singleton.cpp

// Brandon Robare 
// Lab 08
// Singleton Design Pattern Example
// Meyers Singleton 

#include <iostream>
#include <fstream>
#include <string>


// singleton ensures only one instance of class exists
// provides a global access point to that instance.
class Logger {
public:
    // uses local static variable for lazy instantiation - Meyers Singleton
    static Logger& getInstance() {
        static Logger instance; 
        return instance;
    }

    // report() appends a message to the log file
    // payload of singleton
    void report(const std::string &msg) {
        if (ofs.is_open()) {
            ofs << msg << std::endl;
        }
    }


    // singleton cannot be duplicated.
    Logger(const Logger&) = delete;
    Logger& operator=(const Logger&) = delete;

private:
    std::ofstream ofs; 

    // only once when getInstance() is first invoked
    Logger() {
        ofs.open("log.txt", std::fstream::out | std::fstream::app);
        if (!ofs) {
            std::cerr << "Error: Failed to open log.txt" << std::endl;
        }
    }

    // file is closed when program terminates
    ~Logger() {
        if (ofs.is_open()) {
            ofs.close();
        }
    }
};


// both functions prints message using the same singleton
void functionA() {
    Logger::getInstance().report("function A");
}

void functionB() {
    Logger::getInstance().report("function B");
}

int main() {
    functionA();
    functionB();
}