On Sep 28, 2006, at 6:09 PM, Andy Little wrote:
The following doesnt work though ( nor with the std:: versions of the Concepts)
Output is : test.cpp: In function 'T sum(T*, int)': test.cpp:28: error: no match for 'operator=' in 'result = Addable<T>::operator+(result, array[i])' test.cpp:19: note: candidates are: typename Assignable<T>::result_type Assignable<T>::operator=(T&, const T&)
Right. Let's take a closer look...
auto concept Addable<typename T> { typename result_type; result_type operator+(T x, T y); };
This says that you can add two T's with operator+, and the result is a value of some type result_type (we don't know what that is).
auto concept Assignable<typename T> { typename result_type; result_type operator=(T& x, T y); };
This says we can assign from a T to a T.
result = result + array[i];
So, result + array[i] returns a value of type Addable<T>::result_type. However, there's no requirement on sum that says that we can assign from an Addable<T>::result_type to a T... we only say that we can assign from a T to a T! There are a couple ways to fix this. You could make the Assignable concept take two parameters (e.g., assign to a T from a U), or you could add a Convertible constraint that says that the result_type needs to be convertible to T, e.g., std::Convertible<Addable<T>::result_type, T> I added this to the where clause, and things work as expected. Doug