C ++类实例化理论

Question

#include <iostream>
using namespace std;

class A {
public:
    A() {
        cout << "Default Ctor" << endl;
    }
};

int main() {
    A(); // <------- Problem

    return 0;
}

It shows Default Ctor on console. My questions

• Is is valid?
• If so, how did it instantiate since I didn't use new or any object?

Answer 1

You are creating a new object with A() .

Is is valid?

Yes it is.

If so, how did it instantiate since I didn't use new or any object?

new simply creates the object in dynamic memory. You're creating the object in automatic memory. Also, just because you didn't give the object a name, doesn't mean it isn't created.

Think of this:

int foo() { return 1; }

int main()
{
   foo();
}

Leaving optimizations aside Did foo() actually return 1 ? Yes it did. Just that you're not using it.

EDIT:

Let's break it down a bit:

A();  //creates a temporary unnamed object in automatic storage

A a;   //creates an object a of type A in automatic storage

A a(); //creates no object, but declare a function a returning A and taking no parameters

A a(A());   //these two are equivalent
A a = A();  //creates a temporary unnamed object and creates an object a in automatic storage
            //using the compiler-generated copy constructor

A a;
a = A();    //creates an object a in automatic storage
            //creates an unnamed temporary object and uses the compiler-generated assignment operator
            //to assign it to a

A a = &A(); //invalid, &A() is the address of a temporary a, which is illegal

Answer 2

Is is valid?
Yes, It is valid

What exactly happens?

A(); 

Creates a temporary nameless object of the type A by calling its default no argument constructor but the object is destructed by the time the next statement is executed since.

If so, how did it instantiate since I didn't use new or any object?
You can create objects on the local/automatic storage or on dynamic storage depending on your usage.

When you use new objects are created on dynamic storage(heap), when you create a object as you have it is created on the local/automatic storage(stack).

So using new only determines where the object will be created not whether it will be created.

What are Temporary Nameless objects?
You do not always need to name an object to instantiate them.
For Ex:
While calling function, when you pass objects by value temporary nameless objects are created and automatically destroyed all the time.These are objects which do not have any name and hence cannot be explicitly referred through program but they do serve the purpose for which they were created.

In simple words,
You are creating a nameless temporary object on the local/automatic storage which does not exist once the statement completes execution.