This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 118e. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2025-11-05

1336. Definition of “converting constructor”

[Voted into the WP at the February, 2012 meeting; moved to DR at the October, 2012 meeting.]

Although the normative wording of 11.4.8.2 [class.conv.ctor] paragraph 1 defining a converting constructor says

A constructor declared without the function-specifier explicit specifies a conversion from the types of its parameters to the type of its class.

implying that a constructor with multiple parameters can be a converting constructor, it would be helpful if the example contained such a constructor.

Proposed resolution (August, 2011):

1. Change the example in 11.4.8.2 [class.conv.ctor] paragraph 1 as follows:

  struct X {
      X(int);
      X(const char*, int =0);
      X(int, int);
  };

  void f(X arg) {
    X a = 1;          // a = X(1)
    X b = "Jessie";   // b = X("Jessie",0)
    a = 2;            // a = X(2)
    f(3);             // f(X(3))
    f({1, 2});        // f(X(1,2))
  }

2. Change the example in 11.4.8.2 [class.conv.ctor] paragraph 2 as follows:

  struct Z {
    explicit Z();
    explicit Z(int);
    explicit Z(int, int);
  };

  Z a;                       // OK: default-initialization performed
  Z a1 = 1;                  // error: no implicit conversion
  Z a3 = Z(1);               // OK: direct initialization syntax used
  Z a2(1);                   // OK: direct initialization syntax used
  Z* p = new Z(1);           // OK: direct initialization syntax used
  Z a4 = (Z)1;               // OK: explicit cast used
  Z a5 = static_cast<Z>(1);  // OK: explicit cast used
  Z a6 = { 3, 4 };           // error: no implicit conversion