fork download

copy

#include <iostream>
#include <iostream>
#include <iomanip>
#include <ctime>
#include <atomic>
#include <mutex>
 
 // Eager-Initialization, Thread Safe - double check locking with atomic type
 
class current_date_time {
 
private:
 //private constuctor and lock variable
    current_date_time() {};
    static std::mutex m_;
 
public:
    //creates global instance on call - saves memory if instance is memory intense
    static std::atomic<current_date_time*>& create_instance();
    //responsible for assignment of single class object with lock variable
    static current_date_time*& initialize_instance(current_date_time*& global_time_variable);
 
    void time_output(){
        std::time_t t = std::time(nullptr);
        std::tm tm = *std::localtime(&t);
        std::cout << std::put_time(&tm, "%c %Z") << "\n";
    }
 
};
 
 
std::mutex current_date_time::m_;
 
std::atomic<current_date_time*>& current_date_time::create_instance(){
    static std::atomic<current_date_time*> global_time_variable = nullptr; //global variable creation
    return global_time_variable;
}
 
 
 
current_date_time*& current_date_time::initialize_instance(current_date_time*& global_time_variable){
 
    if(global_time_variable == nullptr){  //check to see if the variable is empty
        std::lock_guard<std::mutex> lock(m_);  //acquire lock
        if(global_time_variable == nullptr){  //second check if multiple threads acquire lock
            global_time_variable = new current_date_time(); //only one thread executes this line atomically
        }
    }
 
    return global_time_variable;
}
 
 
 
int main() {
 
    current_date_time* eager_initialized_variable = current_date_time::create_instance();
    current_date_time* after_thread_safe_variable = current_date_time::initialize_instance(eager_initialized_variable);
    after_thread_safe_variable->time_output();
 
    return 0;
}

prog.cpp: In static member function ‘static std::atomic<current_date_time*>& current_date_time::create_instance()’:
prog.cpp:35:67: error: use of deleted function ‘std::atomic<_Tp*>::atomic(const std::atomic<_Tp*>&) [with _Tp = current_date_time]’
     static std::atomic<current_date_time*> global_time_variable = nullptr; //global variable creation
                                                                   ^~~~~~~
In file included from prog.cpp:5:0:
/usr/include/c++/6/atomic:328:7: note: declared here
       atomic(const atomic&) = delete;
       ^~~~~~
/usr/include/c++/6/atomic:332:17: note:   after user-defined conversion: constexpr std::atomic<_Tp*>::atomic(std::atomic<_Tp*>::__pointer_type) [with _Tp = current_date_time; std::atomic<_Tp*>::__pointer_type = current_date_time*]
       constexpr atomic(__pointer_type __p) noexcept : _M_b(__p) { }
                 ^~~~~~

Standard output is empty