Question

I’m having trouble getting something to compile. I don’t understand the error thrown out
by the compiler. Some code to illustrate the problem is below.

#include <map>

using namespace std;

class Thing
{
public:
    Thing(int n):val(n) {}

    bool operator < (const Thing& rhs) const
    {
        return val < rhs.val;
    }

    int getVal() { return val; }

private:
    int val;
};


int main(int argc, char* argv[])
{
    std::map<Thing, int> mymap;
    Thing t1(1);
    Thing t2(10);
    Thing t3(5);

    mymap[t1] = 1; // OK

    mymap.insert(t1); // Compile error
}

Now the compiler error message:

test.cpp: In function ‘int main(int,
char**)’: test.cpp:34: error: no
matching function for call to
‘std::map,
std::allocator > >::insert(Thing&)’
/usr/include/c++/4.4/bits/stl_map.h:499:
note: candidates are:
std::pair,
std::_Select1st >, _Compare, typename _Alloc::rebind >::other>::iterator, bool> std::map<_Key, _Tp, _Compare,
_Alloc>::insert(const std::pair&) [with _Key = Thing, _Tp = int, _Compare = std::less,
_Alloc = std::allocator >]
/usr/include/c++/4.4/bits/stl_map.h:539:
note: typename
std::_Rb_tree<_Key, std::pair, std::_Select1st >, _Compare, typename _Alloc::rebind >::other>::iterator std::map<_Key, _Tp, _Compare,
_Alloc>::insert(typename std::_Rb_tree<_Key, std::pair, std::_Select1st >, _Compare, typename _Alloc::rebind >::other>::iterator, const std::pair&) [with
_Key = Thing, _Tp = int, _Compare = std::less, _Alloc =
std::allocator >]

What does that mean? Is there another method or operator I need to define in Thing to make it compile?

Answer 1

You need mymap.insert(std::pair<Thing,int>(t1,x));, where x is the value you want to map to t1.