Passing as const and by reference – Worth it?

Passing by const reference is the preferred way to pass around objects as a smart alternative to pass-by-value. When you pass by const reference, you take the argument in by reference (avoiding making any copies of it), but cannot make any changes to the original object (much as would happen when you would take the parameters in by value).

If you consider these three functions:

void VersionOne(Vector v);
void VersionTwo(Vector& v);
void VersionThree(const Vector& v);

There are subtle distinctions between all of them. The first function, for example, will invoke the copy constructor when you pass in a Vector so that it has its own local copy of the Vector. If your copy constructor takes a while to run or does a lot of resource allocation and deallocation, this may be slow, but you can make any changes you want to the parameter without risking any changes propagating back up to the caller. There will also be a destructor call at the end of the function as the argument is cleaned up, and if this is too large a cost it may be advisable to avoid this setup. That said, for small objects it may be perfectly acceptable.

The second version of this function takes in a Vector by reference, which means that the function can make any changes it wants to the Vector and the changes will propagate back up to the caller. Whenever you see a function that takes an argument by non-const reference, like this VersionTwo function, you should assume that it will be modifying the argument, since if it weren’t going to make any modifications, it would be taken by const reference. You will most likely take in the value by reference if you need to make changes to the Vector; for example, by rotating it, scaling it, etc. One tradeoff involved here is that the Vector will not be copied when it is passed into this function, and so you will avoid a call to the copy constructor and destructor. This may have performance implications for your program, though if that’s your reasoning you should probably go with pass by const reference. One thing to note is that following a reference is very similar to following a pointer (in fact, most implementations of references just treat them as though they were automatically-dereferenced pointers), so there may be a small performance hit every time you access the data through the reference. Only profiling can tell you whether or not this is a major problem, though, and I wouldn’t worry about it unless you had a specific reason to think it was at fault.

The final version of this function takes in a Vector by const reference, which, like passing by regular reference, avoids any copying. However, when taking the Vector by const reference, you are prohibited from making any changes to the Vector inside the function, so clients can assume that the Vector will not be modified. (Yes, technically it could be modified if it is poorly-written or has mutable data members, but we’ll ignore that for now. It’s the high-level idea that’s important here). This option would be good if you wanted to be able to inspect the value in the function without copying it and without mutating it.

There is one more difference between pass-by-reference and pass-by-const-reference, and that’s the behavior of the function on rvalues. If you have a temporary Vector object – either you created it explicitly by writing Vector() or by doing some mathematical operation on it like writing v1 + v2 – then you cannot pass that temporary Vector into a function that takes its parameter by reference because references can only bind to lvalues. The idea is that if you have a function like this:

void DoSomething(Vector& v) {
     v.x = 0.0f;
}

Then it doesn’t make sense to write

DoSomething(v1 + v2);

Since this would be changing the x field of a temporary expression. To prevent this, the compiler will refuse to compile this code.

However, C++ makes an exception and lets you pass rvalues into functions that take their argument by const reference, because, intuitively, you shouldn’t be able to modify an object through a const reference. Thus this code is perfectly legal:

void DoSomething(const Vector& v) {
    cout << v.x << endl;
}

DoSomething(v1 + v2);

So, to summarize-

1. Pass-by-value and pass-by-const-reference imply similar things – you want to be able to look at the value without being able to modify it.
2. Any time you could use pass-by-value you could instead use pass-by-const-reference without affecting the correctness of the program. However, there are performance tradeoffs between the indirection of the reference and the cost of copying and destructing the parameter.
3. Pass-by-non-const-reference should be used to indicate “I want to modify the argument.”
4. You cannot pass rvalues into functions that take their arguments by non-const reference.

Hope this helps!