在类内按数组中的属性对对象进行排序，这些对象本身就是另一个类的对象

Question

I am trying to sort an array of objects which are stored within another object and then display the sorted objects. I want to sort these objects by a specific attribute. I'm not exactly sure how I am supposed to run the sort() method.

composer.h

class Composer {
  Public:
    int get_ranking();
    ..
    ..
};

database.h

Class Database {
  Public:
    void DisplayAll(){
      for(int i=0;i<next_slot_;i++){
            composers_[i].Display();
      }
    };

    bool SortByRank(Composer const & a, Composer const & b) {
        return a.get_ranking() < b.get_ranking();
    };

    void DisplayByRank(){
      sort(composers_, composers_+next_slot_, SortByRank);
      DisplayAll();
    };

testdatabase.cpp

int main(){
  Database myDB;

  Composer& comp1 = myDB.AddComposer("Ludwig van", "Beethoven", "Romantic", 1770,
                   "Beethoven was completely deaf during the latter part of his life - he 
                    never heard a performance of his 9th symphony.");
  comp1.Promote(7);

  Composer& comp2 = myDB.AddComposer("Johann Sebastian", "Bach", "Baroque", 1685,
                   "Bach had 20 children, several of whom became famous musicians as well.");
  comp2.Promote(5);

  cout << endl << "all Composers: " << endl << endl;
  myDB.DisplayAll();
  myDB.DisplayByRank();

When I run this I get:

error: passing 'const Composer' as 'this' argument of 'int Composer::get_ranking()' discards qualifiers [-fpermissive] return a.get_ranking() < b.get_ranking();

and

error: no matching function for call to 'sort(Composer [100], Composer*, unresolved overloaded function type)' sort(composers_, composers_+next_slot_, SortByRank);

Adding const to the end of get_ranking() seems to have solved the const correctness error.

Answer 1

You should add const to get_ranking method.

class Composer {
public:
    int get_ranking() const;
};

Which is part of C++ const-correctness. It means that the method does not change internal state of the object (except mutable fields which are rather rarely used) it is called by and allows to call this method by const objects.

Answer 2

Alright, so I did a fair amount of reading and learned a bit.

After resolving my const correctness by changing int get_ranking(); to int get_ranking()const it turned out my SortByRank() still wasn't working. This was because it was a non-static member of Database requiring a this pointer. Simply changing it to: static bool SortByRank(); or defining SortByRank() outside of the Database class solves the issue.