左倾投射 [英] lvalue cast
问题描述
我最近更新了我的gcc版本（到gcc版本3.4.2
（mingw-special）），我得到了一个警告我不太清楚如何最好
解决：
警告：使用强制转换表达式作为左值不推荐使用
调用此警告的代码与PIDL（指向项目ID
列表的指针，这是一个Windows事物并且实际上并不太重要）有关。问题
是我有一个指向结构的指针（typedef''d到ITEMIDLIST）和
为了访问一些数据我必须移动一定数量字节数（
指针真正指向一个具有任意大小的内存块的数组，并且
结构只是一种接口这些块的方式），所以这就是我b $ b做了：
ITEMIDLIST * pidl;
int offset;
...
（字符*）pidl + =偏移;
如何修复警告？优先不使用额外的
变量（例如char *）。
谢谢，
- -
Martijn
http://www.sereneconcepts.nl
" Martijn" <苏********************* @ hot-remove-mail.com>写道：
＃
＃
＃我最近更新了我的gcc版本（到gcc版本3.4.2
＃（mingw-special）），我收到警告我不太确定如何最好
＃solve：
＃
＃警告：不推荐使用强制转换表达式使用强制转换表达式
＃
＃调用此警告的代码与PIDL（指向项目ID的指针）有关/>
#list，这是一个Windows的东西，并不是真的太重要了）。问题
＃是我有一个指向结构的指针（typedef''d到ITEMIDLIST）和
＃order来访问一些我需要移动的数据一定数量的字节（
＃指针确实指向具有任意大小的内存块的数组，并且
＃结构只是一种连接这些块的方法），所以这就是我
＃做了：
＃
＃ITEMIDLIST * pidl;
#int offset;
＃
＃...
＃
＃（char *）pidl + = offset;
＃
＃我可以做些什么来修复警告？优选地，不使用额外的
＃变量（例如char *）。
将类型S的左值V强制转换为类型T的左值，
你可以做类似的事情
*（T *）& V
lvalue S V
指向S& V的指针
左值T *（T *）& V
-
SM瑞恩 http://www.rawbw.com/~wyrmwif/
留给天主教徒来破坏生存。
< blockquote>在文章< 43 *********************** @ news.xs4all.nl>，
Martijn< su *********************@hot-remove-mail.com>写道：
ITEMIDLIST * pidl;
int offset;
...
（char *）pidl + = offset;
你可以做pidl =（ITEMIDLIST *）（（（char *）pidl）+ offset）。
其他人可能会解决这个
类的东西的可移植性问题。我假设你知道对齐问题。
- Richard
Martijn写道：
我最近更新了我的gcc版本（到gcc版本3.4.2
（mingw-special）），我收到警告我不太确定如何最好地解决：
警告：不推荐使用强制转换表达式使用强制转换表达式
调用此警告的代码与PIDL（指向项目的指针）有关id
list，这是一个Windows的东西，并不是真的太重要了）。问题
是我有一个指向结构的指针（typedef''d到ITEMIDLIST）并且为了访问一些数据我必须移动一定数量的字节（
指针确实指向具有任意大小的内存块的数组，并且结构只是一种接口这些块的方式），所以这就是我所做的：
ITEMIDLIST * pidl;
int offset;
...
（char *）pidl + = offset;
我该怎么办修复警告？
优先不使用额外的变量（例如char *）。
pidl =（ITEMIDLIST *）（（char * ）pidl + offset）;
-
pete
Hi,
I recently updated my version of gcc (to gcc version 3.4.2
(mingw-special) ), and I get a warning I am not too sure about how to best
solve:
warning: use of cast expressions as lvalues is deprecated
The code that invokes this warning has to do with PIDLs (pointer to item id
list, which are a Windows thing and not too important really). The problem
is that I have a pointer to a structure (typedef''d to ITEMIDLIST) and in
order to access some data I have to move over a certain amount of bytes (the
pointer really points to an array with arbitrary-sized memory blocks, and
the structure is just a way to interface those blocks), so this is what I
did:
ITEMIDLIST* pidl;
int offset;
...
(char*)pidl += offset;
What can I do to fix the warning? Preferebly without the use of an extra
variable (e.g. a char*).
Thanks,
--
Martijn
http://www.sereneconcepts.nl
"Martijn" <su*********************@hot-remove-mail.com> wrote:
# Hi,
#
# I recently updated my version of gcc (to gcc version 3.4.2
# (mingw-special) ), and I get a warning I am not too sure about how to best
# solve:
#
# warning: use of cast expressions as lvalues is deprecated
#
# The code that invokes this warning has to do with PIDLs (pointer to item id
# list, which are a Windows thing and not too important really). The problem
# is that I have a pointer to a structure (typedef''d to ITEMIDLIST) and in
# order to access some data I have to move over a certain amount of bytes (the
# pointer really points to an array with arbitrary-sized memory blocks, and
# the structure is just a way to interface those blocks), so this is what I
# did:
#
# ITEMIDLIST* pidl;
# int offset;
#
# ...
#
# (char*)pidl += offset;
#
# What can I do to fix the warning? Preferebly without the use of an extra
# variable (e.g. a char*).
To cast an lvalue V of type S to an lvalue of type T,
you can do something like
*(T*)&V
lvalue S V
pointer to S &V
pointer to T (T*)&V
lvalue T *(T*)&V
--
SM Ryan http://www.rawbw.com/~wyrmwif/
Leave it to the Catholics to destroy existence.
In article <43***********************@news.xs4all.nl>,
Martijn <su*********************@hot-remove-mail.com> wrote:
ITEMIDLIST* pidl;
int offset;
...
(char*)pidl += offset;
You could do "pidl = (ITEMIDLIST*)(((char *)pidl) + offset)".
Someone else will probably address thet portability issues with this
sort of thing. I assume you are aware of alignment issues.
-- Richard
Martijn wrote:
Hi,
I recently updated my version of gcc (to gcc version 3.4.2
(mingw-special) ), and I get a warning I am not too sure about how to best
solve:
warning: use of cast expressions as lvalues is deprecated
The code that invokes this warning has to do with PIDLs (pointer to item id
list, which are a Windows thing and not too important really). The problem
is that I have a pointer to a structure (typedef''d to ITEMIDLIST) and in
order to access some data I have to move over a certain amount of bytes (the
pointer really points to an array with arbitrary-sized memory blocks, and
the structure is just a way to interface those blocks), so this is what I
did:
ITEMIDLIST* pidl;
int offset;
...
(char*)pidl += offset;
What can I do to fix the warning?
Preferebly without the use of an extra variable (e.g. a char*).
pidl = (ITEMIDLIST *)((char *)pidl + offset);
--
pete
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