Object array initialization without default constructor

Question

#include <iostream> class Car { private:   Car(){};   int _no; public:   Car(int no)   {     _no=no;   }   void printNo()   {     std::cout<<_no<<std::endl;   } }; void printCarNumbers(Car *cars, int length) {     for(int i = 0; i<length;i++)          std::cout<<cars[i].printNo(); }  int main() {   int userInput = 10;   Car *mycars = new Car[userInput];   for(int i =0;i < userInput;i++)          mycars[i]=new Car[i+1];   printCarNumbers(mycars,userInput);   return 0; }     

I want to create a car array but I get the following error:

cartest.cpp: In function ‘int main()’: cartest.cpp:5: error: ‘Car::Car()’ is private cartest.cpp:21: error: within this context 

is there a way to make this initialization without making Car() constructor public?

Answer 1

You can use placement-new like this:

class Car {     int _no; public:     Car(int no) : _no(no)     {     } };  int main() {     void *raw_memory = operator new[](NUM_CARS * sizeof(Car));     Car *ptr = static_cast<Car *>(raw_memory);     for (int i = 0; i < NUM_CARS; ++i) {         new(&ptr[i]) Car(i);     }      // destruct in inverse order         for (int i = NUM_CARS - 1; i >= 0; --i) {         ptr[i].~Car();     }     operator delete[](raw_memory);      return 0; } 

Reference from More Effective C++ - Scott Meyers:
Item 4 - Avoid gratuitous default constructors