第 18.11 節
Function objects and predicates
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STL- Function Objects
function object
Function Object Concept
Concept:
- A class that overloads the function call operator typically has its objects referred to as function objects.
- Function objects behave like function calls when using the overloaded
()and are also called functors.
Essence:
Function objects (functors) are classes, not functions.
Function objects usage
Features:
- Function objects can be called like regular functions, can take parameters, and can return values.
- Function objects go beyond the concept of ordinary functions, as they can have their own state.
- Function objects can be passed as parameters.
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;
}
运行结果：
Hello C++
Summary:
- Functor implementations are highly flexible and can be passed as parameters.
predicate
Predicate Concepts
Concept:
- A functor that returns a boolean type is called a predicate.
- If the operator() takes a single parameter, it's known as a unary predicate.
- If the operator() takes two parameters, it is called a binary predicate.
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;
}
运行结果：
找到:6
In summary: a predicate with only one parameter is called a unary predicate.
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;
}
运行结果：
10 20 30 40 50
----------------------------
50 40 30 20 10
Summary: A predicate with only two parameters is called a binary predicate.
built-in function objects
The Significance of Built-in Function Objects
Concept:
- STL includes some built-in function objects.
Category:
- arithmetic functor
- relation functor
- logic functor
Usage:
- The objects generated by these functors are used exactly the same way as regular functions.
- Using built-in function objects requires including the header file
#include<functional>
arithmetic functor
Description:
- Implement arithmetic operations
- Among them, negate is a unary operation, and the others are binary operations.
Function Prototype:
template<class T> T plus<T>// Addition Functortemplate<class T> T minus<T>//subtraction functortemplate<class T> T multiplies<T>//Multiplication functortemplate<class T> T divides<T>//division functortemplate<class T> T modulus<T>//modulo functiontemplate<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;
}
运行结果：
-50
30
Summary: When using built-in function objects, you need to include the header file #include <functional>
relation functor
Description:
- Implement relationship comparison
Function Prototype:
template<class T> bool equal_to<T>//equalstemplate<class T> bool not_equal_to<T>//not equal totemplate<class T> bool greater<T>//greater thantemplate<class T> bool greater_equal<T>//greater than or equal totemplate<class T> bool less<T>//less thantemplate<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;
}
运行结果：
10 30 50 40 20
50 40 30 20 10
Summary: The most commonly used among relational functors is the greater<> than functor.
logic functor
Description:
- Implement logical operations
Function prototype:
template<class T> bool logical_and<T>//Logic ANDtemplate<class T> bool logical_or<T>//logical ORtemplate<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;
}
运行结果：
1 0 1 0
0 1 0 1
Summary: Logic functors are rarely used in practice, so just be aware of them.