C++ Programming - Constructors and Destructors - Discussion

Compiler always provide parameter-less constructor, so option B is correct. But we can declare constructor as private so that we can create object of class outside member function for ex:constructor and copy constructor in singleton class is private.

How can constructor can't be private? If we consider design patterns, for singleton pattern implementation, constructor should be private.

Option C is correct.

1) Compiler doesn't ALWAYS provide a zero-argument constructor. It provides a zero-arg constructor only when the user hasn't defined any constructor explicitly.

2) Constructor need not be public always. If it private/protected we can create another public member function which can call the private/protected constructor.

The keyword "ALWAYS" changes the answer here.

When we want to execute the particular part of the program we can declare constructor as private or protected.

It can be any of them.

But compiler can automatically provide the default constructor.

A class without any public constructor is not totally pointless, if it is a base class, and it has a protected constructor. Its' child class may be instantiated.

A class without any public or protected constructor is totally pointless. Neither this class nor its' child class is instatiatable, because the constructor of the child class must call the constructor of the base class, which is inaccessible from the child class if that constructor is private.

A is true, from the standard (paraphrased). The default constructor is a special member function. Special member functions are still member functions.

B is false. The compiler only provides a zero member default constructor if you don't provide a constructor yourself.

Example:

class B {
private:
int i;
public:
B(int x) { i = x; }
};

int main() {
B b; // COMPILER ERROR HERE...
return 0;
}

c) is also false. The following is valid c++
class A {
A() {}
};


@Daniel Sandor. I think you're wrong when you say that: "A class without any public or protected constructor is totally pointless".

You can do this and create instances using public factory methods.

Here option (B) seems to be ambiguous.

The compiler will provide zero argument constructor by default only when we haven't declared any parametrized constructor.

But.

If has declared any parametrized constructor. In our C++ program, then in that case compiler won't provide any zero argument constructor by default. There will be a compilation error.

It seems to that the answer B is incorrect. Let us assume that we define a constructor with parameters. In such situation a compiler would not generates the default constructor!