c++ - warning C4018: '<' : signed/unsigned mismatch

Question

On migrating a project from vc++ 6.0 to vs 2008, I get the below warnings. Can anyone please explain me why this warning occurs?

inline tstring& tstring::trim()
{
    long lDelFront = 0, lDelBack = 0;

    while (lDelFront < length() && at(lDelFront) == ' ')      //C4018 Warning
        lDelFront++;

    if ( lDelFront > 0 )
    {
        erase(0, lDelFront);
    }

    while (lDelBack < length() &&                       //C4018 Warning
            at(length() - lDelBack - 1) == ' ')
        lDelBack++;

    if ( lDelBack > 0 )
    {
        erase(length() - lDelBack, lDelBack);
    }

    return *this;
}

Thanks! Ankush

Answer 1

length() probably returns size_t or unsigned long and you are comparing it with signed long. Change

long lDelFront = 0, lDelBack = 0;

to

size_t lDelFront = 0;
size_t lDelBack = 0;

to avoid signed/unsigned comparison