转换泛型或void指针指向结构... [英] Casting a generic or void pointer to point to a struct...
问题描述
首先，我要感谢所有花时间回答我关于根据存储在
变量中的数值创建数组的问题的问题。
我在阅读完回复并进行更多研究后意识到，我真正需要的东西在C中被称为动态数组。基本上，
你超越数组表示法并使用带有内存的指针获得带有malloc（）或calloc（）的
。我认为这将完全符合我的需要。
这提出了另一个问题。我不确定我用什么语法将b
用于将一个gneeric或void指针强制转换为我已定义的
结构。例如，如果我已经定义了new_data_type，那么结构
之前在源代码文件中，以下代码是否有效？
/ *创建一个通用指针，指向块中的第一个元素br />
使用calloc（）函数获得的内存。 * /
void generic_pointer = calloc（number_of_elements，size_of_element）;
/ *如果内存已成功分配，则转换为通用
指向正确数据类型的指针。 * /
if（generic_pointer！= NULL）
（struct new_data_type *）generic_pointer
else
/ *我们遇到了问题。 * /
再次感谢大家的帮助。
Scott Huey
First, I would thank all of those that took the time to answer my
question about creating an array based on a numeric value stored in a
variable.
I realize after reading the responses and doing some more research,
that what I really need is known in C as a "dynamic array". Basically,
you surpass the array notation and use pointers with memory obtained
with malloc() or calloc(). I think this will do just what I needed.
That has brought up another question though. I''m not sure what syntax I
would use to cast a gneeric or void pointer to a struct that I have
defined. For example, if I have defined the "new_data_type" struct
previously in the source code file, would the following code be valid?
/* Create a generic pointer to the first element in the block of
memory obtained with the calloc() function. */
void generic_pointer = calloc(number_of_elements, size_of_element);
/* If the memory has been sucessfully allocated, cast the generic
pointer to the correct data type. */
if(generic_pointer != NULL)
(struct new_data_type *)generic_pointer
else
/* We''ve got problems. */
Thanks again for the help everyone.
Scott Huey
推荐答案
re ***** ***********@gmail.com 写道：
首先，我要感谢所有花时间的人回答我的
关于根据存储在
变量中的数值创建数组的问题。
我读完之后意识到回答并做了一些更多的研究，我真正需要的是在C中称为动态数组。基本上，
你超越数组表示法并使用带有内存的指针获得带有malloc（）或calloc（）的
。我认为这将完全符合我的需要。
这提出了另一个问题。我不确定我用什么语法将b
用于将一个gneeric或void指针强制转换为我已定义的
结构。例如，如果我已经定义了new_data_type，那么结构
之前在源代码文件中，以下代码是否有效？
/ *创建一个通用指针，指向块中的第一个元素br />
使用calloc（）函数获得的内存。 * /
void generic_pointer = calloc（number_of_elements，size_of_element）;
/ *如果内存已成功分配，则转换为通用
指向正确数据类型的指针。 * /
if（generic_pointer！= NULL）
（struct new_data_type *）generic_pointer
else
/ *我们遇到了问题。 * /
再次感谢大家的帮助。
Scott Huey
First, I would thank all of those that took the time to answer my
question about creating an array based on a numeric value stored in a
variable.
I realize after reading the responses and doing some more research,
that what I really need is known in C as a "dynamic array". Basically,
you surpass the array notation and use pointers with memory obtained
with malloc() or calloc(). I think this will do just what I needed.
That has brought up another question though. I''m not sure what syntax I
would use to cast a gneeric or void pointer to a struct that I have
defined. For example, if I have defined the "new_data_type" struct
previously in the source code file, would the following code be valid?
/* Create a generic pointer to the first element in the block of
memory obtained with the calloc() function. */
void generic_pointer = calloc(number_of_elements, size_of_element);
/* If the memory has been sucessfully allocated, cast the generic
pointer to the correct data type. */
if(generic_pointer != NULL)
(struct new_data_type *)generic_pointer
else
/* We''ve got problems. */
Thanks again for the help everyone.
Scott Huey
struct new_data_type * ptr = calloc（number_of_elements，
size_of_element）;
if（ptr == NULL）
{
/ *错误* /
}
其他
{
/ *做点什么* / < br $>
}
MQ
struct new_data_type * ptr = calloc(number_of_elements,
size_of_element);
if(ptr == NULL)
{
/*error */
}
else
{
/*do something */
}
MQ
MQ写道：
MQ wrote:
re ****** **********@gmail.com 写道：
首先，我要感谢所有花时间回答的人我的
关于根据存储在
变量中的数值创建数组的问题。
读完回复后我才意识到并做了一些更多的研究，我真正需要的是在C中称为动态数组。基本上，
你超越数组表示法并使用带有内存的指针获得带有malloc（）或calloc（）的
。我认为这将完全符合我的需要。
这提出了另一个问题。我不确定我用什么语法将b
用于将一个gneeric或void指针强制转换为我已定义的
结构。例如，如果我已经定义了new_data_type，那么结构
之前在源代码文件中，以下代码是否有效？
/ *创建一个通用指针，指向块中的第一个元素br />
使用calloc（）函数获得的内存。 * /
void generic_pointer = calloc（number_of_elements，size_of_element）;
/ *如果内存已成功分配，则转换为通用
指向正确数据类型的指针。 * /
if（generic_pointer！= NULL）
（struct new_data_type *）generic_pointer
else
/ *我们遇到了问题。 * /
再次感谢大家的帮助。
Scott Huey
First, I would thank all of those that took the time to answer my
question about creating an array based on a numeric value stored in a
variable.
