Question

I am working on a recursive map class called group_by which models the SQL namesake.

For example, gb is a group_by object which will store pointers to foo grouped by std::string, int, and char key types, in that order.

group_by<foo,std::string,int,char>  gb;

group_by provides an at( I const& key ) accessor method which can be used to look inside current level map. Chaining at() calls to retrieve deeper maps works fine.

auto& v = gb.at( k1 ).at( k2 ).at( k3 ).get_vec();

PROBLEM

I would like to create an alternative of at() called at_variadic( Args const& ...args ) which can retrieve deeper maps all in one call, without chaining.

auto& w = gb.at_variadic( k1, k2 );
auto& x = gb.at_variadic( k1, k2, k3 );

However, I am running into some issues. First of all, I don’t know how to specify the return type, since it depends on the variadic arguments. Maybe use decltype(), somehow?

WORKING ANSWER

Ecatmur’s answer below outlined a good approach.

I had to play around with terminal case of group_by<> to make the compiler happy, but the code below, heavily based on Ecatmur’s answer, seems to work fine with gcc 4.7.2.

#include <cassert>
#include <map>
#include <vector>
#include <iostream>

template< typename T, typename... Args >
struct group_by
{
    using child_type = T;

    std::vector<T*>  m_vec;

    void insert( T* t ) 
    {
        m_vec.push_back( t );
    }

    child_type&
    at( size_t i )
    {
        return *m_vec[i];
    }
};

template< typename T, typename I, typename... Args >
struct group_by<T,I,Args...>
{  
    using child_type = group_by<T,Args...>;

    std::map<I,child_type>  m_map;

    void insert( T* t ) 
    {
        m_map[ *t ].insert( t );
    }

    child_type& at( I const& key ) 
    {
    return m_map.at( key );
    }

    template<typename... Ks>
    auto
    at( I const& i, Ks const&...ks )
    -> decltype( m_map.at( i ).at( ks... ) )
    {
        return m_map.at( i ).at( ks... );
    }
};

// -----------------------------------------------------------------------------

struct foo
{
    std::string  s;
    int          i;
    char         c;

    operator std::string() const { return s; }
    operator int        () const { return i; }
    operator char       () const { return c; }

    bool operator==( foo const& rhs ) const
    {
        return s==rhs.s && i==rhs.i && c==rhs.c;
    }
};

int main( int argc, char* argv[] )
{
    foo f1{ "f1", 1, 'z' };
    foo f2{ "f2", 9, 'y' };
    foo f3{ "f3", 3, 'x' };
    foo f4{ "f1", 4, 'k' };

    group_by<foo,std::string,int,char>  gb;

    gb.insert( &f1 );
    gb.insert( &f2 );
    gb.insert( &f3 );
    gb.insert( &f4 );

    std::string k1{ "f1" };
    int         k2{ 1    };
    char        k3{ 'z'  };

    auto& a = gb.at( k1 ).at( k2 ).at( k3 ).at( 0 );
    auto& b = gb.at( k1 ).at( k2 ).m_map;
    auto& c = gb.at( k1 ).m_map;
    auto& d = gb.at( k1, k2 ).m_map;
    auto& e = gb.at( k1, k2, k3 ).m_vec;
    auto& f = gb.at( k1, k2, k3, 0 );

    assert( a==f1 );
    assert( b.size()==1 );
    assert( c.size()==2 );
    assert( d.size()==1 );
    assert( e.size()==1 );
    assert( f==f1 );

    return 0;
}

Answer 1

Chained method calls are essentially recursive, so you need to implement at recursively:

child_type& at( I const& key ) {
    return m_map.at( key );
}

template<typename J, typename... Ks>
auto at(const I &i, const J &j, const Ks &...ks)
-> decltype(m_map.at(i).at(j, ks...)) {
    return m_map.at(i).at(j, ks...);
}

Note that since at requires at least 1 argument, the variadic form takes at least 2 parameters. This is significantly easier to implement than dispatching on sizeof..., and should be easier to read.