转载至:http://blog.csdn.net/caroline_wendy/article/details/15378055
算法谓词, 即标准库算法传递的参数, 可以指定算法的操作, 如std::sort, 默认是从小到大, 通过谓词可以修改从大到小.
本文包含基本的5种谓词模式: 函数,函数指针,lambda表达式,函数对象,库定义的函数对象.
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;
代码:
-
- 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.
代码:
- 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;
代码:
- 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.
代码:
-
- 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<functional>,包含基本的算法和逻辑操作.
代码:
- std::stable_sort(sv.begin(), sv.end(), std::less<std::string>());
所有方法代码(Eclipse CDT; GCC 4.7.1):
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- #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;
- };
-
-
- bool isLarger (const std::string &s1, const std::string &s2) {
- return s1.size() > s2.size();
- }
-
-
- 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;
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- 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;
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- 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