Summary

The scope of a for loop control declaration in C is different from that in C++. In particluar, while in C the declaration establishes its own scope in which the scope of the body of the for statement is nested, in C++ the two are one and the same. The practical implication of this difference is that while in C a declaration in the body can hide the for loop declaration, in C++ such a re-declaration would be ill-formed. The following example demonstrates the difference:

        static inline int f (void) {
            for (int i = 0; ; ) {
                long i = 1;   // valid C, invalid C++
                // ...
                return i;     // (perhaps unexpectedly) returns 1 in C
            }
        }
      

During a discussion of this difference on the mailing list (starting with post C22WG14.13355), it was noted that the re-declaration could lead to subtle bugs.

The incompatibility between rules used by the two languages also makes writing headers intended to be used by both C and C++ that contain inline functions more prone to error than necessary.

In addition, it was noted (by Larry Jones in SC22WG14.13359) that the intent was for C99, where the ability to declare a for loop control variable was first added, to follow the C++ rules, but that it had been missed that the C++ rules ultimately adopted by ISO/IEC 14882:1998 changed from those of The Annotated C++ Reference Manual that was initially used to craft the C rules.

Suggested Technical Corrigendum

The author recognizes that changing the C rules could render some existing programs invalid. However, it is likely that such programs are broken/buggy and thus a breaking change would result in correcting such latent bugs.

Therefore, the proposed corrigendum suggests to align the C rules with those of C++ by adding a new paragraph to section 6.2.1 Scopes of identifiers as follows.

Names declared in clause-1 of the for statement are local to the for statement and shall not be redeclared in a subsequent condition of that statement nor in the outermost block of the controlled statement.

Note: the text of the paragraph is aligned with the corresponding paragraph 4 of section 3.3.3 Block scope of ISO/IEC 14882:2014 (and section 3.3.2 Block scope of ISO/IEC 14882:1998).