Why that partial template specialization is allowed?

Question

I've looked at a partial template specialization argument-list and stumbled upon such an example

template <typename>
class function;
template <typename ReturnValue, typename Args,typename Args2>
class function<ReturnValue(Args,Args2)>
{
    //smth
};

What exactly means ReturnValue(Args,Args2) (couse it's not a type as I know) and by what rules can I write similar things?

Thanks

This is [function signature template](https://stackoverflow.com/questions/4642079/function-signature-like-expressions-as-c-template-arguments). Details actually can be found in this [book](https://en.wikipedia.org/wiki/Modern_C%2B%2B_Design) — Victor Gubin, Apr 25 '19 at 08:33
`ReturnValue(Args,Args2)` *is* a type (a function returning type `ReturnValue` and taking two arguments of type `Args` and `Args2`). — WhozCraig, Apr 25 '19 at 08:35

score 4 · Accepted Answer · answered Apr 25 '19 at 08:34

4

couse it's not a type as I know

Actually, it is. ReturnValue(Args,Args2) is the type "function taking Args and Arg2 and returning ReturnValue". Take this example:

void foo(int, char);
typedef void FunctionType(int, char);

Here, FunctionType is indeed the type void(int, char), which is also the type of foo.

answered Apr 25 '19 at 08:34

Angew is no longer proud of SO

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But why I can't modify the example above like this: ```class function``` ? – Poarthur Apr 25 '19 at 08:44
3

@Poarthur Because `function` has *one* template parameter, while you'd be passing *two* arguments: the type `ReturnValue(Args)`, and the type `Args2`. – Angew is no longer proud of SO Apr 25 '19 at 08:48

Why that partial template specialization is allowed?

1 Answers1