Should the deleted default constructor be in Public or Private?

Question

I had to delete the default constructor to ensure that the parameterized constuctor is used always. That's when I wanted to know if my deleted default constructor should be under public or private access specifier.

I just wrote a sample code to test this. I tried to access the deleted default constructor.

class A
{
public:
    A(const int val)
    {
        assign = val;
    }
private:
    A() = delete;
    int assign;
};

int main()
{
    A obj1(5);
    A obj2;
}

main.cpp: In function ‘int main()’:

main.cpp:35:7: error: ‘A::A()’ is private within this context

A obj2;

main.cpp:28:5: note: declared private here

A() = delete;

main.cpp:35:7: error: use of deleted function ‘A::A()’

A obj2;

main.cpp:28:5: note: declared here

A() = delete;

"I had to delete the default constructor to ensure that the parameterized constuctor is used always." No. The default constructor is not declared if you have other constructors. — L. F., Jul 29 '19 at 10:52

Guillaume D · Accepted Answer · 2019-07-29T15:54:41.283

You don't need to use the deleted ctor.

class A
{
public:
    A(const int &val) : assign(val){}
private:
    int assign;
};
int main()
{
    A a;
    return 0;
}

gives error: no matching function for call to ‘A::A()’

and

class A
{
public:
    A() = delete;
    A(const int &val) : assign(val){}
private:
    int assign;
};
int main()
{
    A a;
    return 0;
}

gives error: use of deleted function ‘A::A()’

Concerning the use of delete, it could be useful with inheritance.

class A
{
public:
    A() : assign(0){}
    A(int val) : assign(val){}
private:
    int assign;
};

class B : public A
{
public:
    B() = delete;
    B(int val) : A(val){};
};

class C : public A
{

};

class D : public A
{
public:
    D() = delete;
};


int main()
{
    A a1; //works
    A a2(5); //works

    //B b1; -- does not work : error: use of deleted function ‘B::B()’
    B b2(3); //works

    C c1; //works, creates a default ctor
    //C c2(7); -- does not work : no matching function for call to ‘C::C(int)’

    //D d1; -- does not work :error: use of deleted function ‘D::D()’
    //D d2(2); -- does not work: error: no matching function for call to ‘D::D(int)’
    return 0;
}

Note that D can't be instantiated. Quite useless though :)

If that is the case, when would the deleted default constructor be useful? — Boanerges, Jul 29 '19 at 11:51
https://stackoverflow.com/questions/13654927/why-explicitly-delete-the-constructor — Guillaume D, Jul 29 '19 at 11:52
https://stackoverflow.com/questions/45866127/deleted-constructor-vs-no-constructor — Guillaume D, Jul 29 '19 at 11:54

score 0 · Answer 2 · answered Jul 29 '19 at 10:59

0

It's better to define default constructor with public access:

class A
{
public:
    A() = delete;
    A(const int &val) : assign(val){}
private:
    int assign;
};

Now you get a better error with this:

int main (void){
    A obj;
}

answered Jul 29 '19 at 10:59

Ghasem Ramezani

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Should the deleted default constructor be in Public or Private?

2 Answers2