轉載至:http://blog.csdn.net/caroline_wendy/article/details/15378055
算法謂詞, 即标準庫算法傳遞的參數, 可以指定算法的操作, 如std::sort, 預設是從小到大, 通過謂詞可以修改從大到小.
本文包含基本的5種謂詞模式: 函數,函數指針,lambda表達式,函數對象,庫定義的函數對象.
1. 函數(function)謂詞
通過傳遞函數名, 比對二進制謂詞(binary predicates), 根據函數提供的政策, 輸出值;
代碼:
[cpp] view plain copy print ?
- bool isLarger (const std::string &s1, const std::string &s2) {
- return s1.size() > s2.size();
- }
- ......
- std::stable_sort(sv.begin(), sv.end(), isLarger);
2. 函數指針(function pointer)謂詞
建立一個函數指針, 傳入算法, 使用指針代替函數名, 用法類似函數謂詞.
代碼:
[cpp] view plain copy print ?
- bool (*pf) (const std::string &s1, const std::string &s2);
- pf = &isLarger;
- std::stable_sort(sv.begin(), sv.end(), *pf);
3. Lambda表達式(lambda expression)謂詞
Lambda表達式格式: [capture list] (parameter list) -> return type { function body }
需要比對謂詞數, 一進制(unary) 或 二進制(binary), 也可以通過[capture list]傳遞函數的變量;
代碼:
[cpp] view plain copy print ?
- std::stable_sort(sv.begin(), sv.end(),
- [](const std::string& s1, const std::string& s2){ return s1.size()>s2.size(); });
4. 函數對象(Function Object)謂詞
類中重載函數的調用"()", 使類可以被調用, 并且傳入算法謂詞中, 進行使用.
代碼:
[cpp] view plain copy print ?
- 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. 庫定義的函數對象(Library-Defined Function Object)謂詞
使用标準庫定義的函數對象, 充當算法中的謂詞, 包含在#include<functional>,包含基本的算法和邏輯操作.
代碼:
[cpp] view plain copy print ?
- std::stable_sort(sv.begin(), sv.end(), std::less<std::string>());
所有方法代碼(Eclipse CDT; GCC 4.7.1):
[cpp] view plain copy print ?
- #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;
- 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;
- }
輸出:
[plain] view plain copy print ?
- 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