I need a "holder" class. It is supposed to store objects references.

Like: holder.A = a; // Gets a reference!

Sample code bellow including the compiler error:

class A
{
};

class Holder
{
public:
    A& MyA; // I want to store a reference.
};

int main()
{
    A testA;
    Holder holder; // Compiler error: the default constructor of "Holder" cannot be referenced -- it is a deleted function
    holder.A = testA; // I was hopping to get testA reference.
}

CodePudding user response：

The problem is that since your class Holder has a reference data member MyA, its default constructor Holder::Holder() will be implicitly deleted.

This can be seen from Deleted implicitly-declared default constructor that says:

The implicitly-declared or defaulted (since C 11) default constructor for class T is undefined (until C 11)defined as deleted (since C 11) if any of the following is true:

• T has a member of reference type without a default initializer (since C 11).

(emphasis mine)

To solve this you can provide a constructor to initialize the reference data member MyA as shown below:

class A
{
};

class Holder
{
public:
    A& MyA; 
    //provide constructor that initialize the MyA in the constructor initializer list
    Holder( A& a): MyA(a)//added this ctor
    {
        
    }
};

int main()
{
    A testA;
    Holder holder(testA); //pass testA
    
}

Working Demo

CodePudding user response：

References (MyA) should be bound to something and can't be empty, since you don't tell what it should bind to, the default constructor is ill formed.

prog.cc: In function 'int main()': prog.cc:23:12: error: use of deleted function 'Holder::Holder()' 23 | Holder holder; // Compiler error: the default constructor of "Holder" cannot be referenced -- it is a deleted function | ^~~~~~ prog.cc:14:7: note: 'Holder::Holder()' is implicitly deleted because the default definition would be ill-formed: 14 | class Holder

You need to add a constructor specifying how to initialized MyA

Holder(A &obj):MyA(obj){}

and then create holder objects

A testA;
Holder holder(testA);