C++ 谓词(predicate) 详解

转自：https://blog.csdn.net/caroline_wendy/article/details/15378055

1.函数(function)谓词
通过传递函数名, 匹配二元谓词(binary predicates), 根据函数提供的策略, 输出值;

/*Function Predicate*/
bool isLarger (const std::string &s1, const std::string &s2) {
    return s1.size() > s2.size();
}
......
std::stable_sort(sv.begin(), sv.end(), isLarger);

2.函数指针(function pointer)谓词
建立一个函数指针, 传入算法, 使用指针代替函数名, 用法类似函数谓词.

bool (*pf) (const std::string &s1, const std::string &s2);
pf = &isLarger;
std::stable_sort(sv.begin(), sv.end(), *pf);

3.Lambda表达式(lambda expression)谓词
Lambda表达式格式: [capture list] (parameter list) -> return type { function body }
需要匹配谓词数, 一元(unary) 或 二元(binary), 也可以通过[capture list]传递函数的变量;

std::stable_sort(sv.begin(), sv.end(),
    [](const std::string& s1, const std::string& s2){ return s1.size()>s2.size(); });

4.函数对象(Function Object)谓词
类中重载函数的调用"()", 使类可以被调用, 并且传入算法谓词中, 进行使用.

/*Function Object*/
class LargerString {
public:
    bool operator() (const std::string& a, const std::string& b) {
        return a.size() > b.size();
    }
};
......
std::stable_sort(sv.begin(), sv.end(), LargerString());

5.库定义的函数对象(Library-Defined Function Object)谓词
使用标准库定义的函数对象, 充当算法中的谓词, 包含在#include,包含基本的算法和逻辑操作.

std::stable_sort(sv.begin(), sv.end(), std::less<std::string>());

所有代码：

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

using namespace std;

class PrintString {
public:
    PrintString (std::ostream &o = std::cout, char c = ' ') : os(o), sep(c) { }
    void operator() (const std::string &s) const { os << s << sep; }
private:
    std::ostream &os;
    char sep;
};

/*Function Predicate*/
bool isLarger (const std::string &s1, const std::string &s2) {
    return s1.size() > s2.size();
}

/*Function Object*/
class LargerString {
public:
    bool operator() (const std::string& a, const std::string& b) {
        return a.size() > b.size();
    }
};

int main (void) {

    std::vector<std::string> sv = {"Beauty", "Girl", "Lady", "Women", "Pretty"};

    std::stable_sort(sv.begin(), sv.end(), isLarger);
    std::cout << "Function Predicate : ";
    std::for_each(sv.begin(), sv.end(), PrintString(std::cout));
    std::cout << std::endl;

    std::stable_sort(sv.begin(), sv.end(),
            [](const std::string& s1, const std::string& s2){ return s1.size()>s2.size(); });
    std::cout << "Lambda Expression Predicate : ";
    std::for_each(sv.begin(), sv.end(), PrintString(std::cout));
    std::cout << std::endl;

    bool (*pf) (const std::string &s1, const std::string &s2);
    pf = &isLarger;
    std::stable_sort(sv.begin(), sv.end(), *pf);
    std::cout << "Function Pointer Predicate : ";
    std::for_each(sv.begin(), sv.end(), PrintString(std::cout));
    std::cout << std::endl;

    std::stable_sort(sv.begin(), sv.end(), LargerString());
    std::cout << "Function Object Predicate : ";
    std::for_each(sv.begin(), sv.end(), PrintString(std::cout));
    std::cout << std::endl;


    std::stable_sort(sv.begin(), sv.end(), std::larger<std::string>());
    std::cout << "Library-Defined Function Object Predicate : ";
    std::for_each(sv.begin(), sv.end(), PrintString(std::cout));
    std::cout << std::endl;

    return 0;

}

输出：

Function Predicate : Beauty Pretty Women Girl Lady
Lambda Expression Predicate : Beauty Pretty Women Girl Lady
Function Pointer Predicate : Beauty Pretty Women Girl Lady
Function Object Predicate : Beauty Pretty Women Girl Lady
Library-Defined Function Object Predicate : Women Pretty Lady Girl Beauty