C++, multiple definition

Question

I need to define a constant containing an environment variable (Linux, g++). I would prefer to use string, but std::getenv needs *char

Answer 1

You are including the header twice. Once from DataLocation.cpp (where it finds HEADER_DATALOCATION_H not yet defined and thus defines ENV_APPL_ROOT), and once from Test.cpp (where it agains finds HEADER_DATALOCATION_H not yet defined and thus defines ENV_APPL_ROOT again.) The "header protection" only protects a header file being included multiple times in the same compilation unit.

You need:

extern const char* ENV_APPL_ROOT;

in the header file, and

const char* ENV_APPL_ROOT = "APPL_ROOT";

in one .cpp file.