Why is this for loop not correct? - c++

Question

Visual Studio is telling me that this for loop isn't correct.
Error Messages are:
type bool unexpected
ok is undeclared identifier
missing ; before }
infos:
-recordset.Select return a long -MoveNext a bool
for (size_t i = 0, bool ok = recordset.Select(Adress::getSQLStatement() + "Where A05.recid = %ld", i); ok; ok = recordset.MoveNext(), i++) {
at(i).Save(recordset);
}

Answer 1

It's as StenSoft said. But you can define an anonymous structure in the loops first statement, and initialize that.
#include <iostream>
using namespace std;
int main() {
for (struct {size_t i; bool ok;} s = {0, true}; s.ok; ++s.i) {
s.ok = s.i < 10;
cout << s.i;
}
return 0;
}
But IMHO, while it works, it's more trouble than it's worth. Better restructure you code.

Answer 2

First off, you can of course rewrite your loop like so:
{
bool ok = recordset.Select(...);
for (std::size_t i = 0; ok; ok = recordset.MoveNext(), ++i)
{
/* ... */
}
}
But the meta lesson here is that almost all loops are for-loops, and when you think your structure is different, think again. There's probably a rewrite in terms of for-loops that makes your logic clearer. Your present code doesn't distinguish an initial error from a "no more records" error later. With the new code, that's now explicitly possible:
if (bool ok = recordset.select(...))
{
for (std::size_t i = 0; ok; ok = recordset.MoveNext(), ++i) { /* ... */ }
}
else
{
// handle initial error
}
I would probably even get rid of the redundant ok variable:
if (recordset.select(...))
{
for (std::size_t i = 0; ; ++i)
{
/* ... */
if (!recordset.MoveNext()) break;
}
}
else
{
// handle initial error
}