Question

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.

Answer 1

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