Re: User defined attributes use

[ilya-stromberg]

On Monday, 16 September 2013 at 07:36:13 UTC, simendsjo wrote:

I don't have a full example without adding a lot of code, but this partial

example might give you the gist of it.


// This is the type that validates
struct matches(string mustMatch)
{
    alias re = ctRegex!(mustMatch);

    static string[] validate(T)(const ref T t)
    {
        static if(!isSomeString!T)

static assert(0, "matches only works on strings, not "~T.stringof);

        return match(t, re).empty ? ["no match"] : null;
    }
}

// and this is the code that runs all validators for a variable
void validate(alias T)(ref Appender!(string[]) app)
{
    static if(isTupleWrapper!T)
    {
        validate!(T.Head)(app);
        validate!(T.Tail)(app);
    }
    else
    {
        foreach(memberAttr; getValidaterAttrs!T)
        {
            foreach(attr; memberAttr.Tail)
            {
                foreach(msg; attr.validate(T))
                    if(msg.length)
                        app.put(msg);
            }
        }
    }
}

// .. And here is some of the plumbing

string[] validate(Vars...)()
{
    auto app = appender!(string[])();
    validate!Vars(app);
    return app.data();
}

// The getMembersAndAttributesWhere are templates in my little library that isn't released. Uses quite some custom __traits stuff, but it's basically __traits(getAttributes

template getValidaterAttrs(alias T)
{

alias getValidaterAttrs = TypeTuple!(getMembersAndAttributesWhere!(T, isValidationAttr).Elements, getMembersAndAttributesWhere!(TypeOf!T, isValidationAttr).Elements);

}

// Well.. Incomplete
template isValidationAttr(alias T)
{
    enum isValidationAttr = hasMember!(TypeOf!T, "validate");
}

Can I explicitly specify when I can use attribute? Something like
this:

@attribute("field")
struct matches(string mustMatch)
{
}

string wrongAttribute
{
}

class Foo
{
    @matches("[0-9]+")
    string someNumber; //OK, it's a field
}

@matches("[0-9]+") //Error, it's a class, not a field
class Bar
{
}

@wrongAttribute //Error, this attribute doesn't exist
class C
{
}