函数对象（Function Objects）

Objects of class types that define the call operator are often referred to as function objects that is, they are objects that act like functions.（定义了调用操作符 的类，其对象常称为函数对象 ，即它们是行为类似函数 的对象。）

 

定义BET_cls类，用于测试给定string对象的长度是否与其边界相匹配；
定义GT_cls类，用于测试给定string对象的长度是否在给定值以上；
读入文本文件，使用BET_cls类和GT_cls类，统计长度在1到9之间（含1和9）及10以上的单词的数目（排除相同的单词）。

[1]
使用自己定义的函数对象类 ，BET_cls和GT_cls

#include <iostream> #include <fstream> #include <vector> #include <algorithm> #include <string> using namespace std; #define LOCAL class BET_cls { public: BET_cls(string::size_type len1, string::size_type len2) { if (len1<len2) { minLength=len1; maxLength=len2; } else { minLength=len2; maxLength=len1; } } bool operator() (const string &s) { return s.size()>=minLength && s.size()<=maxLength; } private: string::size_type minLength, maxLength; }; class GT_cls { public: GT_cls(size_t val=0):bound(val) {} bool operator() (const string &s) { return s.size()>=bound; } private: string::size_type bound; }; bool isShorter(const string &s1, const string &s2) { return s1.size()<s2.size(); } string make_plural(size_t ctr, const string &word, const string &ending) { return (ctr==1) ? word : word+ending; } int main(int argc, char* argv[]) { #ifdef LOCAL freopen("output.txt", "a+", stdout); #endif if (argc<2) { cerr<<"No input file!"<<endl; return EXIT_SUCCESS; } ifstream inFile; inFile.open(argv[1]); if (!inFile) { cerr<<"Can not open input file!"<<endl; return EXIT_SUCCESS; } vector<string> words; string word; while (inFile>>word) { words.push_back(word); } sort(words.begin(), words.end()); words.erase(unique(words.begin(), words.end()), words.end()); vector<string>::size_type wc=0; wc=count_if(words.begin(), words.end(), BET_cls(1, 9)); cout<<wc<<" "<<make_plural(wc, "word", "s") <<" are of sizes 1 through 9 inclusive"<<endl; wc=count_if(words.begin(), words.end(), GT_cls(10)); cout<<wc<<" "<<make_plural(wc, "word", "s") <<" 10 characters or longer"<<endl; return 0; } /* In Anhui province , 560,000 students will take the exams , but only 240,000 will gain places at University . Competition is fierce ! You must shoulder your parent's expectations , the great trust of your school and the hopes of our Motherland ! output: 32 words are of sizes 1 through 9 inclusive 4 words 10 characters or longer */

[2]
改为使用标准库函数对象 代替GT_cls来查找指定长度的单词
[注意]
因为没有可用于直接比较字符串长度的标准库函数对象，所以不能直接使用count_if算法，改用greater_equal对象 循环实现单词计数。

#include <iostream> #include <fstream> #include <vector> #include <algorithm> #include <string> #include <functional>// note, this header includes the std function objects using namespace std; #define LOCAL class BET_cls { public: BET_cls(string::size_type len1, string::size_type len2) { if (len1<len2) { minLength=len1; maxLength=len2; } else { minLength=len2; maxLength=len1; } } bool operator() (const string &s) { return s.size()>=minLength && s.size()<=maxLength; } private: string::size_type minLength, maxLength; }; class GT_cls { public: GT_cls(size_t val=0):bound(val) {} bool operator() (const string &s) { return s.size()>=bound; } private: string::size_type bound; }; bool isShorter(const string &s1, const string &s2) { return s1.size()<s2.size(); } string make_plural(size_t ctr, const string &word, const string &ending) { return (ctr==1) ? word : word+ending; } int main(int argc, char* argv[]) { #ifdef LOCAL freopen("output.txt", "a+", stdout); #endif if (argc<2) { cerr<<"No input file!"<<endl; return EXIT_SUCCESS; } ifstream inFile; inFile.open(argv[1]); //inFile.open("1.txt"); if (!inFile) { cerr<<"Can not open input file!"<<endl; return EXIT_SUCCESS; } vector<string> words; string word; while (inFile>>word) { words.push_back(word); } sort(words.begin(), words.end()); words.erase(unique(words.begin(), words.end()), words.end()); vector<string>::size_type wc=0; wc=count_if(words.begin(), words.end(), BET_cls(1, 9));// std has not this function object cout<<wc<<" "<<make_plural(wc, "word", "s") <<" are of sizes 1 through 9 inclusive"<<endl; //wc=count_if(words.begin(), words.end(), GT_cls(10));// my function object //wc=count_if(words.begin(), words.end(), bind2nd(greater<string>(), 10));// error // the following way is right ! wc=0; greater_equal<string::size_type> grep;// std function object for (vector<string>::const_iterator iter=words.begin(); iter!=words.end(); ++iter) { if (grep(iter->size(), 10)) ++wc; } cout<<wc<<" "<<make_plural(wc, "word", "s") <<" 10 characters or longer"<<endl; return 0; } /* In Anhui province , 560,000 students will take the exams , but only 240,000 will gain places at University . Competition is fierce ! You must shoulder your parent's expectations , the great trust of your school and the hopes of our Motherland ! output: 32 words are of sizes 1 through 9 inclusive 4 words 10 characters or longer */