Using _crtBreakAlloc to find memory leaks - identifier "_crtBreakAlloc" is unidentified

_crtBreakAlloc will be reported as unidentified if the ucrtbased.dll symbols are not loaded. I had this problem because I do not automatically load my symbols. You can go in your module list and manually Load symbols for ucrtbased.dll and then _crtBreakAlloc should show up and work.

_crtBreakAlloc is a macro under VS2015 that is replaced by a call to a function returning a pointer to an int. Tracking a variable in the watch window doesn't seem an option.
Better insert in your (debug) code something like this:

_crtBreakAlloc = 18;

{,,ucrtbased.dll}*__p__crtBreakAlloc()

works for Visual Studio 2017