I'm pretty sure I'm have a 'Doh!' moment here, but why does this compile?

Surely the last line of main is gibberish; the value type of m is pair, not S, so int_from_S

is not a valid function?

#include <boost/range.hpp>

#include <boost/range/adaptor/transformed.hpp>

#include <map>

struct S

{

S( ) { }

int i;

};

int int_from_S( S s ) { return s.i; }

int main( )

{

std::map<unsigned, S> m;

m[0]=S();

m | boost::adaptors::transformed(int_from_S);

}