How to sort C++ map keys with std::greater?
c++
Solution
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:
Use `std::make_pair()` function:
#include <utility>
...
void Row::addNumber(int num, int pos)
{
numMap.insert(std::make_pair(num, pos));
}
Directly use `std::pair` constructor:
void Row::addNumber(int num, int pos)
{
numMap.insert(std::pair<int, int>(num, pos));
}
Problem
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.