无法推断模板参数

Question

Part of the class I am implementing looks like this:

    struct Cord
    {
        int x_cord;
        int y_cord;
        Cord(int x = 0,int y = 0):x_cord(x),y_cord(y) {}
        bool operator()(const Cord& cord) const
        {
            if (x_cord == cord.x_cord)
            {
                return y_cord < cord.y_cord;
            }
            return x_cord < cord.x_cord;
        }
    };
class Cell
    {

    };
std::map<Cord,Cell> m_layout;

I can`t compile the code above getting

error C2784: 'bool std::operator <(const std::basic_string<_Elem,_Traits,_Alloc> &,const _Elem *)' : could not deduce template argument for 'const std::basic_string<_Elem,_Traits,_Alloc> &' from 'const Layout::Cord'

Any advices?

Answer 1

Your operator() should be operator< :

    bool operator<(const Cord& cord) const
    {
        if (x_cord == cord.x_cord)
        {
            return y_cord < cord.y_cord;
        }
        return x_cord < cord.x_cord;
    }

operator< is what std::map uses to order its keys.

Answer 2

You could fix this by providing an operator<(const Cord&, const Cord&) :

// uses your operator()
bool operator<(const Cord& lhs, const Cord& rhs) { return lhs(rhs);)

or re-naming operator()(const Cord& cord) const to operator<(const Cord& cord) const

Answer 3

You are using your class in a map and it needs to define operator< for it.

// ...
bool operator<(const Cord& cord) const
{
  if (x_cord == cord.x_cord)
    return y_cord < cord.y_cord;
  return x_cord < cord.x_cord;
}
// ...