I'd like to iterate over a std::map using BOOST_FOREACH and edit the values. I can't quite get it.

typedef std::pair<int, int> IdSizePair_t;
std::map<int,int> mmap;    
mmap[1] = 1;
mmap[2] = 2;
mmap[3] = 3;
BOOST_FOREACH( IdSizePair_t i, mmap )
    i.second++;
// mmap should contain {2,3,4} here

Of course this doesn't change anything because I'm not iterating by reference. So I substitute this line instead (as per the example in the Boost docs):

BOOST_FOREACH( IdSizePair_t &i, mmap )

and I get the compiler error:

error C2440: 'initializing' : 
cannot convert from 'std::pair<_Ty1,_Ty2>' to 'IdSizePair_t &'
    with
    [
        _Ty1=const int,
        _Ty2=int
    ]

Any suggestions?