Question

To compile my C++ code I use the -W flag, which causes the warning:

warning: comparison of unsigned expression < 0 is always false

I believe this was considered as a bug and was fixed on version GCC 4.3, but I’m using GCC 4.1

Code that is obviously offending here:

void FieldGroup::generateCreateMessage (const ApiEvent::GroupData &data, omsgstream &result) const {
  dblog << debug;

  // Write out the data fields we care about, in the order they were specified
  for (size_t index = 0; index < fields.size(); ++index) {
    size_t esIndex = clsToES[index];
    if (esIndex < 0 || esIndex >= data.fields.length()) {
      ostringstream buf;
      buf << "Invalid field " << index << " (index in ES data set " << esIndex << ", " << data.fields.length() << " fields returned)";
      throw InvalidDataException (buf.str());
    }
    fields[index].writeData (data.fields[esIndex], result);
  }
}

Warning I’m getting:

dbtempl.cpp: In member function ‘void ECONZ::FieldGroup::generateCreateMessage(const nz::co::econz::eventServer::ApiEvent::GroupData&, ECONZ::omsgstream&) const’:
dbtempl.cpp:480: warning: comparison of unsigned expression < 0 is always false

How can i possibly stop these warnings from appearing? I don’t want to remove the -W flag.

Answer 1

You are testing if a positive value is below 0.

A size_t is unsigned, so at least 0.

This can never happen and the compiler optimize things out by just removing the test. The warning is here to tell you because if someone does that, it might be a mistake.

In your case, you might just remove the test, it should be fine.