[Boost-bugs] [Boost C++ Libraries] #3270: Ambiguous conversion, even though no conversion exists

Subject: [Boost-bugs] [Boost C++ Libraries] #3270: Ambiguous conversion, even though no conversion exists
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2009-07-16 14:29:57

#3270: Ambiguous conversion, even though no conversion exists
-------------------------------------------------+--------------------------
 Reporter:  Olaf Lenz <lenzo@…>                  |       Owner:  dgregor 
     Type:  Bugs                                 |      Status:  new     
Milestone:  Boost 1.40.0                         |   Component:  function
  Version:  Boost 1.39.0                         |    Severity:  Problem 
 Keywords:                                       |  
-------------------------------------------------+--------------------------
 Cheers from Germany!

 Look at the following code:
 {{{
 #include <boost/function.hpp>

 struct A {};

 struct B {
   void f(const A &a) const {}
   void f(boost::function< void () > a) {}
 };

 int main() {
   A a;
   B b;
   b.f(a);
 }
 }}}

 Compiling this, I get the error:
 {{{
 > g++ boosttest.cpp -o boosttest
 boosttest.cpp: In function ‘int main()’:
 boosttest.cpp:13: error: ISO C++ says that these are ambiguous, even
 though the worst conversion for the first is better than the worst
 conversion for the second:
 boosttest.cpp:6: note: candidate 1: void B::f(const A&) const
 boosttest.cpp:7: note: candidate 2: void B::f(boost::function<void ()(),
 std::allocator<void> >)
 }}}

 When I define B::f() in the first version non-const, the program compiles
 without any problems.
 Usually, the problem in these cases is, that the conversion from an A to a
 const A object is seen as an operation as bad as the conversion of the
 object to another, therefore it is ambiguous and can be healed by using
 "explicit" in the conversion operator of the other class. Unfortunately,
 in this case, there does not even exist a conversion from "class A" to the
 function type! To test that, just comment the first definition of B::f():

 {{{
   f(const A &a) const {}
 }}}

 compilation fails with:
 {{{
 > g++ boosttest.cpp -o boosttest
 [clip]
 boosttest.cpp:13:   instantiated from here
 /usr/include/boost/function/function_template.hpp:136: error: no match for
 call to ‘(A) ()’
 }}}

 I think that this can be considered a bug. Or what else can I do in this
 case?

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/3270>
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