This is a minimum example of how global variables defined in header files result in separate instances in each translation unit that includes the header.
C++17's inline variables makes this less of a problem, and for built-in types
I'm not sure it ever mattered, but for std::string in, say, C++11, there is
a real, if irrelevant, distinction.
This is supporting material for a proposed change to some code.
This is a demo of the difference between putting const std::string
definitions in a header file, versus merely declaring them in a header
file and putting their definitions in one dedicated translation unit (i.e.
.cpp file).
To see const std::string fish with two separate addresses,
$ make violating
$ ./violating
unit_1 &fish is 0x55e573aae160
unit_2 &fish is 0x55e573aae1a0To see const std::string fish with a unique address,
$ make following
$ ./following
unit_1 &fish is 0x55879221b160
unit_2 &fish is 0x55879221b160