How to get the number of fields in a boost-hana adapted struct?-CodePudding

Say I have the following struct:

struct MyStruct
{
    int field1;
    float field2;
};

I would like to obtain the number of fields in the struct using boost-hana.

#include <boost/hana/adapt_struct.hpp>


BOOST_HANA_ADAPT_STRUCT(MyStruct, position, field1, field2);

// this code does not work:
constexpr std::size_t NUMBER_OF_FIELDS = boost::hana::length<MyStruct>();

static_assert(NUMBER_OF_FIELDS  == 2);

How to get the number of fields in a boost-hana adapted struct?

CodePudding user response：

Hana specifically aims to simplify meta-programming by lifting type functions to the constexpr domain. In plain English: you shouldn't be using length<> as a "type function template" but as a normal function:

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struct MyStruct
{
    int position;
    int field1;
    float field2;
};

#include <boost/hana.hpp>

BOOST_HANA_ADAPT_STRUCT(MyStruct, position, field1, field2);

constexpr std::size_t NUMBER_OF_FIELDS = boost::hana::size(MyStruct{});
static_assert(NUMBER_OF_FIELDS  == 3);

int main(){}