在C ++中for循环中的变量的重写 [英] Redeclaration of a variable in a for-loop in C++
问题描述
当尝试为多个平台编译以下（简化）代码时，我发现它在某些方面失败，即IBM的xlC_r。进一步的调查发现，它也失败了来和ang。它用g ++和Solaris的CC编译成功。
这里是代码：
int main（）
{
int a1 [1];
bool a2 [1];
for（int * it = a1，* end = a1 + 1; it！= end; ++ it）{
// ...
bool * jt = a2，* end = a2 + 1;
// ...
}
}
xlC_r错误：
main.cpp，第8.25行：1540-0400（S）end
main.cpp，行6.25：1540-0425（I）end在main.cpp的第6行定义。
clang错误：
main.cpp：8：25：error：redefinition的'end'类型不同
bool * jt = a2，* end = a2 + 1;
^
main.cpp：6：25：注意：以前的定义在这里
for（int * it = a1，* end = a1 + 1; it！= end; ++ it ）{
^
comeau错误：
$ b b
ComeauTest.c，第8行：错误：end，在for循环初始化中声明，可能不
在此范围内重新声明
bool * jt = a2，* end = a2 + 1;
^
问题是为什么是错误？
通过2003年的标准，它说如下（6.5.3）：
语句
for（for-init-statement; condition; expression）语句
相当于
{
for-init-statement;
while（condition）{
statement;
expression;
}
}
，但在for-init语句中声明的名称与在条件中声明的
声明性区域相同
此外，它说（6.5） .1）：
当while语句的条件是声明时，变量的范围
声明从声明
（3.3.1）延伸到while语句的结尾。 while语句
while（T t = x）语句
等价于
标签：
{
T t = x;
if（t）{
语句;
goto label;
}
}
同样，我不确定这是否相关，因为在条件中没有声明。所以给定从6.5.3的等价重写，我的代码应该是相同的：
int main（）
{
int a1 [1];
bool a2 [1];
{
int * it = a1，* end = a1 + 1;
while（it！= end）{
// ...
bool * jt = a2，* end = a2 + 1;
// ...
++ it;
}
}
}
解决方案标准有点模棱两可。您引用的代码等同于而循环意味着有一个内部作用域，其中循环中的声明可以隐藏条件中的声明;但是标准也说（引用C ++ 11，因为我没有C ++ 03方便）：
6.4 / 2条件规则适用于选择语句和 for 和 while 语句
6.4 / 3如果在条件控制的子句的最外面的块中重新声明了名称，那么重新声明名字的声明是不成立的。
在for-init语句中声明的6.5.3 / 1个名称与在条件
中声明的声明域相同。
它们之间的意思是不能重命名这些名称。
旧的for-init语句放入循环外的声明区域。这意味着你的代码是有效的，但这不会：
for（int i = ...; .. 。...）{...}
for（int i = ...; ...; ...）{...} //错误：redeclaration的i
When trying to compile the following (simplified) code for multiple platforms, I found that it was failing on some, namely IBM's xlC_r. Further investigation has found that it also fails on comeau and clang. It compiles successfully with g++ and Solaris's CC.
Here is the code:
int main()
{
int a1[1];
bool a2[1];
for (int *it = a1, *end = a1+1; it != end; ++it) {
//...
bool *jt = a2, *end = a2+1;
//...
}
}
xlC_r error:
"main.cpp", line 8.25: 1540-0400 (S) "end" has a conflicting declaration.
"main.cpp", line 6.25: 1540-0425 (I) "end" is defined on line 6 of "main.cpp".
clang error:
main.cpp:8:25: error: redefinition of 'end' with a different type
bool *jt = a2, *end = a2+1;
^
main.cpp:6:25: note: previous definition is here
for (int *it = a1, *end = a1+1; it != end; ++it) {
^
comeau error:
"ComeauTest.c", line 8: error: "end", declared in for-loop initialization, may not
be redeclared in this scope
bool *jt = a2, *end = a2+1;
^
The question is why is this an error?
Looking through the 2003 standard, it says the following (6.5.3):
The for statement
for ( for-init-statement; condition; expression ) statement
is equivalent to
{
for-init-statement;
while ( condition ) {
statement;
expression;
}
}
except that names declared in the for-init-statement are in the same
declarative-region as those declared in condition
Here there are no names declared in condition.
Further, it says (6.5.1):
When the condition of a while statement is a declaration, the scope
of the variable that is declared extends from its point of declaration
(3.3.1) to the end of the while statement. A while statement of the form
while (T t = x) statement
is equivalent to
label:
{
T t = x;
if (t) {
statement;
goto label;
}
}
Again, I'm not sure this is relevant, as there is no declaration in the condition. So given the equivalent re-write from 6.5.3, my code should be the same as:
int main()
{
int a1[1];
bool a2[1];
{
int *it = a1, *end = a1+1;
while (it != end) {
//...
bool *jt = a2, *end = a2+1;
//...
++it;
}
}
}
Which obviously would allow end to be re-declared.
解决方案 The standard is somewhat ambiguous. The code you quote as being equivalent to a while loop implies that there is an inner scope where declarations inside the loop could hide declarations in the condition; however the standard also says (quoting C++11, since I don't have C++03 handy):
6.4/2 The rules for conditions apply both to selection-statements and to the for and while statements
6.4/3 If the name is re-declared in the outermost block of a substatement controlled by the condition, the declaration that re-declares the name is ill-formed.
6.5.3/1 names declared in the for-init-statement are in the same declarative-region as those declared in the condition
which between them imply that the names can't be redeclared.
Older (pre-1998) versions of the language put declarations in the for-init-statement into the declarative region outside the loop. This meant that your code would be valid, but this wouldn't:
for (int i = ...; ...; ...) {...}
for (int i = ...; ...; ...) {...} // error: redeclaration of i
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