c++ illogical >= comparison when dealing with vector.size() most likely due to size_type being unsigned

Question

I could use a little help clarifying this strange comparison when dealing with vector.size() aka size_type

vector<cv::Mat> rebuiltFaces;
int rebuildIndex = 1;
cout << "rebuiltFaces size is " << rebuiltFaces.size() << endl;

while( rebuildIndex >= rebuiltFaces.size() ) {
    cout << (rebuildIndex >= rebuiltFaces.size()) << " , " << rebuildIndex << " >= " << rebuiltFaces.size() << endl;
    --rebuildIndex;
}

And what I get out of the console is

rebuiltFaces size is 0
1 , 1 >= 0
1 , 0 >= 0
1 , -1 >= 0
1 , -2 >= 0
1 , -3 >= 0

If I had to guess I would say the compiler is blindly casting rebuildIndex to unsigned and the +- but is causing things to behave oddly, but I'm really not sure. Does anyone know?

Answer 1

As others have pointed out, this is due to the somewhat counter-intuitive rules C++ applies when comparing values with different signedness; the standard requires the compiler to convert both values to unsigned. For this reason, it's generally considered best practice to avoid unsigned unless you're doing bit manipulations (where the actual numeric value is irrelevant). Regretfully, the standard containers don't follow this best practice.

If you somehow know that the size of the vector can never overflow int, then you can just cast the results of std::vector<>::size() to int and be done with it. This is not without danger, however; as Mark Twain said: "It's not what you don't know that kills you, it's what you know for sure that ain't true." If there are no validations when inserting into the vector, then a safer test would be:

while ( rebuildFaces.size() <= INT_MAX
        && rebuildIndex >= (int)rebuildFaces.size() )

Or if you really don't expect the case, and are prepared to abort if it occurs, design (or find) a checked_cast function, and use it.