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Pass by reference more expensive than pass by value

I want to know which is better, sending parameters by value or by reference in C++. I heard that there are cases where sending by value is faster than sending by reference. Which are these cases?

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  • When you send by value - copy of object is created (time and space consuming). So instead it is better to send by const& or &(in case if you want modify the object) Commented Nov 8, 2012 at 9:34
  • @UmNyobe Maybe here Want Speed? Pass by Value.. But there is also a discussion here: stackoverflow.com/questions/2108084/… Commented Nov 8, 2012 at 9:36
  • A reference is implemented using a pointer, so if the parameter is smaller than a pointer then passing it by value may be faster. Commented Nov 8, 2012 at 9:36
  • yes, but there are cases for basic types, for example int or float where it is slower to send by reference than by value. I'd like to know which bytes does a type to have in order to be faster sending by value than reference Commented Nov 8, 2012 at 9:37
  • "I'd like to know which bytes does a type to have in order to be faster sending by value than reference". Depends more on : which assembly instructions are used to make the value available to the function and to access this value. Commented Nov 8, 2012 at 9:47

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As a general rule, you should pass POD types by value and complex types by const reference.

That said, a good place where you pass complex types by value is where you would need a copy of the object inside the function anyway. In that case, you have two choices:

  • pass the argument as a const reference and create a local copy inside the function

  • pass the argument by value (the compiler creates the local copy).

The second option is generally more efficient. For an example, see the copy&swap idiom.

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The obvious case is when the parameter is equal to or smaller than a pointer in size and trivial to copy -- then you would pass by value. However, this is a age-old discussion and quite a long answer is required to answer it correctly for a given architecture. There are also many corner cases (e.g. RVO).

There's more to the question than speed -- semantics should be your first priority.

See also: Is it better in C++ to pass by value or pass by constant reference?

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