Proper implementation of global configuration - c++

Question

My goal is to have global constants in a C++ game I'm working on (to represent some graphics info and the like). My current implementation is to toss them all in a .h and include them everywhere. This works, except that every time I change a setting, the entire code base must be recompiled.
So, my next idea was to toss them in some configuration txt file and parse them in, that way no code is actually changed when settings change. The parser was simple enough, and I could put the values into the constants, but because the parser was a code block, the constants were no longer global.
Is there a good way to solve this? Perhaps some way to make them global despite being in a block or some way to avoid recompiling everything when changing settings?

Answer 1

The way I used solve this is to put the variables in a separate global namespace, which is in a header file named something like config.h, then include that file everywhere.
// In config.h
#ifndef CONFIG_H
#define CONFIG_H
namespace config
{
extern int some_config_int;
extern std::string some_config_string;
bool load_config_file();
}
#endif
Then in a source file, you define the variable and also set them to a default value. This source file also have the code to load the variables from your configuration file.
// In config.cpp
namespace config
{
int some_config_int = 123;
std::string some_config_string = "foo";
}
bool config::load_config_file()
{
// Code to load and set the configuration variables
}
Now in every source file you need the configuration variables, include config.h and access them like config::some_config_int.
However, there is no "proper" way of solving this, all ways that work are proper in my eyes.

Answer 2

Another way to do this would be to create a singleton class.
#include <fstream>
#include <map>
#include <string>
class ConfigStore
{
public:
static ConfigStore& get()
{
static ConfigStore instance;
return instance;
}
void parseFile(std::ifstream& inStream);
template<typename _T>
_T getValue(std::string key);
private:
ConfigStore(){};
ConfigStore(const ConfigStore&);
ConfigStore& operator=(const ConfigStore&);
std::map<std::string,std::string> storedConfig;
};
Here the configuration is saved in a map, meaning as long as parseFile can read the file and getValue can parse the type there is no need to recompile the config class if you add new keys.
Usage:
std::ifstream input("somefile.txt");
ConfigStore::get().parseFile(input);
std::cout<<ConfigStore::get().getValue<std::string>(std::string("thing"))<<std::endl;

Answer 3

What about creating functions that return your constants that you can specify in a .cxx file? For example:
// foo.h
const int BAR();
// foo.cxx
const int BAR() {
return 10;
};

Answer 4

put only the declarations in head file and put the definitions in a cpp file. then you change the definitions in cpp file will not cause all code recompiled