Question

why in C++, for objects A,B

//interface, case #1 class A {  B bb; }  A::A() { //constructor bb = B(); }   //interface, case #2 class A {  B *bb; }  A::A() { //constructor bb = new B(); } 

Why case #2 work but not #1??

Edit: I got it now. But for case #1, if an instance of A is freed, will its bb also be automatically freed? Case #2 you have to explicitly call bb = NULL right?

Answer 1

The question as for me was how to make

A::A() { //constructor bb = new B(); } 

but without new. And I suppose the code in the question is not a real code. Just why ‘new’ works but simple assignment doesn’t.

And my answer is following. If I understood the question in a wrong way or the answer itself is wrong – please let me know.

Change

A::A() { //constructor bb = B(); } 

to

A::A():     bb() {    // some logic } 

and to have a consistence in the case with ‘new’ it is better to implement like

A::A():     bb( new B() ) {     // some logic } 

In these both cases when ‘some logic’ will start its execution you can be sure that object bb either initialized or exception will be thrown.

To make your case compilable B should has implemented assign operator.

To your edit: Your guess about case 1 is correct. in case 2 you have to call delete bb in the class destructor.

Please leave a message of the ‘-1’ reason. I am really confused.