std::collate::~collate

From Cppreference

Destructs a std::collate facet. This destructor is protected and virtual (due to base class destructor being virtual). An object of type std::collate, like most facets, can only be destroyed when the last std::locale object that implements this facet goes out of scope or if a user-defined class is derived from std::collate and implements a public destructor.

[edit] Example

#include <iostream>
#include <locale>
struct Destructible_collate : public std::collate<wchar_t>
{
    Destructible_collate(std::size_t refs = 0) : collate(refs) {}
    ~Destructible_collate() {}; // public dtor
};
int main()
{
    Destructible_collate dc;
    // std::collate<wchar_t> c;  // compile error: protected destructor
}