Is It Better in C++ to Pass by Value or Pass by Reference-To-Const

Is it better in C++ to pass by value or pass by reference-to-const?

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.

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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.

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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.

Why not always pass by const reference in C++?

When an argument is passed by value it is modifiable and copying it may be elided. For example, the canonical way to implement the assignment operator looks like this:

T& T::operator= (T value) {
    value.swap(*this);
    return *this;
}

At first sight it may look inefficient because a T is being copied. However, it would be copied anyway, i.e., if a copy is needed one will be created either way:

T& T::operator= (T const& value) {
    T(value).swap(*this); // has to do a copy right here
    return *this;
}

However, for the first version, it may be possible not to create copy at all, for example

T f() { return T(); }
// ...
T x = ...;
x = f();

When assigning the result of f() which is of type T to x the compiler may decide that it doesn't need to copy the result of f() and instead pass it into the assignment operator directly. In that case, if the assignment operator takes the argument by const& the compiler has to create a copy inside the assignment operator. In the implementation taking the argument by value it can elide the copy! In fact, the return from f() can already elide the copy, i.e., the call to f() and the following assignment may just involve the default construction of the object! ... and for many modern compilers that is, indeed, the case!

Put differently: if you need to copy an argument, getting it passed by value may avoid the need to create a copy. Also, you can std::move() from value arguments but not from const& arguments.

Why pass by const reference instead of by value?

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.

When is a const reference better than pass-by-value in C++11?

The general rule of thumb for passing by value is when you would end up making a copy anyway. That is to say that rather than doing this:

void f(const std::vector<int>& x) {
    std::vector<int> y(x);
    // stuff
}

where you first pass a const-ref and then copy it, you should do this instead:

void f(std::vector<int> x) {
    // work with x instead
}

This has been partially true in C++03, and has become more useful with move semantics, as the copy may be replaced by a move in the pass-by-val case when the function is called with an rvalue.

Otherwise, when all you want to do is read the data, passing by const reference is still the preferred, efficient way.

Why pass by value and not by const reference?

There are situations where you don't modify the input, but you still need an internal copy of the input, and then you may as well take the arguments by value. For example, suppose you have a function that returns a sorted copy of a vector:

template <typename V> V sorted_copy_1(V const & v)
{
    V v_copy = v;
    std::sort(v_copy.begin(), v_copy.end());
    return v;
}

This is fine, but if the user has a vector that they never need for any other purpose, then you have to make a mandatory copy here that may be unnecessary. So just take the argument by value:

template <typename V> V sorted_copy_2(V v)
{
    std::sort(v.begin(), v.end());
    return v;
}

Now the entire process of producing, sorting and returning a vector can be done essentially "in-place".

Less expensive examples are algorithms which consume counters or iterators which need to be modified in the process of the algorithm. Again, taking those by value allows you to use the function parameter directly, rather than requiring a local copy.

Where should I prefer pass-by-reference or pass-by-value?

There are four main cases where you should use pass-by-reference over pass-by-value:

1. If you are calling a function that needs to modify its arguments, use pass-by-reference or pass-by-pointer. Otherwise, you’ll get a copy of the argument.
2. If you're calling a function that needs to take a large object as a parameter, pass it by const reference to avoid making an unnecessary copy of that object and taking a large efficiency hit.
3. If you're writing a copy or move constructor which by definition must take a reference, use pass by reference.
4. If you're writing a function that wants to operate on a polymorphic class, use pass by reference or pass by pointer to avoid slicing.

Difference between const reference and pass by value in c++? [duplicate]

Passing by value, basicaly creates a local copy of the argument, so passing by reference is better, but you might want to avoid the function to alter the variable you pass by reference, let's say you just want to print it on the screen, then you mark it as const so it cannot be changed, and you avoid copying it making your program a bit more efficient. There can be other reasons to pass by reference, like, for instance, if some object cannot be copied.

Performance cost of passing by value vs. by reference or by pointer?

It depends on what you mean by "cost", and properties of the host system (hardware, operating system) with respect to operations.

If your cost measure is memory usage, then the calculation of cost is obvious - add up the sizes of whatever is being copied.

If your measure is execution speed (or "efficiency") then the game is different. Hardware (and operating systems and compiler) tend to be optimised for performance of operations on copying things of particular sizes, by virtue of dedicated circuits (machine registers, and how they are used).

It is common, for example, for a machine to have an architecture (machine registers, memory architecture, etc) which result in a "sweet spot" - copying variables of some size is most "efficient", but copying larger OR SMALLER variables is less so. Larger variables will cost more to copy, because there may be a need to do multiple copies of smaller chunks. Smaller ones may also cost more, because the compiler needs to copy the smaller value into a larger variable (or register), do the operations on it, then copy the value back.

Examples with floating point include some cray supercomputers, which natively support double precision floating point (aka double in C++), and all operations on single precision (aka float in C++) are emulated in software. Some older 32-bit x86 CPUs also worked internally with 32-bit integers, and operations on 16-bit integers required more clock cycles due to translation to/from 32-bit (this is not true with more modern 32-bit or 64-bit x86 processors, as they allow copying 16-bit integers to/from 32-bit registers, and operating on them, with fewer such penalties).

It is a bit of a no-brainer that copying a very large structure by value will be less efficient than creating and copying its address. But, because of factors like the above, the cross-over point between "best to copy something of that size by value" and "best to pass its address" is less clear.

Pointers and references tend to be implemented in a similar manner (e.g. pass by reference can be implemented in the same way as passing a pointer) but that is not guaranteed.

The only way to be sure is to measure it. And realise that the measurements will vary between systems.

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