You need to specify a value for const member variable, It is actually not very common (afaik) to use const member variable in c++, you can either use a #define preprocessor, a static const member variable, or a const variable in a namespace :
In the game, however, I usually put these ‘const’ config values that are used by many object instances, in the Game Singleton. Or, if I want the user to change it via ini file (quite common in mod-able games) then I will put them in a special config class :