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Function Objects and Predicates

About 6 min read

1.STL - Function Objects

1.1.Function object

1.1.1.Function object concept

Concept:

  • overloadingfunction call operatorclass, whose objects are often calledfunction object
  • function objectwhen using overloaded (), the behavior is similar to a function call, also calledfunctor

Essence:

A function object (functor) is aclass， not a function

1.1.2.Function object usage

Features:

  • When a function object is used, it can be called like an ordinary function, can take parameters, and can have a return value.
  • Function objects go beyond the concept of ordinary functions; a function object can have its own state.
  • A function object can be passed as a parameter.

Example:

#include <iostream>
#include <string>

using namespace std;

//1、函数对象在使用时，可以像普通函数那样调用, 可以有参数，可以有返回值
class MyAdd
{
public :
	int operator()(int v1,int v2)
	{
		return v1 + v2;
	}
};

void test01()
{
	MyAdd myAdd;
	cout << myAdd(10, 10) << endl;
}

//2、函数对象可以有自己的状态
class MyPrint
{
public:
	MyPrint()
	{
		count = 0;
	}
	void operator()(string test)
	{
		cout << test << endl;
		count++; //统计使用次数
	}

	int count; //内部自己的状态
};
void test02()
{
	MyPrint myPrint;
	myPrint("hello world");
	myPrint("hello world");
	myPrint("hello world");
	cout << "myPrint调用次数为： " << myPrint.count << endl;
}

//3、函数对象可以作为参数传递
void doPrint(MyPrint &mp , string test)
{
	mp(test);
}

void test03()
{
	MyPrint myPrint;
	doPrint(myPrint, "Hello C++");
}

int main() {

	//test01();
	//test02();
	test03();


	return 0;
}

Operation result:

Hello C++

Summary:

  • Functors are very flexible in their syntax and can be passed as parameters.

1.2.predicate

1.2.1.Predicate concept

Concept:

  • A functor that returns a bool type is called apredicate
  • If operator() accepts one parameter, then it is called a unary predicate
  • If operator() accepts two parameters, then it is called a binary predicate

1.2.2.Unary predicate

Example:

#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

//1.一元谓词
struct GreaterFive{
	bool operator()(int val) {
		return val > 5;
	}
};

void test01() {

	vector<int> v;
	for (int i = 0; i < 10; i++)
	{
		v.push_back(i);
	}

	vector<int>::iterator it = find_if(v.begin(), v.end(), GreaterFive());
	if (it == v.end()) {
		cout << "没找到!" << endl;
	}
	else {
		cout << "找到:" << *it << endl;
	}

}

int main() {

	test01();


	return 0;
}

Operation result:

找到:6

Summary: A predicate with only one parameter is called a unary predicate.

1.2.3.binary predicate

Example:

#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

//二元谓词
class MyCompare
{
public:
	bool operator()(int num1, int num2)
	{
		return num1 > num2;
	}
};

void test01()
{
	vector<int> v;
	v.push_back(10);
	v.push_back(40);
	v.push_back(20);
	v.push_back(30);
	v.push_back(50);

	//默认从小到大
	sort(v.begin(), v.end());
	for (vector<int>::iterator it = v.begin(); it != v.end(); it++)
	{
		cout << *it << " ";
	}
	cout << endl;
	cout << "----------------------------" << endl;

	//使用函数对象改变算法策略，排序从大到小
	sort(v.begin(), v.end(), MyCompare());
	for (vector<int>::iterator it = v.begin(); it != v.end(); it++)
	{
		cout << *it << " ";
	}
	cout << endl;
}

int main() {

	test01();


	return 0;
}

Operation result:

10 20 30 40 50
----------------------------
50 40 30 20 10

Summary: A predicate with only two parameters is called a binary predicate.

1.3.Built-in function objects

1.3.1.Significance of built-in function objects

Concept:

  • STL has some built-in function objects

Category:

  • arithmetic functors

  • relational functors

  • logical functors

Usage:

  • The objects produced by these function objects are used in exactly the same way as ordinary functions.
  • To use built-in function objects, you need to include the header file #include<functional>

1.3.2.arithmetic functor

Function description:

  • Implement the four arithmetic operations
  • Among them, negate is a unary operation, while the others are binary operations

Functor prototype:

  • template<class T> T plus<T> // Addition functor
  • template<class T> T minus<T> // Subtraction functor
  • template<class T> T multiplies<T> // Multiplication functor
  • template<class T> T divides<T> // Division functor
  • template<class T> T modulus<T> // Modulus functor
  • template<class T> T negate<T> // Negation functor

Example:

#include <iostream>
#include <functional>

using namespace std;

//negate
void test01()
{
	negate<int> n;
	cout << n(50) << endl;
}

//plus
void test02()
{
	plus<int> p;
	cout << p(10, 20) << endl;
}

int main() {

	test01();
	test02();


	return 0;
}

Operation result:

-50
30

Summary: When using built-in function objects, the header file must be included #include <functional>

1.3.3.Relation functor

Function description:

  • Implementation relationship comparison

Functor prototype:

  • template<class T> bool equal_to<T> //Equal to
  • template<class T> bool not_equal_to<T> //Not equal to
  • template<class T> bool greater<T> //Greater than
  • template<class T> bool greater_equal<T> //Greater than or equal to
  • template<class T> bool less<T> //Less than
  • template<class T> bool less_equal<T> //Less than or equal to

Example:

#include <iostream>
#include <functional>
#include <vector>
#include <algorithm>

using namespace std;

class MyCompare
{
public:
	bool operator()(int v1,int v2)
	{
		return v1 > v2;
	}
};
void test01()
{
	vector<int> v;

	v.push_back(10);
	v.push_back(30);
	v.push_back(50);
	v.push_back(40);
	v.push_back(20);

	for (vector<int>::iterator it = v.begin(); it != v.end(); it++) {
		cout << *it << " ";
	}
	cout << endl;

	//自己实现仿函数
	//sort(v.begin(), v.end(), MyCompare());
	//STL内建仿函数  大于仿函数
	sort(v.begin(), v.end(), greater<int>());

	for (vector<int>::iterator it = v.begin(); it != v.end(); it++) {
		cout << *it << " ";
	}
	cout << endl;
}

int main() {

	test01();


	return 0;
}

Operation result:

10 30 50 40 20
50 40 30 20 10

Summary: The most commonly used relational functor is greater<>, for greater-than.

1.3.4.logical functor

Function description:

  • Implement logical operations

Function prototype:

  • template<class T> bool logical_and<T> //logical AND
  • template<class T> bool logical_or<T> //logical OR
  • template<class T> bool logical_not<T> //logical NOT

Example:

#include <iostream>
#include <vector>
#include <functional>
#include <algorithm>

using namespace std;

void test01()
{
	vector<bool> v;
	v.push_back(true);
	v.push_back(false);
	v.push_back(true);
	v.push_back(false);

	for (vector<bool>::iterator it = v.begin();it!= v.end();it++)
	{
		cout << *it << " ";
	}
	cout << endl;

	//逻辑非  将v容器搬运到v2中，并执行逻辑非运算
	vector<bool> v2;
	v2.resize(v.size());
	transform(v.begin(), v.end(),  v2.begin(), logical_not<bool>());
	for (vector<bool>::iterator it = v2.begin(); it != v2.end(); it++)
	{
		cout << *it << " ";
	}
	cout << endl;
}

int main() {

	test01();


	return 0;
}

Operation result:

1 0 1 0
0 1 0 1

Summary: Logical functors are rarely used in practice; just understand them.