c++ - 在 C++ 中的 for 循环中重新声明变量

Question

当尝试为多个平台编译以下(简化的)代码时，我发现它在某些平台上失败了，即 IBM 的 xlC_r。进一步的调查发现，它在 comeau 和 clang 上也失败了。使用 g++ 和 Solaris 的 CC 编译成功。

代码如下:

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", 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 错误:

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 错误:

"ComeauTest.c", line 8: error: "end", declared in for-loop initialization, may not
          be redeclared in this scope
          bool *jt = a2, *end = a2+1;
                          ^

问题是为什么这是一个错误？

查看 2003 年标准，它说以下 (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

这里没有在条件中声明名称。

此外，它说 (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;
        }
    }

同样，我不确定这是否相关，因为条件中没有声明。因此，鉴于 6.5.3 的等效重写，我的代码应该与以下内容相同:

int main()
{
    int a1[1];
    bool a2[1];

    {
        int *it = a1, *end = a1+1;
        while (it != end) {
            //...
            bool *jt = a2, *end = a2+1;
            //...
            ++it;
        }
    }
}

这显然会允许重新声明 end。

Answer 1

标准有些模糊。您引用的代码等效于 while 循环意味着存在一个内部范围，循环内的声明可以隐藏条件中的声明；但是标准也说(引用 C++11，因为我手边没有 C++03):

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

它们之间的含义是不能重新声明名称。

该语言的较旧(1998 年之前)版本将 for-init-statement 中的声明放入循环外的声明区域。这意味着您的代码将是有效的，但这不会:

for (int i = ...; ...; ...) {...}
for (int i = ...; ...; ...) {...}  // error: redeclaration of i