Inline Variables (C++17)

// Old way: header file
// config.h
extern const int max_size;  // Declaration in header

// config.cpp
const int max_size = 100;   // Definition in one cpp file

// C++17: inline variables - define in header, safe to include everywhere
// config.h
inline constexpr int max_size = 100;
inline const std::string app_name = "MyApp";

// Same instance across all translation units!

// Header-only libraries
// math_constants.h
inline constexpr double pi = 3.14159265358979323846;
inline constexpr double e = 2.71828182845904523536;

// Singleton pattern - simpler with inline
// logger.h
class Logger {
public:
    static Logger& instance();
    void log(const std::string& msg);
};

// Inline variable for the instance
inline Logger& logger = Logger::instance();

// Usage
logger.log("Hello");  // Same logger everywhere