On Fri, Sep 29, 2006 at 11:58:48AM -0400, David Abrahams wrote:
Joel de Guzman <joel@boost-consulting.com> writes:
Here's another problem:
std::map<int*, int*> m;
generic code that treats pair<int* int*> as an iterator_range will treat m's data type incorrectly.
No, generic code doesn't look at types and say, "which of two concepts with no refinement relationship does this type fulfill?" and then take different actions based on the answer. The only such type-based dispatching in generic code comes through algorithm specialization: "this type fulfills concept X, but also concept Y which is a refinement of X, so I'll use that information to do whatever I was going to do more efficiently."
The problem here is that there is one concept, namely a runtime sequence, that pair<int*,int*> fulfills in two unrelated ways. So generic code does need to be able to decide which way to interpret it. Or the concepts of iterator_range and runtime sequence have to be separated, which is unfortunate, since iterator_range is a very natural refinement of a runtime sequence. Geoffrey