How do I declare several variables in a for (;;) loop in C?

Question

I thought one could declare several variables in a for loop:

for (int i = 0, char* ptr = bam; i < 10; i++) { ... } 

But I just found out that this is not possible. GCC gives the following error:

error: expected unqualified-id before 'char'

Is it really true that you can't declare variables of different types in a for loop?

Answer 1

You can (but generally shouldn't) use a local struct type.

for ( struct { int i; char* ptr; } loopy = { 0, bam };       loopy.i < 10 && * loopy.ptr != 0;       ++ loopy.i, ++ loopy.ptr )     { ... } 

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Since C++11, you can initialize the individual parts more elegantly, as long as they don't depend on a local variable:

for ( struct { int i = 0; std::string status; } loop;       loop.status != "done"; ++ loop.i )     { ... } 

This is just almost readable enough to really use.

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C++17 addresses the problem with structured bindings:

for ( auto [ i, status ] = std::tuple{ 0, ""s }; status != "done"; ++ i ) 

Answer 2

It's true that you can't simultaneously declare and initialize declarators of different types. But this isn't specific to for loops. You'll get an error if you do:

int i = 0, char *ptr = bam; 

too. The first clause of a for loop can be (C99 §6.8.5.3) "a declaration" or a "void expression". Note that you can do:

int i = 0, *j = NULL; for(int i = 0, *j = NULL;;){} 

because i and *j are both of type int. The exact syntax for a declaration is given in §6.7