Abstract

This proposal recommends the relaxing of [dcl.fct]/11; specifically, the prohibition of defining (anonymous) types as return values. This proposal is considered contingent on P0224, as obvious problems otherwise arise.

(Note: references made to the existing draft standard are made against N4567.)

Rationale

The concept of multiple return values is well known. At present, however, C++ lacks a good mechanism for implementing the same. std::tuple is considered clunky by many and, critically, creates sub-optimal API by virtue of the returned values being unnamed, forcing developers to rely on supplemental documentation to explain their purpose. Aggregates represent an improvement, being self-documenting, but the need to provide external definitions of the same is awkward and, worse, pollutes their corresponding namespace with entities that may be single use. Proposals such as N4560 present a complicated mechanism for providing tagged (self-documenting) tuple-like types, which may be necessary in some cases, but still represent a non-trivial amount of complexity that ideally should not be required.

Proposals such as P0144 in particular represent a significant step toward support of multiple return values as first class citizens. This proposals, along with P0197 and other current directions show an encouraging movement away from the traditional std::pair and std::tuple towards comparable concepts without requiring the explicit types. (We expect, however, that the standard template library types will remain useful for algorithms where the identity of the elements is unimportant, while it is important to be able to name at least the outer, if not complete, type. In that respect, we hypothesize that we may in the future see the ability to construct a std::tuple from any tuple-like, as also suggested in P0197.) On their own, however, these proposals risk exacerbating the problem that this proposal aims to address.

It has been suggested on multiple occasions that the optimal solution to the above issues is to return an anonymous struct. This solves the problems of clutter and self-documentation, but runs afoul of a much worse issue; because the struct is anonymous, it can be difficult to impossible to give its name a second time in order to separate the declaration and definition of the function that wishes to use it.

The discussion on this topic lead to an interesting question, which is addressed in P0224: why is it necessary to repeat the return value at all? If P0224 is accepted, the problem of naming the return type is immediately eliminated, and along with it, a major reason why returning anonymous structs is not currently permitted.

Proposal

The use of auto to indicate an inferred return type (as proposed by P0224) provides an optimal solution to the following problem:

// foo.h
struct { int id; double value; } foo();

How does one now provide an external definition for foo()? With P0224, the solution is simple:

// foo.cpp
auto foo()
{
  ...
  return { id, value };
}

Recent and proposed changes in C++ significantly mitigate the reasons to prohibit an anonymous struct defined as a return type. Constructing the return result is a non-issue, since the type name may now be elided, and the combination of auto variable declarations, decltype, and P0224, permit implicit naming of the type where necessary. In short, the prohibition ([dcl.fct]/11) against defining types in return type specifications has become largely an artificial and arbitrary restriction which we propose to remove.

We additionally note that this prohibition is already not enforced by at least one major compiler (MSVC), and is enforced sporadically in others (see What about defining types in function pointer types?).

Proposed Wording

(Proposed changes are specified relative N4567.)

Change [dcl.fct]/11 (8.3.5.11) as follows:

Interactions

Definition of an anonymous class-type as a return value type is currently ill-formed (although not universally enforced by existing major compilers). Accordingly, this change will not affect existing and conforming code, and may cause existing but non-conforming code to become conforming. This proposal does not make any changes to other existing language or library features; while conceivable that some library methods might benefit from the feature, such changes are potentially breaking, and no such changes are proposed at this time.

Implementation and Existing Practice

The proposed feature is at least already partly implemented by MSVC and (to a lesser extend) GCC and ICC. The curious, partial support in GCC and ICC (see What about defining types in function pointer types?) suggests that the existing prohibition may already be largely artificial, and that removing it would accordingly be a simple matter.

Discussion

// Declare a function pointer type which returns an anonymous struct
using ReturnsAnonymousStruct = struct { int result; } (*)();

// Define a function using the same
int bar(ReturnsAnonymousStruct f) { return ((*f)()).result; }

// Provide a mechanism to obtain the return type of a function
template <typename T> struct ReturnType;

template <typename T, typename... Args>
struct ReturnType<T (*)(Args...)>
{
    using result_t = T;
};

// Declare a function that is a ReturnsAnonymousStruct
ReturnType<ReturnsAnonymousStruct>::result_t foo() { return {0}; }

// Use the function
int main()
{
    return bar(&foo);
}

struct { int result; } foo() { ... }
struct { int result; } bar()
{
  return foo();
}

struct { int result; } bar()
{
  // The use of '...' here implies that the compiler stores the result of
  // 'foo()' in a temporary, which is unpacked into a parameter pack and then
  // expanded into an expression list which is used to form an initializer
  // list which in turn forms the return value of 'bar'. This syntax should
  // be taken as illustrative only; we do not anticipate that this would be
  // the exact syntax used should such a feature be added.
  return { foo()... };
}

auto foo()
{
  return { .x = 3, .y = 2 }; // deduce: struct { int x, y; }
}

Future Directions

In the Discussion section above, we presented a utility for extracting the return type from a function pointer type. The facility as presented has significant limitations; namely, it does not work on member functions and the several variations (e.g. CV-qualification) which apply to the same. We do not here propose a standard library implementation of this facility, which presumably would cover these cases, however there is room to imagine that such a facility could be useful, especially if the proposals we present here are adopted. (David Krauss points out that std::reference_wrapper can be used to similar effect... on some compilers. However, imperfect portability and the disparity between intended function and use for this result suggest that this is not the optimal facility for the problem.)

Another consideration that seems likely to come up is if we should further simplify the syntax for returning multiple values (conceivably, this could apply to both anonymous structs and to std::pair / std::tuple). Some have suggested allowing that the struct keyword may be omitted. In light of P0151, we can conceive that allowing the syntax <int x, double y> foo() might be interesting. At this time, we prefer to focus on the feature here presented rather than risk overextending the reach of this proposal. However, if this proposal is accepted, it represents an obvious first step to considering such features in the future.

Acknowledgments

We wish to thank everyone on the std-proposals forum, especially Bengt Gustafsson and Tim Song, for their valuable feedback and insights.

References

• N4567 Working Draft, Standard for Programming Language C++

  http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4567.pdf

• N4560 Extensions for Ranges

  http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4560.pdf

• P0144 Structured Bindings

  http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0144r0.pdf

• P0151 Proposal of Multi-Declarators (aka Structured Bindings)

  http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0151r0.pdf

• P0197 Default Tuple-like Access

  http://wg21.link/p0197

• P0224 Implicit Return Type

  http://wg21.link/p0224