One more foolishness of the C++ Standard
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From: Vladimir Grigoriev on 22 Jan 2010 10:39 I wanted to say that for example for the std::find algorithm the requirement of EqualityComparabality is not correctly or clear formulated because it is not obligatory that typename std::iterator_traits<InputIterator>::value_type is the same as T in the algorithm definition.:) In my example with Person an object of the type Person is converted to size_t and size_t is EqualityComparable indeed! Vladimir Grigoriev
From: Vladimir Grigoriev on 22 Jan 2010 11:35 So the resume is that the both forms of the algorithm std::find use a predicate. Only the first form of the std::find uses a special predicate which is written as an operator-function and as a result of such approach there is no need to specify it as a parameter in the first form of std::find. Vladimir Grigoriev
From: sasha on 22 Jan 2010 14:55 Duane Hebert wrote: > > I guess I don't understand what you mean by the STL algorithms using > constructors. Can you give me an example of one that has to construct > something? What I meant to say of an algorithm needs to iterate over an array of IDs, but there is no constructor that takes just an ID, then there is a problem.
From: Ulrich Eckhardt on 25 Jan 2010 03:42 Vladimir Grigoriev wrote: > However try to provide that a + a will not compute when a + b, b + a, and > b + b will compute for two classes when one is derived from another.. Simple, provide these three overloads and no others: b operator+(a const& l, b const& r); b operator+(b const& l, a const& r); b operator+(b const& l, b const& r); However, I can't think of a reason for this either. Think of this: b some_b; a& a_ref = some_b; a_ref+a_ref; // now what? struct c: b {...}; c some_c; b& b_ref = some_c; b_ref+b_ref; // and here? Generally, operator+ is something that rather fits for value types. Public inheritance usually means the type is rather an entity type, which don't like being treated as a value. Uli -- C++ FAQ: http://parashift.com/c++-faq-lite Sator Laser GmbH Geschäftsführer: Thorsten Föcking, Amtsgericht Hamburg HR B62 932
From: Vladimir Grigoriev on 25 Jan 2010 05:58
Ulrich, I forgot to add that in the derived class the constructor for the base class must be. B( const A & ); In this case two conversions can be: B ==> A and A ==> B and an ambiguous reference can occur. Vladimir Grigoriev .. "Ulrich Eckhardt" <eckhardt(a)satorlaser.com> wrote in message news:i42037-u4l.ln1(a)satorlaser.homedns.org... > Vladimir Grigoriev wrote: >> However try to provide that a + a will not compute when a + b, b + a, and >> b + b will compute for two classes when one is derived from another.. > > Simple, provide these three overloads and no others: > > b operator+(a const& l, b const& r); > b operator+(b const& l, a const& r); > b operator+(b const& l, b const& r); > > However, I can't think of a reason for this either. Think of this: > > b some_b; > a& a_ref = some_b; > a_ref+a_ref; // now what? > > struct c: b {...}; > c some_c; > b& b_ref = some_c; > b_ref+b_ref; // and here? > > Generally, operator+ is something that rather fits for value types. Public > inheritance usually means the type is rather an entity type, which don't > like being treated as a value. > > Uli > > -- > C++ FAQ: http://parashift.com/c++-faq-lite > > Sator Laser GmbH > Geschaftsfuhrer: Thorsten Focking, Amtsgericht Hamburg HR B62 932 |