This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 110b. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2023-01-15


126. Exception specifications and const

Section: 14.5  [except.spec]     Status: TC1     Submitter: Martin von Loewis     Date: 8 June 1999

The standard is inconsistent about constness inside exception specifications.

    struct X {};
    struct Y:X {};

    const Y bar() {return Y();}

    void foo()throw(const X)
    {
      throw bar();
    }
It is unclear whether calling foo will result in a call to std::unexpected. According to 14.5 [except.spec] paragraph 7, only two cases are treated specially with regard to inheritance: If "class X" appears in the type-id-list, or if "class X*" appears in the type-id-list. Neither is the case here, so foo only allows exceptions of the same type (const X). As a result, std::unexpected should be called.

On the other hand, the intent of exception specification appears to allow an implementation of this example as

    void foo()
    try{
      throw bar();
    }catch(const X){
      throw;
    }catch(...){
      std::unexpected();
    }
According to 14.4 [except.handle] , this replacement code would catch the exception, so std::unexpected would not be called.

Suggested resolution: Change 14.5 [except.spec] paragraph 7 to read

A function is said to allow all exception objects of all types E for which one of the types T in the type-id-list would be a handler, according to 14.4 [except.handle] .

Proposed resolution (10/00):

Replace 14.5 [except.spec] paragraph 7 with the following:

A function is said to allow an exception of type E if its exception-specification contains a type T for which a handler of type T would be a match (14.4 [except.handle]) for an exception of type E.