C++ – When to Prefer Pass-by-Value Over Pass-by-Const-Reference

It used to be generally recommended best practice¹ to use pass by const ref for all types, except for builtin types (char, int, double, etc.), for iterators and for function objects (lambdas, classes deriving from std::*_function).

This was especially true before the existence of move semantics. The reason is simple: if you passed by value, a copy of the object had to be made and, except for very small objects, this is always more expensive than passing a reference.

With C++11, we have gained move semantics. In a nutshell, move semantics permit that, in some cases, an object can be passed “by value” without copying it. In particular, this is the case when the object that you are passing is an rvalue.

In itself, moving an object is still at least as expensive as passing by reference. However, in many cases a function will internally copy an object anyway — i.e. it will take ownership of the argument.²

In these situations we have the following (simplified) trade-off:

1. We can pass the object by reference, then copy internally.
2. We can pass the object by value.

“Pass by value” still causes the object to be copied, unless the object is an rvalue. In the case of an rvalue, the object can be moved instead, so that the second case is suddenly no longer “copy, then move” but “move, then (potentially) move again”.

For large objects that implement proper move constructors (such as vectors, strings …), the second case is then vastly more efficient than the first. Therefore, it is recommended to use pass by value if the function takes ownership of the argument, and if the object type supports efficient moving.

A historical note:

In fact, any modern compiler should be able to figure out when passing by value is expensive, and implicitly convert the call to use a const ref if possible.

In theory. In practice, compilers can’t always change this without breaking the function’s binary interface. In some special cases (when the function is inlined) the copy will actually be elided if the compiler can figure out that the original object won’t be changed through the actions in the function.

But in general the compiler can’t determine this, and the advent of move semantics in C++ has made this optimisation much less relevant.

¹ E.g. in Scott Meyers, Effective C++.

² This is especially often true for object constructors, which may take arguments and store them internally to be part of the constructed object’s state.

There are two main considerations. One is the expense of copying the passed object and the second is the assumptions that the compiler can make when the object is a a local object.

E.g. In the first form, in the body of f it cannot be assumed that a and b don't reference the same object; so the value of a must be re-read after any write to b, just in case. In the second form, a cannot be changed via a write to b, as it is local to the function, so these re-reads are unnecessary.

void f(const Obj& a, Obj& b)
{
    // a and b could reference the same object
}

void f(Obj a, Obj& b)
{
    // a is local, b cannot be a reference to a
}

E.g.: In the first example, the compiler may be able to assume that the value of a local object doesn't change when an unrelated call is made. Without information about h, the compiler may not know whether an object that that function has a reference to (via a reference parameter) isn't changed by h. For example, that object might be part of a global state which is modified by h.

void g(const Obj& a)
{
    // ...
    h(); // the value of a might change
    // ...
}

void g(Obj a)
{
    // ...
    h(); // the value of a is unlikely to change
    // ...
}

Unfortunately, this example isn't cast iron. It is possible to write a class that, say, adds a pointer to itself to a global state object in its constructor, so that even a local object of class type might be altered by a global function call. Despite this, there are still potentially more opportunities for valid optimizations for local objects as they can't be aliased directly by references passed in, or other pre-existing objects.

Passing a parameter by const reference should be chosen where the semantics of references are actually required, or as a performance improvement only if the cost of potential aliasing would be outweighed by the expense of copying the parameter.