Count of parameters in a parameter pack? Is there

问题:

I was just wondering if there was anything in the C++0x std lib already available to count the number of parameters in a parameter pack? I'd like to get rid of the field_count in the code below. I know I can build my own counter, but it just seems like this would be an obvious thing to include in the C++0x std lib, and I wanted to be sure it wasn't already there :) Home-grown counter implementations are most welcome too.

template<const int field_count, typename... Args> struct Entity {
    const tuple<Args...> data;
    const array<const char*, field_count> source_names;

    Entity() : data() {
    }
};

回答1:

Yes, you can use sizeof.... From the C++0x FCD (§5.3.3/5):

The identifier in a sizeof... expression shall name a parameter pack. The sizeof... operator yields the number of arguments provided for the parameter pack identifier. The parameter pack is expanded (14.5.3) by the sizeof... operator. [Example:

template<class... Types>
struct count {
    static const std::size_t value = sizeof...(Types);
};

—end example ]

回答2:

Here's a link that may help you. Sample source from link:

template<typename... Args> struct count;

template<>
struct count<> {
    static const int value = 0;
};

template<typename T, typename... Args>
struct count<T, Args...> {
    static const int value = 1 + count<Args...>::value;
};