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?

your code does not compile, since you cannot assign pointers (as returned by getExampleObject and getExample2Object) to non-pointer variables ec and ec2. — SirDarius, Jun 20 '12 at 10:14

score 9 · Answer 1 · answered Jun 20 '12 at 10:17

9

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.

answered Jun 20 '12 at 10:17

hmjd

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1

Thanks. That's what I thought but this made me doubt myself: [link](http://stackoverflow.com/questions/1404305/c-reference-in-constructor) – user1468729 Jun 20 '12 at 10:30
@user1468729, in that question `myclass` has `std::string` member, not a reference to a `std::string`. – hmjd Jun 20 '12 at 10:33
So does my first ExampleClass. Or am I missing something here? – user1468729 Jun 20 '12 at 10:43
@user1468729, and your `ExampleClass` is fine. The code in the other question is legal, I think the question was asking _what_ if I stored reference. If you notice, the accepted (and correct) answer to that question also state to store a copy. – hmjd Jun 20 '12 at 10:47

score 1 · Answer 2 · answered Jun 20 '12 at 10:19

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.

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

2 Answers2