How a progressbar should be correctly updated within a nested loop in C++?

Question

I use MS Visual Studio 2010, C++,PPL library for parallel calculations and Qt library.

Concurrency::parallel_for (size_t(0), m_Engines.size(), [&](size_t i)
{
    for (size_t j = 1;j <= m_Iterations;j++)
    {
        Compute(i);//some time-cosuming thing           
    }
});

Let's imagine we've got 3 independent engines with m_Iterations for each one. The progress bar for each engine will look like:

progress 1: ||||||||

progress 2: |||||

progress 3: |||||||||||

And I wanna have the single progressbar (instead of the shown above) which shows the backward progressbar (which shows minimal value among them):

total progress: |||||

Let us assume that we have QProgressBar * pProgressbar, CoreApplication::processEvents() "code snippets". How should I modify the code in order to implement the idea of a single progress bar for the inner loop using Concurrency Runtime techniques.

Answer 1

In order to show the minimal time, you can use QWaitCondition in side the function or you can write separate function to notify one m_Iterations finish. Lets assume , currently you are executing 3 parallel calculation engine , write QWaitCondition to signal other iterations do their jobs.