Count of parameters in a parameter pack? Is there a C++0x std lib function for this?

c++ templates c++11 std variadic
Brett Rossier picture Brett Rossier · Sep 2, 2010 · Viewed 12.6k times · Source

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() {
    }
};

Answer

James McNellis picture James McNellis · Sep 2, 2010

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 ]

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