Java style reference returns in C++

Question

Consider this Java code snippet:

Vertex a = graph.addVertex(null);
Vertex b = graph.addVertex(null);

Is it possible to do the same in C++? AFAIK only option in C++ would be:

Vertex& a = graph.addVertex(NULL);

Or maybe this:

typedef Vertex& Vertexref;
Vertexref a = graph.addVertex(NULL);

Of course returning a copy of the object is also possible.

But Java syntax seems more straightforward to me. Would it be possible to have it in C++?

You can modify the Vertex class to store shared pointers to its data, if you don't really care about performance. — Benjamin Lindley
– Benjamin Lindley, Commented Jan 1, 2014 at 18:14
Don't typedef a reference, or the people who have to maintain your code will hunt you down. — Alan Stokes
– Alan Stokes, Commented Jan 1, 2014 at 18:22
@Benjamin shared_ptr performance is pretty good, and usually good enough - especially since the compiler will elide many of the copies. — Alan Stokes
– Alan Stokes, Commented Jan 1, 2014 at 18:26
I do not intend to typedef a reference. I just wanted to make clear it is possible in order to have a syntax more similar to Java. I am aware of the drawbacks of doing so. — jbgs
– jbgs, Commented Jan 1, 2014 at 18:27

Lightness Races in Orbit · Accepted Answer · 2014-01-01 18:10:29Z

5

You may think the Java syntax is more straightforward, but have you tried returning a copy of an object in Java? That's far less straightforward. C++ makes you opt in to references, whereas Java makes you opt in to not references. Swings and roundabouts.

You have already shown us the two ways to write this in C++, and they are both very straightforward.

answered Jan 1, 2014 at 18:10

Lightness Races in Orbit

387k77 gold badges672 silver badges1.1k bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

jbgs Over a year ago

It also seems wrong to me. Sometimes I wonder why C++ doesn't provide a mechanism to return references without needing the caller to explicitly create a reference.

Steve Jessop Over a year ago

@jbgs: Are you saying, you wonder why C++ doesn't provide a mechanism for a variable that looks like a value of type Vertex, but if it was initialized using a call to addVertex is actually a reference to type Vertex? Aside from implementation difficulty, the reason is that this would be incredibly difficult to read. In C++ Vertex a; always declares a value (if Vertex is a class type). In Java Vertex a; always declares a reference. Why would you want a feature in either language to make it difficult to tell which you have? Explicitly stating the difference is good.

jbgs Over a year ago

Why not? When you call a function there's no way to know if you are passing by reference or by value (unless you have a look at function's signature). Cannot we have the same issue for return values? I agree it might be inconvenient anyway.

Lightness Races in Orbit Over a year ago

Sure, let's take an inconvenience, and apply it somewhere else too. That's fun.

Thomas Padron-McCarthy · Accepted Answer · 2014-01-20 09:20:49Z

Java objects are referenced by pointers, even if it's not visible in the syntax in the same way as in C++. Your Java function doesn't really return a reference (in the C++ sense, not just a synonym for pointer), but it returns a pointer, and that pointer is returned by value. So the equivalent C++ would be something like this:

Vertex* a = graph.addVertex(null);
Vertex* b = graph.addVertex(null);

It is a mistake to think of the "references" in Java as the same thing as the "references" in C++. The Java "references" are pointers. C++ "references" are aliases for variables, which is not the same thing as a pointer, even though they are usually (but not always) implemented "under the hood" using pointers.

To clarify: This is not just terminology nitpicking. Your C++ program will not behave the same as your Java program, if you just translate Java references to C++ references. To make an equivalent C++ program, you need to use pointers.

Collectives™ on Stack Overflow

Java style reference returns in C++

2 Answers 2

4 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

4 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related