Just wondering why this and the above variables are not initialized by default.

Initially it was just num_in / num_out I've left empty for some reason (as those are always initialized in ctor), but now it doesn't looks so good.

Didn't get your point, now it is initialized in ctor. What's the problem ?

The problem is when a new ctor is added (who knows), these fields may left uninitialized

Partial initialization isn't supported in C++11

class cv::gimpl::ie::RequestPool {
struct RequestPool;

Warning on "Custom Win" builder:

C:\build\precommit_custom_windows\opencv\modules\gapi\src\backends\ie\giebackend.cpp(497): warning C4099: 'cv::gimpl::ie::RequestPool': type name first seen using 'struct' now seen using 'class' [C:\build\precommit_custom_windows\build\modules\gapi\opencv_gapi.vcxproj]

"Custom Mac":

/build/precommit_custom_mac/opencv/modules/gapi/src/backends/ie/giebackend.cpp:497:1: warning: 'RequestPool' defined as a class here but previously declared as a struct; this is valid, but may result in linker errors under the Microsoft C++ ABI [-Wmismatched-tags]

Fixed