questions about default constructor

[nsly]

//baseClass.h
class baseClass {
public:
baseClass() {};
}


//derivedClass
class derivedClass : public baseClass {
public:
derivedClass() {};

...other function and members

}

but the question is why complier (vs.net) tell me that error C2512: derivedClass: no appropriate default constructor available, when I try to declare like this:

baseClass bc = new derivedClass();

shall I say that an empty constructor expicitly defined by user is not the way to do this?

[Paul McKenzie]

Originally posted by nsly
but the question is why complier (vs.net) tell me that error C2512: derivedClass: no appropriate default constructor available, when I try to declare like this:

baseClass bc = new derivedClass();
There are two problems with your code:

1) Operator new returns a pointer to an object, not an object.

baseClass *bc = new derivedClass();

2) Even if you did declare a pointer, your code is ill-formed. Your baseClass must have a virtual destructor. If not, deleting the *bc object invokes undefined behavior.

Regards,

Paul McKenzie

[nsly]

if I remove all the empty ctor I created (because I don't have anyting to do within the ctor), I still got that error.

but what I assume is the compiler Must create one default for me.

so could I initiate one instance of the class using new (like baseClass bc = new derivedClass(); )?

[Graham]

Read what Paul said: new returns a pointer and you are declaring an object. When you put

baseclass b = new derived;

you are trying to invoke the constructor baseclass::baseclass(derived*), which doesn't exist.

[nsly]

sorry for wrong typing.

yes u r right, I do use pointer. baseClass * bc = new derivedClass();

the second problem is not really related to my question, even though I know this is. (that code is just a example).

[Andreas Masur]

[Moved thread]