Return type deduction for normal functions

Introduction

This proposal also resolves core DRs 975 (lambda return type deduction from multiple return statements), 1048 (inconsistency between auto and lambda return type deduction), and 1588 (deducing cv-qualified auto).

ChangeLog

• Add decltype(auto)
• Allow auto in trailing-return-type
• Prohibit deduction from an initializer-list

• Prohibit virtual auto
• Require decltype(auto) to stand alone
• Clarify explicit instantiation declaration

• Add friend and address overloading examples
• Resolve core issue 1588
• Clarify sfinae and instantiation time

Redeclaration

struct A {
  auto f(); // forward declaration
};
auto A::f() { return 42; }

The proposed wording allows non-defining declarations so long as all declarations have the same declared type, without considering the deduced type.

auto f(); // return type is unknown
auto f() { return 42; } // return type is int
auto f(); // redeclaration
int  f(); // error, declares a different function

template <class T> auto g(T t); // forward declaration
template <class T> auto g(T t) { return t; } // return type is deduced at instantiation time
template <class T> auto g(T t); // redeclaration

Of course, using such a function in an expression when only a forward declaration has been seen is ill-formed:

auto f();    // return type is unknown
int i = f(); // error, return type of f is unknown

template <class T> auto f(T t) { return t; } // #1
template auto f(int); // OK
template char f(char); // error, no matching template
template<> auto f(double); // OK, forward declaration with unknown return type

template <class T> T f(T t) { return t; } // OK, not functionally equivalent to #1
template char f(char); // OK, now there is a matching template
template auto f(float); // OK, matches #1

Multiple returns

The limitation on return type deduction to function bodies consisting of a single return statement is inconvenient for lambdas (DR 975), but is likely to be even more of an annoyance on normal functions.

auto iterate(int len) // error, body isn't "return expr;"
{
  for (int i = 0; i < len; ++i)
    if (search (i))
      return i;
  return -1;
}

Recursion

auto h() { return h(); } // error, return type of h is unknown

auto sum(int i) {
  if (i == 1)
    return i;          // return type deduced to int
  else
    return sum(i-1)+i; // ok to call it now
}

Instantiation

template <class T> auto f(T t) { return t; } // return type deduced at instantiation time
typedef decltype(f(1)) fint_t; // must instantiate f<int> to deduce return type

SFINAE

More general deduction

template <class T> struct A { static T t; };
template <class T> auto& f() { return A::t; } // returns by reference

Such a function must have a return statement, however, since there is no way to get void from auto&.

auto& f() { } // error, no return statement

Difference from decltype

Unfortunately, there is no way to get the effect of decltype with an auto return type; plain auto never deduces to a reference, and auto&& always deduces to a reference. This is a significant problem, as it means that forwarding functions can't use auto. We could consider using decltype semantics instead of the existing auto semantics, but that would mean giving different deduction semantics to auto depending on whether the declaration is of a variable or a function, and making auto functions different from lambdas.

Therefore, I propose to also allow decltype(auto) to get the decltype semantics without having to repeat the expression. For simplicity of specification and orthogonality I propose to allow it everywhere that plain auto is allowed, except for introducing a trailing-return-type. It occurs to me that the difference in meaning of decltype depending on the form of the expression (e.g. parenthesized or not) might be more surprising in this context, but I think it would be even more surprising if this worked differently from C++11 decltype.

noexcept

A few people have requested that an auto function should have a deduced noexcept-specifier as well as return type. I don't think that tying the two deductions together is appropriate, but if people want general deduction of noexcept-specifiers, EWG might wish to revisit N3207.

auto in trailing-return-type

[]()->auto& { return f(); }

Deducing std::initializer_list

Proposed wording

decltype-specifier:

decltype ( expression )

decltype ( auto )

For an expression e, The type denoted by decltype(e) is defined as follows:

• if e is an unparenthesized id-expression or an unparenthesized class member access (5.2.5), decltype(e) is the type of the entity named by e. If there is no such entity, or if e names a set of overloaded func- tions, the program is ill-formed;
• otherwise, if e is an xvalue, decltype(e) is T&&, where T is the type of e;
• otherwise, if e is an lvalue, decltype(e) is T&, where T is the type of e;
• otherwise, decltype(e) is the type of e.

