Binary Predicate(双参判断式)的用途是：比较两个参数的特定属性
我们先建一个领域模型类:
Person.h
#ifndef _Domain_Models_Person_H_
#define _Domain_Models_Person_H_ #include <iostream>
#include <string>
#include <deque> using namespace std; class Person
{
private:
string fn; // first name
string ln; // last name
int age;
public:
Person() { }
Person(const string& f, const string& n)
: fn(f), ln(n)
{
}
string firstname() const;
string lastname() const;
int getAge() const;
void setAge(int a);
static bool sortByName(const Person& p1, const Person& p2);
static bool sortByAge(const Person& p1, const Person& p2);
static void sortDequeByName(deque<Person> &persons);
static void sortDequeByAge(deque<Person> &persons);
}; #endif
Person.cpp
#include <algorithm>
#include "Person.h" inline string Person::firstname() const
{
return fn;
} inline string Person::lastname() const
{
return ln;
} inline int Person::getAge() const
{
return age;
} void Person::setAge(int a)
{
age = a;
} /* binary function predicate:
* - returns whether a person is less than another person
*/
bool Person::sortByName(const Person& p1, const Person& p2)
{
/* a person is less than another person
* - if the last name is less
* - if the last name is equal and the first name is less
*/
return p1.lastname()<p2.lastname() ||
(p1.lastname() == p2.lastname() &&
p1.firstname()<p2.firstname());
} // another binary predicate
bool Person::sortByAge(const Person& p1, const Person& p2)
{
return p1.getAge() < p2.getAge();
} void Person::sortDequeByName(deque<Person> &persons)
{
// sort elements
sort(persons.begin(), persons.end(), // range
Person::sortByName); // sort criterion
} void Person::sortDequeByAge(deque<Person> &persons)
{
// sort elements
sort(persons.begin(), persons.end(), // range
Person::sortByAge); // sort criterion
}
然后，我们需要测试下我们的领域模型
为了方便输出模型，先给我们的测试类加入运算符重载的友元函数
class PredicateTest : public TestBase
{
friend ostream& operator<< (ostream& s, const Person& p);
...
}
实现如下:
ostream& operator<< (ostream& s, const Person& p)
{
s << "[" << p.lastname() << ", " << p.firstname() << ", " << p.getAge() << "]";
return s;
}
测试代码如下:
void PredicateTest::binaryPredicate()
{
// create some persons
Person p1("nicolai", "josuttis");
Person p2("ulli", "josuttis");
Person p3("anica", "josuttis");
Person p4("lucas", "josuttis");
Person p5("lucas", "otto");
Person p6("lucas", "arm");
Person p7("anica", "holle");
p1.setAge();
p2.setAge();
p3.setAge();
p4.setAge();
p5.setAge();
p6.setAge();
p7.setAge(); // insert person into collection coll
deque<Person> coll;
coll.push_back(p1);
coll.push_back(p2);
coll.push_back(p3);
coll.push_back(p4);
coll.push_back(p5);
coll.push_back(p6);
coll.push_back(p7); // print elements
cout << "deque before sort():" << endl;
deque<Person>::iterator pos;
for (pos = coll.begin(); pos != coll.end(); ++pos)
{
cout << *pos << endl;
} cout << "-- Sort By Name --" << endl;
Person::sortDequeByName(coll); // print elements
cout << "deque after sort():" << endl;
for (pos = coll.begin(); pos != coll.end(); ++pos)
{
cout << *pos << endl;
} cout << "-- Sort By Age --" << endl;
Person::sortDequeByAge(coll); // print elements
cout << "deque after sort():" << endl;
for (pos = coll.begin(); pos != coll.end(); ++pos)
{
cout << *pos << endl;
}
}
运行结果:
---------------- binaryPredicate(): Run Start ----------------
deque before sort():
[josuttis, nicolai, 20]
[josuttis, ulli, 30]
[josuttis, anica, 18]
[josuttis, lucas, 2]
[otto, lucas, 22]
[arm, lucas, 35]
[holle, anica, 95]
-- Sort By Name --
deque after sort():
[arm, lucas, 35]
[holle, anica, 95]
[josuttis, anica, 18]
[josuttis, lucas, 2]
[josuttis, nicolai, 20]
[josuttis, ulli, 30]
[otto, lucas, 22]
-- Sort By Age --
deque after sort():
[josuttis, lucas, 2]
[josuttis, anica, 18]
[josuttis, nicolai, 20]
[otto, lucas, 22]
[josuttis, ulli, 30]
[arm, lucas, 35]
[holle, anica, 95]
---------------- binaryPredicate(): Run End ----------------