c++ – Passing as const and by reference – Worth it?

c++ – 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 werent 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 thats 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 wouldnt 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 well ignore that for now. Its the high-level idea thats 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 thats 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 doesnt 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 shouldnt 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!

Do you want to copy the objects passed to the function?
If you pass the objects Vector a and Vector b by value, you have to deal with the overhead of copying them. For small structures the overhead is negligible.

If you pass the objects by reference or by pointer, you dont have this overhead. However, passing by pointer or reference potentially allows for modification of the object:

The const keyword in front of the object name is used to guarantee that your function does not modify the objects that are passed to the function by reference or pointer. Not only will this tell other programmers that your function is safe to use, it is also strictly enforced by the compiler. The const keyword does not have an impact on performance when using it for this purpose.

If you have large objects, pass them by const reference or const pointer.
If you want to modify the object that is passed to the function, use a reference or a pointer.
If your object is a small structure you can pass it by value.

c++ – Passing as const and by reference – Worth it?

Youll get a lot of answers to this question claiming one thing or another. The truth of the matter is that you need to test it. Your object is fairly small so that passing by value may be as fast or faster than passing by reference. Only by profiling your code will you be able to know.

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