具有以下代码:
#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;
}
我无法编译此代码(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
这有助于编译:
if (iter != employees.end())
const_cast<Employee &>(*iter).setTitle("Supervisor");
问题:我知道map
并将multimap
它们的值存储为pair(const K, V)
其中 K 是键而 V 是值。我们不能改变 K 对象。但是set<T>
并将multiset<T>
他们的对象存储为T
,而不是const T
。那么为什么我需要这个 const 演员?