Is it possible to avoid a copy when returning an argument from a function?

Question

Suppose I have value type with some in-place operation. For example, something like this:

using MyType = std::array<100, int>;

void Reverse(MyType &value) {
  std::reverse(value.begin(), value.end());
}

(The type and operation can be more complicated, but the point is the operation works in-place and the type is trivially copyable and trivially destructible. Note that MyType is large enough to care about avoiding copies, but small enough that it probably doesn't make sense to allocate on the heap, and since it contains only primitives, it doesn't benefit from move semantics.)

I usually find it helpful to also define a helper function that doesn't change the value in-place, but returns a copy with the operation applied to it. Among other things, that enables code like this:

MyType value = Reversed(SomeFunction());

Considering that Reverse() operates in-place, it should be logically possible to calculate value without copying the result from SomeFunction(). How can I implement Reversed() to avoid unnecessary copies? I'm willing to define Reversed() as an inline function in a header if that's what's necessary to enable this optimization.

I can think of two ways to implement this:

inline MyType Reversed1(const MyType &value) {
  MyType result = value;
  Reverse(result);
  return result;
}

This benefits from return-value optimization but only after the argument value has been copied to result.

inline MyType Reversed2(MyType value) {
  Reverse(value);
  return value;
}

This might require the caller to copy the argument, except if it's already an rvalue, but I don't think return-value optimization is enabled this way (or is it?) so there's a copy upon return.

Is there a way to implemented Reversed() that avoids copies, ideally in a way that it's guaranteed by recent C++ standards?

Answer 1

If you do want to reverse the string in-place so that the change to the string you send in as an argument is visible at the call site and you also want to return it by value, you have no option but to copy it. They are two separate instances.

---

One alternative: Return the input value by reference. It'll then reference the same object that you sent in to the function:

MyType& Reverse(MyType& value) {   // doesn't work with r-values
    std::reverse(std::begin(value), std::end(value));
    return value;
}

MyType Reverse(MyType&& value) {   // r-value, return a copy
    std::reverse(std::begin(value), std::end(value));
    return std::move(value);       // moving doesn't really matter for ints
}

---

Another alternative: Create the object you return in-place. You'll then return a separate instance with RVO in effect. No moves or copies. It'll be a separate instance from the one you sent in to the function though.

MyType Reverse(const MyType& value) {
    // Doesn't work with `std::array`s:
    return {std::rbegin(value), std::rend(value)}; 
}

---

The second alternative would work if std::array could be constructed from iterators like most other containers, but they can't. One solution could be to create a helper to make sure RVO works:

using MyType = std::array<int, 26>;

namespace detail {
    template<size_t... I>
    constexpr MyType RevHelper(const MyType& value, std::index_sequence<I...>) {
        // construct the array in reverse in-place:
        return {value[sizeof...(I) - I - 1]...};    // RVO
    }
} // namespace detail

constexpr MyType Reverse(const MyType& value) {
    // get size() of array in a constexpr fashion:
    constexpr size_t asize = std::tuple_size_v<MyType>;

    // RVO:
    return detail::RevHelper(value, std::make_index_sequence<asize>{});
}

Answer 2

Your last option is the way to go (except for the typo):

MyType Reversed2(MyType value)
{
    Reverse(value);
    return value;
}

[N]RVO doesn't apply to return result;, but at least it's implicitly moved, rather than copied.

You'll have either a copy + a move, or two moves, depending on the value category of the argument.

Answer 3

You can use a helper method that turns your in-place operation into something that can work on Rvalues. When I tested this in GCC, it results in one move operation, but no copies. The pattern looks like this:

void Reversed(MyType & m);

MyType Reversed(MyType && m) {
  Reversed(m);
  return std::move(m);
}

Here is the full code I used to test whether this pattern results in copies or not:

#include <stdio.h>
#include <string.h>
#include <utility>

struct MyType {
  int * contents;

  MyType(int value0) {
    contents = new int[42];
    memset(contents, 0, sizeof(int) * 42);
    contents[0] = value0;
    printf("Created %p\n", this);
  }

  MyType(const MyType & other) {
    contents = new int[42];
    memcpy(contents, other.contents, sizeof(int) * 42);
    printf("Copied from %p to %p\n", &other, this);
  }

  MyType(MyType && other) {
    contents = other.contents;
    other.contents = nullptr;
    printf("Moved from %p to %p\n", &other, this);
  }

  ~MyType() {
    if (contents) { delete[] contents; }
  }
};

void Reversed(MyType & m) {
  for (int i = 0; i < 21; i++) {
    std::swap(m.contents[i], m.contents[41 - i]);
  }
}

MyType Reversed(MyType && m) {
  Reversed(m);
  return std::move(m);
}

MyType SomeFunction() {
  return MyType(7);
}

int main() {
  printf("In-place modification\n");
  MyType x = SomeFunction();
  Reversed(x);
  printf("%d\n", x.contents[41]);

  printf("RValue modification\n");
  MyType y = Reversed(SomeFunction());
  printf("%d\n", y.contents[41]);
}

I'm not sure if this lack of copies is guaranteed by the standard, but I would think so, because there are some objects that are not copyable.

Note: The original question was just about how to avoid copies, but I'm afraid the goalposts are changing and now we are trying to avoid both copies and moves. The Rvalue function I present does seem to perform one move operation. However, if we cannot eliminate the move operation, I would suggest that the OP redesign their class so that moves are cheap, or just give up on the idea of this shorter syntax.

Answer 4

There is a trick. It is NOT pretty but it works.

Make Reversed accept not a T, but a function returning T. Call it like that:

MyType value = Reversed(SomeFunction); // note no `SomeFunction()`

Here is a full implementation of Reversed:

template <class Generator>
MyType Reversed(Generator&& g)
{
  MyType t{g()};
  reverse(t);
  return t;
}

This produces no copies or moves. I checked.

If you feel particularly nasty, do this

#define Reversed(x) Reversed([](){return x;})

and go back to calling Reversed(SomeFunction()). Again, no copies or moves. Bonus points if you manage to squeeze it through a corporate code review.

Answer 5

When you write

MyType value = Reversed(SomeFunction());

I see 2 things happen: Reversed will do RVO so it directly writes to value and SomeFunction either gets coppied into the argument for Reversed or creates a temporary object and you pass a reference. No matter how you write it there will be at least 2 objects and you have to reverse from one to the other.

There is no way for the compiler to do what I would call an AVO, argument value optimization. You want the argument to the Reversed function to be stored in the return value of the function so you can do in-place operations. With that feature the compiler could do RVO-AVO-RVO and SomeFunction would create it's return value directly in the final value variable.

But you could do this I think:

MyType &&value = SomeFunctio();
reverse(value);

---

Looking at it another way: Say you do figure out a way for Reveresed to do in-place operations then in

MyType &&value = Reversed(SomeFunction());

the SomeFunction would create a temporary but then the compiler has to extend the lifetime of that temporary to the lifetime of value. This works in direct assignment but how is the compiler supposed to know that Reversed will just pass the temporary through?

Answer 6

From the answers and comments it looks like the consensus is that there is no way to achieve this in C++.

It makes sense that this is the general answer without the implementation of MyType Reversed(MyType) available, since the compiler would have no clue that the return value is the same as the argument, so it would necessarily allocate separate spaces for them.

But it looks like even with the implementation of Reversed() available, neither GCC nor Clang will optimize away the copy: https://godbolt.org/z/KW6Y3vsdf

So I think the short story is that what I was asking for isn't possible. If it's important to avoid the copy, the caller should explicitly write:

MyType value = SomeFunction();
Reverse(value);
// etc.