Having the following code:
#include <iostream>
#include <set>
#include <string>
#include <functional>
using namespace std;
class Employee {
// ...
int _id;
string _name;
string _title;
public:
Employee(int id): _id(id) {}
string const &name() const { return _name; }
void setName(string const &newName) { _name = newName; }
string const &title() const { return _title; }
void setTitle(string const &newTitle) { _title = newTitle; }
int id() const { return _id; }
};
struct compEmployeesByID: public binary_function<Employee, Employee, bool> {
bool operator()(Employee const &lhs, Employee const &rhs) {
return lhs.id() < rhs.id();
}
};
int wmain() {
Employee emplArr[] = {0, 1, 2, 3, 4};
set<Employee, compEmployeesByID> employees(emplArr, emplArr + sizeof emplArr/sizeof emplArr[0]);
// ...
set<Employee, compEmployeesByID>::iterator iter = employees.find(2);
if (iter != employees.end())
iter->setTitle("Supervisor");
return 0;
}
I cannot compile this code having (MSVCPP 11.0):
1> main.cpp
1>d:\docs\programming\test01\test01\main.cpp(40): error C2662: 'Employee::setTitle' : cannot convert 'this' pointer from 'const Employee' to 'Employee &'
1> Conversion loses qualifiers
This helps to compile:
if (iter != employees.end())
const_cast<Employee &>(*iter).setTitle("Supervisor");
The question: I know that map
and multimap
store their values as pair(const K, V)
where K is a key and V is a value. We cannot change the K object. But set<T>
and multiset<T>
store their object as T
, not as const T
. So WHY I NEED THIS CONST CAST??
In C++11 set (and multiset) specify that iterator
as well as const_iterator
is a constant iterator, i.e. you cannot use it to modify the key. This is because any modification of they key risks breaking the set's invariant. (See 23.2.4/6.)
Your const_cast
opens the door to undefined behaviour.
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