The operand of the decltype specifier is an unevaluated operand (Clause 5). [ Example:

const int&& foo();
int i;
struct A { double x; };
const A* a = new A();
decltype(foo()) x1 = i;   // type is const int&&
decltype(i) x2;           // type is int
decltype(a->x) x3;        // type is double
decltype((a->x)) x4 = x3; // type is const double&

—end example ]

For decltype(auto), see 7.1.6.4.

The auto and decltype(auto) type-specifiers designate a placeholder type that will be replaced later, either by deduction from an initializer or by explicit specification with signifies that the type of a variable being declared shall be deduced from its initializer or that a function declarator shall include a trailing-return-type.

The auto type-specifier may placeholder type can appear with a function declarator in the decl-specifier-seq, type-specifier-seq, conversion-function-id, or trailing-return-type, with a trailing-return-type (8.3.5) in any context where such a declarator is valid. If the function declarator includes a trailing-return-type (8.3.5), that specifies the declared return type of the function. If the declared return type of the function contains a placeholder type, the return type of the function is deduced from return statements in the body of the function, if any.

Otherwise, the type of the variable The type of a variable declared using auto or decltype(auto) is deduced from its initializer. The name of the variable being declared shall not appear in the initializer expression. This use of auto is allowed when declaring variables in a block (6.3), in namespace scope (3.3.6), and in a for-init-statement (6.5.3). auto or decltype(auto) shall appear as one of the decl-specifiers in the decl-specifier-seq and the decl-specifier-seq shall be followed by one or more init-declarators, each of which shall have a non-empty initializer.

[ Example:

auto x = 5;                 // OK: x has type int
const auto *v = &x, u = 6;  // OK: v has type const int*, u has type const int
static auto y = 0.0;        // OK: y has type double
auto int r;                 // error: auto is not a storage-class-specifier
auto f() -> int;             // OK: f returns int
auto g() { return 0.0; }     // OK: g returns double
auto h();                    // OK, h's return type will be deduced when it is defined

— end example ]

The auto type-specifier A placeholder type can also be used in declaring a variable in the condition of a selection statement (6.4) or an iteration statement (6.5), in the type-specifier-seq in the new-type-id or type-id of a new-expression (5.3.4), in a for-range-declaration, and in declaring a static data member with a brace-or-equal-initializer that appears within the member-specification of a class definition (9.4.2).

A program that uses auto or decltype(auto) in a context not explicitly allowed in this section is ill-formed.

Once the type of a declarator-id has been determined according to 8.3, the type of the declared variable using the declarator-id When a variable declared using a placeholder type is initialized, or a return statement occurs in a function declared with a return type that contains a placeholder type, the deduced return type or variable type is determined from the type of its initializer. In the case of a return with no operand, the initializer is considered to be void(). Let T be the declared type of the variable or return type of the function.

If the placeholder is the auto type-specifier, the deduced type is determined using the rules for template argument deduction. Let T be the declared type that has been determined for a variable identifier d. If the deduction is for a return statement and the initializer is a braced-init-list (8.5.4), the program is ill-formed. Otherwise, Obtain P from T by replacing the occurrences of auto with either a new invented type template parameter U or, if the initializer is a braced-init-list (8.5.4), with std::initializer_list<U>. The type deduced for the variable d is then the deduced A determined Deduce a value for U using the rules of template argument deduction from a function call (14.8.2.1), where P is a function template parameter type and the initializer for d is the corresponding argument. If the deduction fails, the declaration is ill-formed. Otherwise, the type deduced for the variable or return type is obtained by substituting the deduced U into P. [ Example:

auto x1 = { 1, 2 };    // decltype(x1) is std::initializer_list<int>
auto x2 = { 1, 2.0 };  // error: cannot deduce element type

— end example ]

If the placeholder is the decltype(auto) type-specifier, the declared type of the variable or return type of the function shall be the placeholder alone. The type deduced for the variable or return type is determined as described in 7.1.6.2, as though the initializer had been the operand of the decltype. [ Example:

int i;
int&& f();
auto           x3a = i;        // decltype(x3a) is int
decltype(auto) x3d = i;        // decltype(x3d) is int
auto           x4a = (i);      // decltype(x4a) is int
decltype(auto) x4d = (i);      // decltype(x4d) is int&
auto           x5a = f();      // decltype(x5a) is int
decltype(auto) x5d = f();      // decltype(x5d) is int&&
auto           x6a = { 1, 2 }; // decltype(x6a) is std::initializer_list<int>
decltype(auto) x6d = { 1, 2 }; // error, { 1, 2 } is not an expression
auto          *x7a = &i;       // decltype(x7a) is int*
decltype(auto)*x7d = &i;       // error, declared type is not plain decltype(auto)

