std::remove_if – lambda, not removing anything from the collection

You need to call erase on the iterator returned from remove_if, It should look something like this:

auto new_end = std::remove_if(MyDisplayDevices.begin(), MyDisplayDevices.end(),
                              [](const DisplayDevice3d& device)
                              { return device.Modes.size() == 0; });

MyDisplayDevices.erase(new_end, MyDisplayDevices.end());