Concepts vs template constraints - the practical approach

[Norbert Nemec]

Nice! That really improves things! Looking forward to playing with it a 
little more...



On 20.11.2011 20:31, Denis Shelomovskij wrote:
> Your example, a bit improved:
> ---
> // In std.concept, e.g.
> template verify(alias concept, T) {
> //static assert(isConcept!concept, concept.stringof ~ " is not a
> concept"); //the test for particular concept's concepts can be added
> mixin concept!T;
> }
>
> template verify(alias concept) {
> mixin verify!(concept, typeof(this));
> }
>
> template satisfy(alias concept, T) {
> enum satisfy = __traits(compiles, verify!(concept, T));
> }
>
> // User code
> template myConcept(A) {
> static assert(is(A.type), "Error 1: !is(A.type)");
> static assert(A.len >= 0, "Error 2: A.len < 0");
> static assert(is(typeof(A.init[0]) == A.type), "Error 3:
> typeof(A.init[0]) != A.type");
> }
>
> struct MyClass(T,int R) {
> alias T type;
> enum len = R;
>
> T[R] value;
>
> T opIndex(int idx) {
> return value[idx];
> }
>
> mixin verify!myConcept;
> }
>
> void myFunction(A)(A arr) {
> mixin verify!(myConcept, A);
> }
>
> void myOverlodedFunction(A)(A arr) if(satisfy!(myConcept, A)) {
> // Do something
> }
>
> void myOverlodedFunction(A)(int i, A arr) if(satisfy!(myConcept, A)) {
> // Do something
> }
>
> void myOverlodedFunction(T...)(T) {
> static assert(0, "Error: here should be some user-defined error
> message(s) based on T");
> }
>
> unittest {
> MyClass!(int,4) x;
> myFunction(x);
> //myFunction(1); //Error: static assert "Error 1: !is(A.type)"
> myOverlodedFunction(x);
> myOverlodedFunction(3, x);
> //myOverlodedFunction(3, 2); //Error: static assert "Error: here should
> be..."
> }
> ---