c++ multimap insert more than two values

Question

I have found that it's possible to declare such std::multimap:

multimap < u_int32_t, u_int32_t,string> lines;

If it's possible to declare it then it should be possible to insert too

But I wonder how?

I have tried std::pair, but it seems I need something like std::triple.

I know it's possible to decrale some struct and hold into that struct a few values. But I would rather prefer to do it directly. Moreover because it's possible to declare it.

EDIT
I did serious mistake and it turned out I really understood multimap wrong.
Screams of people here and downvotes made me to reread documentation. Now I use it so:

struct container {
u_int32_t  size_in_blocks;
string name_of_file;
};
            //size_of_file
multimap <  u_int32_t, container> lines;
       // first value is used as a key for sorting
       // second value is just a storage

container d;// initialization
lines.insert ( std::pair<u_int32_t,container>( total_size_bytes, d) );

Thanks all!

What is your intention? Why do you think you need a `multimap`? — kirelagin, Jun 03 '13 at 14:12
ok, I need multimap but not map, because a few files can have the same size, so for avoiding collisions I use multimap. — Tebe, Jun 03 '13 at 14:22
OK, that's a valid usecase for multimap. Now, just use it as a normal map. Why do you pass it three template arguments? — kirelagin, Jun 03 '13 at 14:24
I don't pass now three arguments. I DID pass, because I misunderstood it. — Tebe, Jun 03 '13 at 14:25

score 5 · Answer 1 · answered Jun 03 '13 at 14:05

template < class Key,                                     // multimap::key_type
           class T,                                       // multimap::mapped_type
           class Compare = less<Key>,                     // multimap::key_compare
           class Alloc = allocator<pair<const Key,T> >    // multimap::allocator_type
           > class multimap;

The third template argument is for comparator. Your code doesn't make any sense. Having a multimap with three types doesn't make any sense either. It maps from keys to values, what the third type is supposed to mean?

score 5 · Accepted Answer · answered Jun 03 '13 at 14:08

This is wrong:

multimap < u_int32_t, u_int32_t,string> lines;

The template parameters for multimap are listed at en.cppreference.com:

template<
    class Key,
    class T,
    class Compare = std::less<Key>,
    class Allocator = std::allocator<std::pair<const Key, T> >
> class multimap;

The first template parameter is the key, the second is the type stored, and the third is the comparator.

You have specified std::string as the comparator. Clearly this won't do what you want, and I'm somewhat suprised this even compiles. basic_string does have an operator< -- that must be why it compiles.

I think you are confused as to what multimap really is. multimap is not something that can be used to map between a key and one of mopre different kinds of values. multimap is the same as map in that it maps between a single key and a value, except the difference is that with multimap you can have more than just one value mapped to a single key.

*"basic_string does have an operator< -- that must be why it compiles."* - This won't make it work either, since the comparator is invoked with the function call operator `operator()` and not `operator<`. Otherwise correct answer, of course. — Christian Rau, Jun 03 '13 at 14:15

c++ multimap insert more than two values

2 Answers2