QRegularExpressionMatch memory consumption

Question

here is part of my code:

    int Parser::SomeFunction(const QString &line, int start, int& fieldStart, int& fieldLength ) const
{
    int end;
......
    if (0 == m_pRegExp)
    {
......
    }
    else
    {
#ifdef KNOWN_PATTERN
        end = 19;
#else

        QRegularExpressionMatch match = m_pRegExp->match(line,  start);
        if (!match.hasMatch())
        {
            return 0;
        }

        // currently we are requiring match to be found exactly at the position 'start'
        if (match.capturedStart() != start)
        {
            return 0;
        }

        end = match.capturedEnd();
#endif
    }
.....
}

The program loads a text file and parses its lines one by one. The whole purpose of code in 'else' scope is to calculate where a field ends (integer 'end') in the next line passed to the function. When I compile with KNOWN_PATTERN #defined and load some test file, for which I know 'end' should become 19, my program consumes about 400 MB less memory than when compiled without KNOWN_PATTERN #defined. 400 MB is what all lines of my test file occupy in the memory (I can can calculate it based of file size and also I watched memory consumption when file was loaded and before parsing started). So it seems to me that QRegularExpressionMatch creates a copies of each line and does not release it. What am I missing here? Thanks!

Answer 1

Task Manager and similar are very blunt tools for observing memory allocation by the C++ runtime.

In particular, the runtime doesn't necessarily release deallocated memory back to the OS, as OS memory allocation is relatively slow on many platforms, but holds on to it for itself.