— end example ]

If the list of declarators contains more than one declarator, the type of each declared variable is determined as described above. If a function with a declared return type that contains a placeholder type has multiple return statements, the return type is deduced for each return statement. In either case, if the type deduced for the template parameter U is not the same in each deduction, the program is ill-formed. [ Example:

const auto &i = expr;

The type of i is the deduced type of the parameter u in the call f(expr) of the following invented function template:

template <class U> void f(const U& u);

— end example ]

If a function with a declared return type that uses a placeholder type has no return statements, the return type is deduced as though from a return statement with no operand at the closing brace of the function body. [ Example:

auto  f() { } // OK, return type is void
auto* g() { } // error, cannot deduce auto* from void()

— end example ]

If the type of an entity with an undeduced placeholder type is needed to determine the type of an expression, the program is ill-formed. But once a return statement has been seen in a function, the return type deduced from that statement can be used in the rest of the function, including in other return statements. [ Example:

auto n = n; // error, n's type is unknown
auto f();
void g() { &f; } // error, f's return type is unknown
auto sum(int i) {
  if (i == 1)
    return i;  // sum's return type is int
  else
    return sum(i-1)+i; // OK, sum's return type has been deduced
}

—end example]

Return type deduction for a function template with a placeholder in its declared type occurs when the definition is instantiated even if the function body contains a return statement with a non-type-dependent operand. [ Note: So any use of a specialization of the function template will cause an implicit instantiation. Any errors that arise from this instantiation are not in the immediate context of the function type, and can result in the program being ill-formed. —end note ] [ Example:

template <class T> auto f(T t) { return t; } // return type deduced at instantiation time
typedef decltype(f(1)) fint_t; // instantiates f<int> to deduce return type
template<class T> auto f(T* t) { return *t; }
void g() { int (*p)(int*) = &f; } // instantiates both 'f's to determine return types, chooses second

—end example]

Redeclarations or specializations of a function or function template with a declared return type that uses a placeholder type shall also use that placeholder, not a deduced type. [ Example:

auto f();
auto f() { return 42; } // return type is int
auto f(); // OK
int f();  // error, cannot be overloaded with auto f()
decltype(auto) f(); // error, auto and decltype(auto) don't match

template <typename T> auto g(T t) { return t; } // #1
template auto g(int);      // OK, return type is int
template char g(char);     // error, no matching template
template<> auto g(double); // OK, forward declaration with unknown return type

template <class T> T g(T t) { return t; } // OK, not functionally equivalent to #1
template char g(char);     // OK, now there is a matching template
template auto g(float);    // still matches #1

void h() { return g(42); } // error, ambiguous

template <typename T> struct A {
  friend T frf(T);
};
auto frf(int i) { return i; } // not a friend of A<int>

A function declared with a return type that uses a placeholder type shall not be virtual (10.3).

An explicit instantiation declaration (14.7.2) does not cause the instantiation of an entity declared using a placeholder type, but it also does not prevent that entity from being instantiated as needed to determine its type. [ Example:

template <typename T> auto f(T t) { return t; }
extern template auto f(int); // does not instantiate f<int>
int (*p)(int) = f;           // instantiates f<int> to determine its return type, 
                             // but an explicit instantiation definition is still required somewhere in the program

—end example ]

If a lambda-expression does not include a lambda-declarator, it is as if the lambda-declarator were (). If a lambda-expression does not include a trailing-return-type, it is as if the trailing-return-type denotes the following type:

• if the compound-statement is of the form

  { attribute-specifier-seqopt return expression ; }

  the type of the returned expression after lvalue-to-rvalue conversion (4.1), array-to-pointer conver- sion (4.2), and function-to-pointer conversion (4.3);
• otherwise, void.

The lambda return type is auto, which is replaced by the trailing-return-type if provided and/or deduced from return statements as described in 7.1.6.4. [ Example:

auto x1 = [](int i){ return i; }; // OK: return type is int
auto x2 = []{ return { 1, 2 }; }; // error: deducing return type from braced-init-list the return type is void (a
                                  // braced-init-list is not an expression)
int j;
auto x3 = []()->auto&& { return j; }; // OK: return type is int&

— end example ]

Except for inline functions, declarations with types deduced from their initializer or return value (7.1.6.4), and class template specializations, explicit instantiation declarations have the effect of suppressing the implicit instantiation of the entity to which they refer.

References