错误：“新”不能出现在常量表达式中

Question

class A
{
    int data;

public:
        void display()
        {
            cout<<"Value is "<<data;
        }
        void set_data(int x)
        {
            this->data = x;
        }
        A object = new A();
};

When I run the above code, I get the error stating "new cannot appear in constant expression". Why is it so?

Answer 1

Operator new returns a pointer but A is not a pointer type. You want A* :

A* object = new A();

You also want to move the above statement outside your class body and place it into appropriate function such as main() :

int main() {
    A* p = new A();
    // do work
    delete p;
}

That being said you either don't need a pointer at all and you can simply use an object with automatic storage duration:

A object;

Or you want to consider using a smart pointer such as std::unique_ptr :

std::unique_ptr<A> p = std::make_unique<A>();

Answer 2

class A
{
public:

    A * object = new A(); // In any case not: "A object = new A();"
};   

Or:

class A
{
public:

    A object;
};

-

See (let's assume, for a moment, that you don't get the error), in both cases, on the first construction of A object, it creates another A object as a data-member. This A data-member (let's call it object.object ) creates in its turn another A as its data-member (let's call it object.object.object), and so to infinity (or until no more memory). I mean, as a data-member, it can't be either as A* object = new A(); , or as A object;

-

I am not sure what was your intention, but if you want to link one A-object to another A-object, the class should be something like that:

class A
{
public:

    A * object = nullptr
};  

Answer 3

you have to make object of class A into main() .

void main(){
     A object;
}

Answer 4

First of all, you cannot create an object in the class declaration. Class declaration is like a blue print of the class. It is to say these are the components of my class - variables and member functions. You cannot instantiate anything inside it as no memory is allocated during this stage.

Note that you can instantiate an object inside one of the member function including constructor. These are called during object creation when memory is allocated.

Even if you use this statement inside a constructor you will go into an infinite loop as the constructor calls its constructor and so on until you have memory overflow.

You can declare the object in main like this:

int main() {
    A obj = new A();
    //other operations
} //Object A is destroyed once you come out of main.

Or dynamically like this

int main() {
   A* obj = new A(); //dynamic allocation
   //other operations
   delete obj; //explicitly destroy
}