Question

I’m creating a std::map<int, int> in C++ that I’d prefer to have they keys sorted from highest to lowest instead of the default sort order. My research lead me to std::greater which looked promising but when trying to use it I’m getting a compile error:

invalid type argument of unary ‘*’ (have ‘int’)

My map declaration is:

std::map<int, int, std::greater<int> > numMap;

And the error is getting thrown from this function:

void Row::addNumber(int num, int pos) {
    numMap.insert(num, pos);
}

Answers to similar questions such as this include parenthesis in the declaration, i.e. std::greater() – but when I include those I get multiple errors regarding a function returning a function.

Answer 1

The problem – call of std::map::insert member function with invalid parameters: there are two integer values provided; but there must be std::pair<int, int>. Please see the reference: std::map::insert.

Preferable option

For convenience (just not to repeat the map type parameters), create a typedef for the map:

typedef std::map<int, int> IntMap;

The std::map has type definition for std::pair (pair representation) – std::map::value_type.
So, for example, if there is a std::map<int, int> the std::map::value_type would be std::pair<int, int>.

Use the std::map::value_type constructor (IntMap::value_type in this case):

class Row {
public:
    void Row::addNumber(int num, int pos)
    {
        m_numMap.insert(IntMap::value_type(num, pos));
    }

private:
    typedef std::map<int, int> IntMap;
    IntMap m_numMap;
};

Alternatives:

1. Use std::make_pair() function:

   #include <utility>
   
   ...
   
   void Row::addNumber(int num, int pos)
   {
       numMap.insert(std::make_pair(num, pos));
   }
   

2. Directly use std::pair constructor:

   void Row::addNumber(int num, int pos)
   {
       numMap.insert(std::pair<int, int>(num, pos));
   }