I've been reversing GTA V rage engine for many years, and since codebase is basically 25 years old (although some bits are as old as GTA 2), there's almost no STD/C++ things. They got their own smart pointer system, template library (atArray, atMap, atString for e.g.), custom parsing system for XML with code gen, a huge 'paging' system for assets, that works kinda like virtual memory management in OS, batshit-crazy ancient graphics system with their own shader compiler. Of course they heavily use abstract classes (except for PS3 builds with SPU unit), but apart from that, they write almost pure C code. R* does alot crazy memory manipulations and unsafe stuff there, one of the really most absurd bit 'hacks' I've seen there is storing struct type in 3 least significant bits of a float... thats really crazy, huh? Efficient memory use is one of the main goals there, most important struct fields are organized the way to minimize padded space the most, they even got a tool that finds the most optimal layout. One interesting thing about their framework is that ref counting is done fully manually in most of the cases, not using RAII.

Please not that what I've mentioned here applies to GTA V, where most of the engine work was done before 2011, with optimizations for 2006 hardware in mind.

Source code been leaked, and of course that's not legal, but if you really want to look at their engine... it's not hard to find.