Does C++ treat Class Objects like value types if i

问题:

Sample code:

MyItemType a;
MyItemType b;
a.someNumber = 5;
b = a;

cout << a.someNumber << endl;
cout << b.someNumber << endl;

b.someNumber = 10;

cout << a.someNumber << endl;
cout << b.someNumber << endl;

The output:

5
5
5
10

If a and b were reference types, the last 2 lines would have been 10 and 10 instead of 5 and 10 I guess.

Does this mean when you do a declaration like this:

AClassType anInstance;

it is treated like a value type?

------Here is MyItemType.h------------

#ifndef MYITEMTYPE_H
#define MYITEMTYPE_H

class MyItemType{

public:
    int someNumber;
    MyItemType();
};

MyItemType::MyItemType(){
}

#endif  /* MYITEMTYPE_H */

回答1:

It is not treated like a value type, in fact it is.

While in Java object variables store references to objects, in C++ there is an important difference between an object and its reference. Assignment is by default really by value.

If you want a variable to be just a reference, you use either a reference or a pointer type, depending what you want to with it. These types are declared T* and T&.

To illustrate this a little more:

In Java, when you say MyClass obj, an object is created, but a reference/pointer is stored in the variable obj.

In C++, MyClass obj creates the object and will stored it in obj. If you want to work with references/pointers, you need to declare variables explicity as MyClass* objPointer or MyClass& objReference.

回答2:

Basically, yes (if you consider the C++ equivalent to mean the same as in Java).

AClassType anInstance; is an object, not a reference. MyItemType a and MyItemType b are different objects, they reside in different memory space, so obviously changes to one won't affect the other.

When you do a=b, you don't reference one object with the other, but, in this case, do a member-wise assignment. It's basically like saying

a.someNumber = b.someNumber;

回答3:

In C++ objects are referred to as static (stack) variables when they are created without a pointer reference. Dynamic (heap) variables are pointer references that require manual memory management.

In Java or C#, by contrast, almost all objects are reference types which behave like pointers except they are garbage collected whereas value types are a special subset of all objects that are generally immutable. (C++ stack variables are certainly not immutable).

回答4:

While C++ does not call objects value or reference types, the behaviour of value and reference types has equivalence in C++.

Given two objects a and b, a = b:

1. a value type copies the content of b into a keeping them separate objects;
2. a reference type copies the location of b into a, making them refer to the same object.

For C++:

MyClass  a;      // value type
MyClass  b;      // value type
MyClass &c =  a; // reference type (a reference in C++), fixed to a
MyClass *d = &b; // reference type (a pointer in C++)

 a =  b; // copy content of b into a
 c =  b; // copy content of b into a
 d = &a; // set d to refer to a
*d =  b; // copy content of b into a

Pointers/references may be to value objects, objects allocated by new or some other memory management scheme (e.g. malloc or the Win32 CoTaskMalloc).

回答5:

The short explanation is in this key part

b = a;

you are using the copy assignment operator, meaning this sign here =.

the default behaviour of this operator is to apply a memberwise copy, so if you don't define/overload your own operator this row will copy all the values stored in all the members of a in the corresponding members of b.

the new operator it's a complete different story, it's often used to allocate objects on the heap and managing them with a pointer avoiding the stack and unnecessary copies.

回答6:

By default, C++ treats its classes as value types and makes deep (element-wise) copies.

But you can store your member variables inside the class in free storage (on the heap) and customize (override) the behaviour of the assignment-operator (b = a in your code) to show the behaviour of a reference type.

This (and some other 'tricks') is for the example, how the shared_ptr smart pointer is designed. It is a C++ class, where each instance references the same original data, regardless how often it is copied.