Doing two things at once

I like this bug that I found in a colon-separated path splitting function in some of our C++ code:

while (size_t pos = pathstr.find(":") != string::npos) {
    // split string at position pos, i.e. the next colon...

I couldn't see the bug, so I re-wrote it without the cheeky 2-in-1 assign-and-compare:

while (true) {
    size_t pos = pathstr.find(":");
    if (pos == string::npos) break;
    // split string at position pos...

It worked immediately. What happens with the code above is that the path string always gets chopped one character at a time, rather than at the colons. This is because the != operator takes precedence over the assignment (=) operator: the first level of evaluation effectively gives while (size_t pos = true) ... as long as there's a colon somewhere in the string. Automatic type conversion then converts the boolean true to an integer 1 and the string gets chomped 1 character at a time until no more colons are found.

Take the hint: don't try to do two things at once, even if you can.


Comments powered by Disqus