: You''re apparently confused by the last
argument in the call
qsort (..., (numeric ? numcmp:strcmp));
This is an example of what''s often called the "ternary operator"
or the "?:" operator:
condition ? true_result : false_result
This expression first evaluates `condition'' (in this case
`numeric''). If the condition is non-zero (i.e., true), the
value of the expression is `true_result'' (or in this case,
it is `numcmp'', a pointer to the function named numcmp). If
the condition is zero (false), the value is `false_condition'',
or in this case `strcmp''. Other computer languages have had
a similar construct, but have spelled it less confusingly:
if numeric then numcmp else strcmp
.... was the way Algol put it. C''s passion for brevity gets
rid of the `if'' altogether, and spells `then'' and `else'' as
`?'' and '':'', respectively.
The upshot is that this call to qsort() passes as its
fourth argument either a pointer to the numcmp() function or
a pointer to the strcmp() function, depending on the value
of the variable `numeric''. The parentheses that surround the
entire fourth argument are unnecessary, and are presumably
present only for emphasis.
I hope this helps.
And remember: When you graduate to using the "real" qsort(),
you should almost certainly *not* use strcmp() as the final
argument. I believe this is discussed in the FAQ.
--
Eric Sosman
es*****@acm-dot-org.invalid
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