I realize after reading the responses and doing some more research,
that what I really need is known in C as a "dynamic array". Basically,
you surpass the array notation and use pointers with memory obtained
with malloc() or calloc(). I think this will do just what I needed.
That has brought up another question though. I''m not sure what syntax I
would use to cast a gneeric or void pointer to a struct that I have
defined. For example, if I have defined the "new_data_type" struct
previously in the source code file, would the following code be valid?
/* Create a generic pointer to the first element in the block of
memory obtained with the calloc() function. */
void generic_pointer = calloc(number_of_elements, size_of_element);
/* If the memory has been sucessfully allocated, cast the generic
pointer to the correct data type. */
if(generic_pointer != NULL)
(struct new_data_type *)generic_pointer
else
/* We''ve got problems. */
Thanks again for the help everyone.
Scott Huey
struct new_data_type * ptr = calloc（number_of_elements，
size_of_element）;
if（ptr == NULL）
{
/ *错误* /
}
其他
{
/ *做点什么* / < br $>
}
MQ
struct new_data_type * ptr = calloc(number_of_elements,
size_of_element);
if(ptr == NULL)
{
/*error */
}
else
{
/*do something */
}
MQ
更正：size_of_element应替换为sizeof（struct
new_data_type）
MQ
correction: size_of_element should be replaced by sizeof(struct
new_data_type)
MQ
re **************** @ gmail.com 写道：
首先，我要感谢所有花时间回答我关于根据存储在
变量。
我在阅读完回复并做了一些研究后意识到，
我真正需要的是C中已知的动态阵列。基本上，
你超越数组表示法并使用带有内存的指针获得带有malloc（）或calloc（）的
。我认为这将完全符合我的需要。
这提出了另一个问题。我不确定我用什么语法将b
用于将一个gneeric或void指针强制转换为我已定义的
结构。例如，如果我已经定义了new_data_type，那么结构
之前在源代码文件中，以下代码是否有效？
/ *创建一个通用指针，指向块中的第一个元素br />
使用calloc（）函数获得的内存。 * /
void generic_pointer = calloc（number_of_elements，size_of_element）;
/ *如果内存已成功分配，则转换为通用
指向正确数据类型的指针。 * /
if（generic_pointer！= NULL）
（struct new_data_type *）generic_pointer
else
/ *我们遇到了问题。 * /
First, I would thank all of those that took the time to answer my
question about creating an array based on a numeric value stored in a
variable.
I realize after reading the responses and doing some more research,
that what I really need is known in C as a "dynamic array". Basically,
you surpass the array notation and use pointers with memory obtained
with malloc() or calloc(). I think this will do just what I needed.
That has brought up another question though. I''m not sure what syntax I
would use to cast a gneeric or void pointer to a struct that I have
defined. For example, if I have defined the "new_data_type" struct
previously in the source code file, would the following code be valid?
/* Create a generic pointer to the first element in the block of
memory obtained with the calloc() function. */
void generic_pointer = calloc(number_of_elements, size_of_element);
/* If the memory has been sucessfully allocated, cast the generic
pointer to the correct data type. */
if(generic_pointer != NULL)
(struct new_data_type *)generic_pointer
else
/* We''ve got problems. */
不要转换malloc（）或calloc（）的结果。这是不必要的，并且它可以掩盖某些错误。
BTW，它是void * generic_pointer，而不是void generic_pointer"。
另外，请记住，calloc（）并不一定像你想象的那样有用。
可能认为它是。它将分配的内存初始化为
all-bits-zero。对于指针或浮点数据，这不是必须有意义的b $ b;无法保证null
指针或浮点0.0表示为全位零。
通常它''更容易使用malloc（）（它没有初始化
分配的对象），只是确保你不使用任何部分
你没有赋值的对象。
为什么你想要一个通用指针而不是一个指向你的指针
" new_data_type"结构？例如：
struct new_data_type {
/ *会员声明在这里* /
};
struct new_data_type * ptr;
ptr = malloc（number_of_elements * sizeof * ptr）;
有时您确实需要使用void *指针，但通常使用特定的指针类型更好地使用
。
-
Keith Thompson（The_Other_Keith） ks***@mib.org < http://www.ghoti.net/~kst>
圣地亚哥超级计算机中心< *< http://users.sdsc.edu/~kst>
我们必须做点什么。这是事情。因此，我们必须这样做。
Don''t cast the result of malloc() or calloc(). It''s unnecessary, and
it can mask certain errors.
BTW, it''s "void *generic_pointer", not "void generic_pointer".
Also, keep in mind that calloc() isn''t necessarily as useful as you
might think it is. It initializes the allocated memory to
all-bits-zero. For pointers or floating-point data, this is not
necessarily meaningful; there''s no guarantee that either a null
pointer or a floating-point 0.0 is represented as all-bits-zero.
Usually it''s easier to use malloc() (which doesn''t initialize the
allocated object), and just make sure that you don''t use any portion
of the object that you haven''t assigned a value to.
Why do you want a generic pointer rather than a pointer to your
"new_data_type" struct? For example:
struct new_data_type {
/* member declarations here */
};
struct new_data_type *ptr;
ptr = malloc(number_of_elements * sizeof *ptr);
Sometimes you do need to use void* pointers, but quite often it''s
better to use a specific pointer type.
--
Keith Thompson (The_Other_Keith) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.
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