I have a variable which I set early in the program:

double foo = 0.0 ;

The program continues running and later, without ever having touched foo, it becomes some garbage value (say 6.953355807230268e-310).
To track this down I have printed the address of foo, say 0x7fffffffcfa0, and then printed the value at that address with gdb at various points until it has changed:

p *(double *) 0x7fffffffcfa0

Digging into each of the function calls, it seems to happen at the following line:

++bar[name];

Here bar is a mutable std::map<std::string, unsigned int> member variable of some class, and name is a string.
In gdb if I call p bar it returns = std::map with 0 elements.
Hence, calling bar[name] inserts an element into the map.
If I call p bar[name] in gdb it gives 0 (expected) and the value of foo is unchanged.
If I then call p ++bar[name], the value of foo gets changed.

If I step into the line ++bar[name], I can track the change of foo down to /usr/include/c++/11/bits/stl_map.h:497 which reads:

iterator __i = lower_bound(__k);

If I try to step into this function, it changes the value of foo without any steps between.

At this point I am stumped as to what is going on and, unfortunately, I don’t know how to reproduce the problem with a simpler program. Any help is appreciated to try to diagnose and/or treat this issue. In case it is relevant, I’m using g++ 11.4.0 to compile.