How to declare a template type as parameter to functions

[H. S. Teoh via Digitalmars-d-learn]

On Tue, Nov 04, 2014 at 12:29:54AM +0000, Domingo via Digitalmars-d-learn wrote:
[...]
> Like the "simple" function that I tried to write to not duplicate code
> is not worth because I'll need to write one for each type of
> MongoCursor:
> 
> 1 - MongoCursor!(Bson, Bson, typeof(null))
> 2 - MongoCursor!(Bson, Bson, Bson)
> 3 - MongoCursor!(Bson, Bson, int[string])
> 
> protected void sendCollectionListAsDataArrayJson2(T)(T
> collection_list, HTTPServerResponse res)
> 
> What will be "T" on the function above ?

T is a placeholder identifier that can be any accepted type. For
example:

	// Note: only one copy of func needs to be written
	void func(T)(T x) {
		writeln("%s", x);
	}

	void main() {
		func(1); // you can pass an int
		func("x"); // or a string
		func(1.0); // or a float

		struct S {}
		S s;
		func(s); // or a struct
	}


> If I need to write one for each of then My intent of prevent
> duplicated code is dead.
> The function only use code that should work on any combination of
> MongoCursor template.

That's exactly what you need a template function for. Instead of writing
15 copies of the function, one for each different MongoCursor type, you
write only a single function that takes a generic parameter, for
example:

	void sendCollection(T)(MongoCursor!(Bson, Bson, T) list,
		HttpRequest req)
	{
		...
	}

	...
	HttpRequest req;
	MongoCursor!(Bson, Bson, int) x1;
	MongoCursor!(Bson, Bson, float) x2;
	MongoCursor!(Bson, Bson, int[string]) x3;

	// N.B.: you can pass any matching type
	sendCollection(x1, req);
	sendCollection(x2, req);
	sendCollection(x3, req);

Of course, the foregoing assumes that only the last parameter of
MongoCursor varies. If you need to take MongoCursor of *any* combination
of parameters, you can use multiple template parameters, e.g.:

	// Now this will work with MongoCursor!(Bson, Bson,
	// int[string]), MongoCursor!(Json, Bson, Bson),
	// MongoCursor!(string, int, float), etc..
	void sendCollection(T,U,V)(MongoCursor!(T,U,V) list, ...) {
		...
	}


T

-- 
Those who don't understand D are condemned to reinvent it, poorly. -- Daniel N