private constructor

[rajeev kumar singh]

what is the use of private constructor?

[rgb]

private constructors might be similar to private functions where you can make a call to the defined private constructor on one (or more) of your public constructors passing different parameters based on the initialization you needed.

[Banfa]

I have seen private constructors used to prevent external code calling the default constructor when a class really needs to be initialised as it is constructed. The default constructor is defined to as private to prevent it being called externally.

If a class had a static function to allocate instances of itself for some reason then you might want to declare the constructor(s) private to prevent construction by external code.

[Ravi Rupela]

We can use private constructors in two cases

1) when you create Base class say Sports and some derived class of this lets say
Cricket, Football ... Now If you want that no one can create object of Sports Base class .. you only store member varibales into it not member functions.. then declare Sports class constructor in private space.. so derived class objects
Cricket ck ;
Football bl;
Can access data available in base class Sports (public data) .. but no one can create object of Sports class directly.

2) When you want to implement SingleTone class Design patterns in that case you are required to declare constructor in private space.

[Vaddadi]

I think private constructor in C++ can't be created.

#include <iostream>
#include <iomanip>
#include <string>
using namespace std;
enum CarSize{
small = 0, medium = 1, large = 2
};
 
class RentalAccount{
string accountNumber;
string clientName;
int age;
float balance;
// -- Default Constructor
RentalAccount(){}
public:
// -- Constructor
RentalAccount( const string newAccountNumber,
const string newClientName,
const int newAge,
const float newBalance = 0);
 
// -- Methods
 
// -- Set, and get Balance
void SetBalance(const float newBalance);
float GetBalance() const;
 
// -- Calculate Balance
void CalculateBalance(CarSize carSize);
 
// -- Print Client information
void Print() const;
};
 
int main()
{
    RentalAccount ra;
    ra.SetBalance(10.99);
    ra.GetBalance();
    return 0;
}

I think contradicting my statement, there's a way to stop our class being inherited similar to this code, which would be allowed by C++ . Can anyone tell me how?

[vramacha]

Well another use of decalring constructor private is to use the feature of composition. Say there is a class A for which an instantion need not be done make the constructor private and then make the class a member of another class B (composition feature being used here) then at that time private constructors are useful.