How to list.sort(memberFunction);? - c++

Question

I'd like to use the sort function of std::list, and sort with the member function of "myClass" that contains a property I want to use while sorting.
struct node
{
int value;
}
class myClass
{
private:
bool _available;
public:
myClass();
~myClass();
private:
bool compare(const node &first, const node &second)
{
if (_available == true)
return (first.value < second.value);
else
return (first.value > second.value);
}
};
Then when I call : list.sort(myClass::compare);
I got : error: no matching function for call to ‘std::list<node>::sort(<unresolved overloaded function type>)’

Answer 1

The comparison function in problematic because you use a private member variable in it. That means you can't really use a static member function or a free-standing function or a lambda.
You also have a problem in that the comparison function is private, which means you have to sort the collection from inside a myClass object instance.
The first problem can be solved by using std::bind to bind the member function to a specific object instance. The second problem can be solved by making the function public (or by sorting from another member function inside the myClass class).
You can use std::bind like this:
// Definition of `list` here somewhere
myClass myObject;
list.sort(std::bind(&myClass::compare, &myObject));

Answer 2

Because myClass::compare is not a static method. You must make it static to pass it as a predicate for a sort. Or make it global. Also it must be public if it is to be a static class method.
typedef struct s_node
{
int value;
int t_node;
};
class myClass
{
private:
bool _available;
public:
myClass();
~myClass();
static bool compare(const t_node &first, const t_node &second)
{
if (_available == true)
return (first.value < second.value)
else
return (first.value > second.value)
}
};
OR
typedef struct s_node
{
int value;
int t_node;
};
class myClass
{
private:
bool _available;
public:
myClass();
~myClass();
};
bool compare(const t_node &first, const t_node &second)
{
if (_available == true)
return (first.value < second.value)
else
return (first.value > second.value)
}
And you could call it as:
list.sort( myClass:compare ); // first example
or
list.sort( compare ); // second example