Tips when defining global constant in header file

Options of defining global constant in header file

1. #define GLOBAL_CONST_VAR 0xFF
2. const int GLOBAL_CONST_VAR = 0xFF;
3. extern const int GLOBAL_CONST_VAR; and in one source file uses cosnt int GLOBAL_CONST_VAR = 0xFF;

According to Effective C++, ITEM 2:

Prefer const, enum, and inline to #define.

So option 1 is not preferred (which I particularly like to use). In my project, I have a header file that uses three constant string.

// util.hpp
const char * CLIENT_CONNECT_CONFIRM = "CLIENT_CONFIRM";
const char * SERVER_CONNECT_CONFIRM = "SERVER_CONFIRM";
const char * PLAYER_CONNECT_CONFIRM = "PLAYER_CONFIRM";

However, when I run the makefile, it has the error goes like:

/usr/bin/ld: player.o:/home/fg96/ECE650/HW3/util.hpp:14: multiple definition of `CLIENT_CONNECT_CONFIRM'; util.o:/home/fg96/ECE650/HW3/util.hpp:14: first defined here

Which clearly indicates that the CLIENT_CONNECT_CONFIRM are regarded not as the constant, but variable, but why? Reason is that const char * ... is not constant, because const char * means that the value pointer points to cannot change, but the value of the pointer can change (very silly mistake).

The following definition will be just fine:

// all const pointers pointing to const values
const char * const CLIENT_CONNECT_CONFIRM = "CLIENT_CONFIRM";
const char * const SERVER_CONNECT_CONFIRM = "SERVER_CONFIRM";
const char * const PLAYER_CONNECT_CONFIRM = "PLAYER_CONFIRM";

// const pointers is also fine
char * const CLIENT_CONNECT_CONFIRM = "CLIENT_CONFIRM";
char * const SERVER_CONNECT_CONFIRM = "SERVER_CONFIRM";
char * const PLAYER_CONNECT_CONFIRM = "PLAYER_CONFIRM";

Since C++17, inline specifier can also be used on variable. According to the cppreference:

An inline function or variable(since C++17) with external linkage (e.g. not declared static) has the following additional properties: There may be more than one definition of an inline function or variable(since C++17) in the program as long as each definition appears in a different translation unit and (for non-static inline functions and variables(since C++17)) all definitions are identical. For example, an inline function or an inline variable(since C++17) may be defined in a header file that is included in multiple source files.

But we are not allowed to use c++17 so ...

Another possibility would be to use namespace

namespace {
    char * CLIENT_CONNECT_CONFIRM = "CLIENT_CINFIRM";
}

Reference

1. Defining global constant in C++
2. 头文件中定义const全局变量应注意的问题