C++: How do I decide if to pass params by ref or by value?

Question

With C++ how do i decide if i should pass an argument by value or by reference/pointer? (tell me the answer for both 32 and 64bits) Lets take A. Is 2 32bit values more less or equal work as a pointer to a 32bit value?

B to me seems like i always should pass by value. C i think i should pass by value but someone told me (however i haven't seen proof) that processors don't handle values not their bitsize and so it is more work. So if i were passing them around would it be more work to pass by value thus byref is faster? Finally i threw in an enum. I think enums should always be by value

Note: When i say by ref i mean a const reference or pointer (can't forget the const...)

struct A { int a, b; }
struct B { int a; }
struct C { char a, b; }
enum D   { a,b,c }
void fn(T a);

Now tell me the answer if i were pushing the parameters many times and the code doesn't use a tail call? (lets say the values isnt used until 4 or so calls deep)

Answer 1

If you want a copy, then pass by value. If you want to change it and you want those changes to be seen outside the function, then pass by reference. If you want speed and don't want to change it, pass by const reference.

Answer 2

These are the rules that I use:

• for native types:
  • by value when they are input arguments
  • by non-const reference when they are mandatory output arguments
• for structs or classes:
  • by const reference when they are input arguments
  • by non-const reference when they are output arguments
• for arrays:
  • by const pointer when they are input arguments (const applies to the data, not the pointer here, i.e. const TYPE *)
  • by pointer when they are output arguments (const applies to the data, not the pointer)

I've found that there are very few times that require making an exception to the above rules. The one exception that comes to mind is for a struct or class argument that is optional, in which case a reference would not work. In that case I use a const pointer (input) or a non-const pointer (output), so that you can also pass 0.

Answer 3

I pass all parameters by reference for consistency, including builtins (of course, const is used where possible).

I did test this in performance critical domains -- worst case loss compared to builtins was marginal. Reference can be quite a bit faster, for non-builtins, and when the calls are deep (as a generalization). This was important for me as I was doing quite a bit of deep TMP, where function bodies were tiny.

You might consider breaking that convention if you're counting instructions, the hardware is register-starved (e.g. embedded), or if the function is not a good candidate for inlining.

Unfortunately, the question you ask is more complex than it appears -- the answer may vary greatly by your platform, ABI, calling conventions, register counts, etc.

Answer 4

I would consider whether you want value or reference semantics before you go worrying about optimizations. Generally you would pass by reference if you want the method you are calling to be able to modify the parameter. You can pass a pointer in this case, like you would in C, but idiomatic C++ tends to use references.

There is no rule that says that small types or enums should always be passed by value. There is plenty of code that passes int& parameters, because they rely on the semantics of passing by reference. Also, you should keep in mind that for any relatively small data type, you won't notice a difference in speed between passing by reference and by value.

That said, if you have a very large structure, you probably don't want to make lots of copies of it. This is where const references are handy. Do keep in mind though that const in C++ is not strictly enforced (even if it's considered bad practice, you can always const_cast it away). There is no reason to pass a const int& over an int, although there is a reason to pass a const ClassWithManyMembers& over a ClassWithManyMembers.

All of the structs that you listed I would say are fine to pass by value if you are intending them to be treated as values. Consider that if you call a function that takes one parameter of type struct Rectangle{int x, y, w, h}, this is the same as passing those 4 parameters independently, which is really not a big deal. Generally you should be more worried about the work that the copy constructor has to do - for example, passing a vector by value is probably not such a good idea, because it will have to dynamically allocate memory and iterate through a list whose size you don't know, and invoke many more copy constructors.

While you should keep all this in mind, a good general rule is: if you want refence semantics, pass by refence. Otherwise, pass intrinsics by value, and other things by const reference.

Also, C++11 introduced r-value references which complicate things even further. But that's a different topic.

Answer 5

Forget the stack size. You should pass by reference if you want to change it, otherwise you should pass by value.

Preventing the sort of bugs introduced by allowing functions to change your data unexpectedly is far more important than a few bytes of wasted stack space.

If stack space becomes a problem, stop using so many levels (such as replacing a recursive solution with an iterative one) or expand your stack. Four levels of recursion isn't usually that onerous, unless your structures are massive or you're operating in the embedded world.

If performance becomes a problem, find a faster algorithm :-) If that's not possible, then you can look at passing by reference, but you need to understand that it's breaking the contract between caller and callee. If you can live with that, that's okay. I generally can't :-)

The intent of the value/reference dichotomy is to control what happens to the thing you pass as a parameter at the language level, not to fiddle with the way an implementation of the language works.

Answer 6

First, reference and pointers aren't the same.

Pass by pointer

Pass parameters by pointers if any/some of these apply:

1. The passed element could be null.
2. The resource is allocated inside the called function and the caller is responsible should be responsible for freeing such a resource. Remember in this case to provide a free() function for that resource.
3. The value is of a variable type, like for example void*. When it's type is determined at runtime or depending on the usage pattern (or hiding implementation - i.e Win32 HANDLE), such as a thread procedure argument. (Here favor c++ templates and std::function, and use pointers for this purpose only if your environment does not permit otherwise.

Pass by reference

Pass parameters by reference if any/some of these apply:

1. Most of the time. (prefer passing by const reference)
2. If you want the modifications to the passed arguments to be visible to the caller. (unless const reference is used).
3. If the passed argument is never null.
4. If you know what is the passed argument type and you have control over function's signature.

Pass by copy

Pass a copy if any/some of these apply:

1. Generally try to avoid this.
2. If you want to operate on a copy of the passed argument. i.e you know that the called function would create a copy anyway.
3. With primitive types smaller than the system's pointer size - as it makes no performance/memory difference compared to a const ref.
4. This is tricky - when you know that the type implements a move constructor (such as std::string in C++11). It then looks as if you're passing by copy.

Any of these three lists can go more longer, but these are - I would say - the basic rules of thumb.

Answer 7

A lot depends on your requirement but best practice is to pass by reference as it reduces the memory foot print.

If you pass large objects by value, a copy of it is made in memory andthe copy constructor is called for making a copy of this.

So it will take more machine cycles and also, if you pass by value, changes are not reflected in the original object.

So try passing them by reference.

Hope this has been helpful to you.

Regards, Ken

Answer 8

Copying objects by value is usually a bad idea - more CPU to do the constructor function; more memory for the actual object. Use const to prevent the function modifying the object. The function signature should tell the caller what might happen to the referenced object.

Things like int, char, pointers are usually passed by value.

As to the structures you outlined, passing by value will not really matter. You need to do profiling to find out, but on the grand scheme of a program you be better off looking elsewhere for increasing performance in terms of CPU and/or memory.

Answer 9

Your complete question is a bit unclear to me, but I can answer when you would use passing by value or by reference.

When passing by value, you have a complete copy of the parameter into the call stack. It's like you're making a local variable in the function call initialized with whatever you passed into it.

When passing by reference, you... well, pass by reference. The main difference is that you can modify the external object.

There is the benefit of reducing memory load for large objects passing by reference. For basic data types (32-bit or 64-bit integers, for example), the performance is negligible.

Generally, if you're going to work in C/C++ you should learn to use pointers. Passing objects as parameters will almost always be passed via a pointer (vs reference). The few instances you absolutely must use references is in the copy constructor. You'll want to use it in the operators as well, but it's not required.