Passing a const reference as a function parameter in a class member function

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Passing a const reference as a function parameter in a class member function



Suppose we have a class called Line that contains the following modifier that assigns a value to a class data member.


Line



void Start(const Point &start);


void Start(const Point &start);



The line class contains two data members called start and end which are both Point objects. Since the start variable in the function parameter is a reference, and I implement it as follows:


start



void Line::Start(const Point &start)
m_start = start;


void Line::Start(const Point &start)
m_start = start;



In the main function I have the following.



Line line;
Point start(1, 1);
line.Start(start);


Line line;
Point start(1, 1);
line.Start(start);



Would the m_start data member now refer directly to the start Point object in the main function since it was passed by reference?


m_start


start


Point





Where is the declaration of m_start? If m_start is a Point then no, the value is copied. If it is a const reference then you have a compile error. You need to initialize const members and references in the initializer list.
– Retired Ninja
Aug 12 at 1:43



m_start


m_start


Point





Have you made any attempt to test this yourself, such as changing the point after and having another class method which outputs data from the passed Point reference to see if it is the same Point?
– Nick A
Aug 12 at 1:45



Point


Point




1 Answer
1



Would the m_start data member now refer directly to the start Point
object in the main function since it was passed by reference?



No. One of the properties of C++ references (and one of the ways in which their behavior differs from that of pointers) is that it's impossible to reseat a reference -- that is, you can't make a reference that was 'referring to' one thing later 'refer to' a different thing. (And of course if your m_start member variable was of type Point rather than type Point &, then there's no reference to attempt-to-reseat anyway)


m_start


Point


Point &



Instead, start's copy-constructor will be invoked to copy start's contents into your m_start member-variable.


start


start


m_start



The upshot is that your Start() method will behave exactly like a similarly-implemented void Line::Start(Point start) method would, with the following exceptions:


Start()


void Line::Start(Point start)



In your const Point & version of Start(), no separate/method-local copy of the Point object is made


const Point &


Start()



In your const Point & version of Start(), trying to modify start from within the method would be a compile-time error (whereas in the pass-by-value version, the code inside Start() could modify the argument, but the modifications would never be visible outside of the Start() function since they'd only modify the local variable/parameter)


const Point &


Start()


start


Start()


Start()






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