On Thu, Oct 20, 2011 at 7:55 AM, lcaminiti <lorcaminiti@gmail.com> wrote:
Hello all,
has_equal_to< std::vector<T> > always returns true_ because == is defined for std::vector<T> even when it is not defined for T. However, then an attempt to compare for equality std::vector<T> will generate a compiler error in this case because there is no == for T.
For example:
#include <boost/type_traits/has_equal_to.hpp> #include <vector> #include <iostream>
struct myvalue {};
//bool operator== ( myvalue const& l, myvalue const& r ) { return false; } // (1)
int main ( ) { typedef std::vector<myvalue> myvector; std::cout << boost::has_equal_to<myvector>::value << std::endl; // always returns true myvector v(1), w(1); std::cout << (v == w) << std::endl; // compiler error because (1) is not defined return 0; }
Shall has_equal_to< std::vector<T> > be smarter and return false_ if T does not have == ? (If not, this case should at least be documented.)
I'd venture to say this is a deficiency in std::vector's implementation of operator==, which ideally should SFINAE-out bindings of T which do not have an operator==. You get the same sort of problem if you have a class with an unbounded conversion constructor template: struct X { template< class T > X(T x) { /*...*/ } }; Here, I believe, boost::is_convertible< Y, X > will evaluate to true, and some bindings of Y to T may result in a compiler error (depending on what's in /*...*/). I *seem* to remember this was a problem with boost::variant, for example, but even if it wasn't, you can imagine how it could be (the template parameter in the constructor should only be bindable to one of the value_types of the variant). In my opinion, the upshot is that std::vector's operator== is almost certainly not a special case, so I don't know know if it's worth specializing has_equal_to on it or not. Maybe, if it's consistently done for all problematic std-defined operators? P.S. Are the strange HTML tags for < and > gone?
Yes, but the above quoted message only appeared when I clicked on the link: http://boost.2283326.n4.nabble.com/boost-type-traits-has-equal-to-lt-std-vec...