[C++] ODR notes

ODR:
https://eel.is/c++draft/basic.def.odr

All definition across the program should be the same.

It's hard.

e.g.

// size of Person might be different inside the program.
class Person{
    std::string first_;
    std::string last_;
# if HAS_MIDDLE_NAME(VAR)
    std::string middle_;
#endif
};

ODRV can be embedded anywhere(DSO, static library, or executable)

1. Multiple, conflicting definitions of the same symbol in more than one TU.
2. Compiling a given header/source file with different compiler settings or #defines
   Debug/Release, (no-)RTTI, mismatched preprocessor values, etc.
3. Overriding operator new/delete in a DSO, but hiding it from the rest of the program.
   The passing C++ objects across that DSO boundary.
4. Multple varying copies of a dependency(e.g. Boost, JPEG, zlib, etc.)

ODRV Behaviors

1. Hard to debug
2. Hard to reproduce
3. Exceptions failing to get caught
4. Crashing in the destructor after passing an object across a DSO boundary.