tree e8ea154061b15e3cdbd0ec278c8bfa67d8dc2de7
parent 1c86a92bc7518adb9956c643122f672d84fd5dc1
author Kelvin Zhang <zhangkelvin@google.com> 1620918884 -0400
committer Kelvin Zhang <zhangkelvin@google.com> 1621972096 -0400

Define constants in header

There should be no behavior changes. There shouldn't even be any change
necessary to users of these constants.

Old way to define constants:
    put 'extern const char[] key' in .h file
    put 'const char[] key = "some string"' in .cpp file
    However, this has 2 drawbacks:
        1. Compiler doesn't know size of string 'key', therefore
        compiler must generate a call to strlen()  to
        construct a std::string_view or std::string from it.
        2. We must repeat the same key name twice. Once in .h file, once
        in .cpp file. Any addition/removal of string constants will
        involve 2 edits as well.

New way to define constants:
    put 'static constexpr const auto& key = "some string value";' in .h file
    This solves both drawbacks mentioned above. Note that, even though
    we are using 'static' keyword here, compiler will only emit 1 copy
    of the string in the final program. See
    https://replit.com/@zhangxp1998/string-constant-demo for demo

Test: th
Change-Id: I5bf287687062b99cda8c2c13faf45ceee5536d02