I want to create custom compare function for std::sort(), to sort some key-value pairs std::pair
Here is my function
template <typename K, typename V>
int comparePairs(const void* left, const void* right){
if((((pair<K,V>*)left)->first) <= (((pair<K,V>*)right)->first))
return 1;
else
return -1;
}
Then, inside some class I have vector of pairs class member:
vector<pair<K,V>> items;
And some method for sort this vector by keys, using std::sort()
std::sort(items.begin(), items.end(), comparePairs<K,V>);
I have compilation errors within , which said
"cannot convert parameter number from 'std::pair<_Ty1,_Ty2>' to 'const void*'"
. What is a mistake?
This question is related to
c++
std::pair already has the required comparison operators, which perform lexicographical comparisons using both elements of each pair. To use this, you just have to provide the comparison operators for types for types K and V.
Also bear in mind that std::sort requires a strict weak ordeing comparison, and <= does not satisfy that. You would need, for example, a less-than comparison < for K and V. With that in place, all you need is
std::vector<pair<K,V>> items;
std::sort(items.begin(), items.end());
If you really need to provide your own comparison function, then you need something along the lines of
template <typename K, typename V>
bool comparePairs(const std::pair<K,V>& lhs, const std::pair<K,V>& rhs)
{
return lhs.first < rhs.first;
}
Your comparison function is not even wrong.
Its arguments should be the type stored in the range, i.e. std::pair<K,V>, not const void*.
It should return bool not a positive or negative value. Both (bool)1 and (bool)-1 are true so your function says every object is ordered before every other object, which is clearly impossible.
You need to model the less-than operator, not strcmp or memcmp style comparisons.
See StrictWeakOrdering which describes the properties the function must meet.
Look here: http://en.cppreference.com/w/cpp/algorithm/sort.
It says:
template< class RandomIt, class Compare >
void sort( RandomIt first, RandomIt last, Compare comp );
bool cmp(const Type1 &a, const Type2 &b);Also, here's an example of how you can use std::sort using a custom C++14 polymorphic lambda:
std::sort(std::begin(container), std::end(container),
[] (const auto& lhs, const auto& rhs) {
return lhs.first < rhs.first;
});
Source: Stackoverflow.com