According to the boost::fusion::map docs:
A map may contain at most one element for each key.
In practice, it is easy to violate this.
I am able to define the following type:
using map_type = fusion::map<
fusion::pair<int, char>
, fusion::pair<int, char>
, fusion::pair<int, char>>;
and instantiate it with these duplicate keys:
map_type m(
fusion::make_pair<int>('X')
, fusion::make_pair<int>('Y')
, fusion::make_pair<int>('Z'));
Iterating over the map using fusion::for_each shows the data structure does indeed contain 3 pairs, and each of the keys is of type int:
struct Foo
{
template<typename Pair>
void operator()(const Pair& p) const
{
std::cout << typeid(typename Pair::first_type).name() << "=" << p.second << '\n';
}
};
fusion::for_each(m, Foo {});
Output:
i=X
i=Y
i=Z
I would have expected a static_assert on key uniqueness, but this is obviously not the case.
Why is this?
How can I ensure that no one can instantiate a
fusion::mapwith duplicate keys?
Full working example: (on coliru)
#include <boost/fusion/container.hpp>
#include <boost/fusion/include/for_each.hpp>
#include <iostream>
namespace fusion = ::boost::fusion;
struct Foo
{
template<typename Pair>
void operator()(const Pair& p) const
{
std::cout << typeid(typename Pair::first_type).name() << "=" << p.second << '\n';
}
};
int main()
{
using map_type = fusion::map<
fusion::pair<int, char>
, fusion::pair<int, char>
, fusion::pair<int, char>>;
map_type m(
fusion::make_pair<int>('X')
, fusion::make_pair<int>('Y')
, fusion::make_pair<int>('Z'));
fusion::for_each(m, Foo {});
return 0;
}
Due to comments below, here are some further details on what I'm actually trying to achieve.
The idea is to automatically generate FIX serialisation code.
A given field type can only exist once in any given FIX message - hence wanting the static_assert
Motivating example: (on coliru)
#include <boost/fusion/container.hpp>
#include <boost/fusion/sequence.hpp>
#include <boost/fusion/include/for_each.hpp>
#include <boost/mpl/transform.hpp>
#include <iostream>
namespace fusion = ::boost::fusion;
namespace mpl = ::boost::mpl;
template<class Field>
struct MakePair
{
using type = typename fusion::result_of::make_pair<Field, typename Field::Type>::type;
};
template<class Fields>
struct Map
{
using pair_sequence = typename mpl::transform<Fields, MakePair<mpl::_1>>::type;
using type = typename fusion::result_of::as_map<pair_sequence>::type;
};
///////////////////////////
template<typename... Fields>
class Message
{
public:
template<class Field>
void set(const typename Field::Type& val)
{
fusion::at_key<Field>(_fields) = val;
}
void serialise()
{
fusion::for_each(_fields, Serialiser {});
}
private:
struct Serialiser
{
template<typename Pair>
void operator()(const Pair& pair) const
{
using Field = typename Pair::first_type;
std::cout << Field::Tag << "=" << pair.second << "|";
}
};
using FieldsVector = fusion::vector<Fields...>;
using FieldsMap = typename Map<FieldsVector>::type;
FieldsMap _fields;
static_assert(fusion::result_of::size<FieldsMap>::value == fusion::result_of::size<FieldsVector>::value,
"message must be constructed from unique types"); // this assertion doesn't work
};
///////////////////////////
#define MSG_FIELD(NAME, TYPE, TAG) \
struct NAME \
{ \
using Type = TYPE; \
static const int Tag = TAG; \
};
MSG_FIELD(MsgType, char, 35)
MSG_FIELD(Qty, int, 14)
MSG_FIELD(Price, double, 44)
using Quote = Message<MsgType, Qty, Price>;
///////////////////////////
int main()
{
Quote q;
q.set<MsgType>('a');
q.set<Qty>(5);
q.set<Price>(1.23);
q.serialise();
return 0;
}
