No hash function for classes containing conversion operation to ordinary type

[Robert Luberda <robert@…>]

Hi

It seems boost::hash no longer supports calculating hashes for classes that are convertible to int. It used to work with previous versions of boost::hash

Please see the following example (based on example from bug#7437), which compiles fine with boost::hash up to version 1.50, but fails to compile with boost 1.51

#include <boost/unordered_map.hpp>

class MyClass
{
public:
    MyClass(int i=0): v(i) {}
    operator int() const {return v;}
private:
    int v;
};

typedef boost::unordered_map<MyClass,std::string> MyMap;

int main(int argc, char* argv[])
{
    MyMap m;
    MyMap::value_type v(MyClass(1),"apples");
    m.insert(v);

    return 0;
}

The error is:

/opt//boost_1_51_0/include/boost/functional/hash/extensions.hpp:257: error: no matching function for call to 'hash_value(const MyClass&)'

[anonymous]

This is deliberate, implicit conversions can cause subtle errors. For a case like this it's pretty easy to add support with something like:

friend std::size_t hash_value(MyClass const& v) { return (int) v; }