This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 115d. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-10-26

1591. Deducing array bound and element type from initializer list

Section: 13.10.3.2  [temp.deduct.call]     Status: CD4     Submitter: Peter Dimov     Date: 2012-12-01

[Moved to DR at the November, 2014 meeting.]

Currently, 13.10.3.2 [temp.deduct.call] paragraph 1 says,

Template argument deduction is done by comparing each function template parameter type (call it P) with the type of the corresponding argument of the call (call it A) as described below. If removing references and cv-qualifiers from P gives std::initializer_list<P'> for some P' and the argument is an initializer list (9.4.5 [dcl.init.list]), then deduction is performed instead for each element of the initializer list, taking P' as a function template parameter type and the initializer element as its argument. Otherwise, an initializer list argument causes the parameter to be considered a non-deduced context (13.10.3.6 [temp.deduct.type]).

It would seem reasonable, however, to allow an array bound to be deduced from the number of elements in the initializer list, e.g.,

  template<int N> void g(int const (&)[N]);
  void f() {
    g( { 1, 2, 3, 4 } );
  }

Additional note (March, 2013):

The element type should also be deducible.

Proposed resolution (February, 2014):

  1. Change 13.10.3.2 [temp.deduct.call] paragraph 1 as follows:

    2. Template argument deduction is done by comparing each function template parameter type (call it P) with the type of the corresponding argument of the call (call it A) as described below. If P is a dependent type, removing references and cv-qualifiers from P gives std::initializer_list<P'> or P'[N] for some P' and N, and the argument is an a non-empty initializer list (9.4.5 [dcl.init.list]), then deduction is performed instead for each element of the initializer list, taking P' as a function template parameter type and the initializer element as its argument, and in the P'[N] case, if N is a non-type template parameter, N is deduced from the length of the initializer list. Otherwise, an initializer list argument causes the parameter to be considered a non-deduced context (13.10.3.6 [temp.deduct.type]). [Example: 

      template<class T> void f(std::initializer_list<T>);
  f({1,2,3});              // T deduced to int
  f({1,"asdf"});           // error: T deduced to both int and const char*

  template<class T> void g(T);
  g({1,2,3});              // error: no argument deduced for T

  template<class T, int N> void h(T const(&)[N]);
  h({1,2,3});              // T deduced to int, N deduced to 3

  template<class T> void j(T const(&)[3]);
  j({42});                 // T deduced to int, array bound not considered

  struct Aggr { int i; int j; };
  template<int N> void k(Aggr const(&)[N]);
  k({1,2,3});              // error: deduction fails, no conversion from int to Aggr
  k({{1},{2},{3}});        // OK, N deduced to 3

  template<int M, int N> void m(int const(&)[M][N]);
  m({{1,2},{3,4}});        // M and N both deduced to 2

  template<class T, int N> void n(T const(&)[N], T);
  n({{1},{2},{3}},Aggr()); // OK, T is Aggr, N is 3

    —end example] For a function parameter pack...

  2. Change the penultimate bullet of 13.10.3.6 [temp.deduct.type] paragraph 5 as follows:

    3. The non-deduced contexts are: