Passing references to a C++ constructor and saving them to reference or non-reference types

Question

I'd like to know if these are basically the same thing.

class ExampleClass {
    public:
        ExampleClass(Object& newObj) : obj(newObj) {}
    private:
        Object obj;
}

class ExampleClass2 {
    public:
        ExampleClass2(Object& newObj) : objReference(newObj) {}
    private:
        Object& objReference;
}

So would this not work with either of the classes?

ExampleClass* getExampleObject() {
    Object obj;
    return new ExampleClass(obj);
}

ExampleClass2* getExample2Object() {
    Object obj;
    return new ExampleClass2(obj);
}

void main() {
    ExampleClass* ec = getExampleObject();
    ExampleClass2* ec2 = getExample2Object();
    //Do something involving the member objects of ec and ec2
}

So are the member objects invalid after both getExampleObject methods? Doesn't the constructor of ExampleClass save a copy of the object referenced in its constructor?

Solution 1

ExampleClass is fine, as it takes a copy of the object referenced in its constructor argument.

ExampleClass2 requires that the object referenced in its constructor argument exist for the lifetime of the ExampleClass2 instance (as it stores a reference to the object, it does not copy it). If it does not, then the ExampleClass2 instance will have a dangling reference as soon as the object to which it refers is destructed. This is the case with the getExample2Object() function as obj is destructed when getExample2Object() returns.

Solution 2

I'd like to know if these are basically the same thing.

No, they are not. Example's constructor takes an object reference and constructs a member object to using the object passed. This object is not a reference but a copy. In case of Example2, you are actually making objReference refer to object passed in ctor's parameter. You are good only as long as newObj's lifetime.

The

Object obj;
return new ExampleClass2(obj);

is the classic case when it will fail. Once the function returns, you will have a dangling reference.