STL中泛型算法find_if()与sort()的应用举例及详解

请认真看下原型：

//find_if()的原型：
template <class InputIterator, class Predicate>
InputIterator find_if ( InputIterator first, InputIterator last, Predicate pred );
//sort()的原型：
template <class RandomAccessIterator>
void sort ( RandomAccessIterator first, RandomAccessIterator last );
template <class RandomAccessIterator, class Compare>
void sort ( RandomAccessIterator first, RandomAccessIterator last, Compare comp );
//find_if()的原型：
template <class InputIterator, class Predicate>
InputIterator find_if ( InputIterator first, InputIterator last, Predicate pred );
//sort()的原型：
template <class RandomAccessIterator>
void sort ( RandomAccessIterator first, RandomAccessIterator last );
template <class RandomAccessIterator, class Compare>
void sort ( RandomAccessIterator first, RandomAccessIterator last, Compare comp );

经常可以看到这种用法：

[cpp] view plaincopyprint?
// sort algorithm example
#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;
bool myfunction (int i,int j) { return (i<j); }
struct myclass {
bool operator() (int i,int j) { return (i<j);}
} myobject;
int main () {
int myints[] = {32,71,12,45,26,80,53,33};
vector<int> myvector (myints, myints+8);               // 32 71 12 45 26 80 53 33
vector<int>::iterator it;
// using default comparison (operator <):
sort (myvector.begin(), myvector.begin()+4);           //(12 32 45 71)26 80 53 33
// using function as comp
sort (myvector.begin()+4, myvector.end(), myfunction); // 12 32 45 71(26 33 53 80)
// using object as comp
sort (myvector.begin(), myvector.end(), myobject);     //(12 26 32 33 45 53 71 80)
// print out content:
cout << "myvector contains:";
for (it=myvector.begin(); it!=myvector.end(); ++it)
cout << " " << *it;
cout << endl;
return 0;
}
// sort algorithm example
#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;
bool myfunction (int i,int j) { return (i<j); }
struct myclass {
bool operator() (int i,int j) { return (i<j);}
} myobject;
int main () {
int myints[] = {32,71,12,45,26,80,53,33};
vector<int> myvector (myints, myints+8);               // 32 71 12 45 26 80 53 33
vector<int>::iterator it;
// using default comparison (operator <):
sort (myvector.begin(), myvector.begin()+4);           //(12 32 45 71)26 80 53 33
// using function as comp
sort (myvector.begin()+4, myvector.end(), myfunction); // 12 32 45 71(26 33 53 80)
// using object as comp
sort (myvector.begin(), myvector.end(), myobject);     //(12 26 32 33 45 53 71 80)
// print out content:
cout << "myvector contains:";
for (it=myvector.begin(); it!=myvector.end(); ++it)
cout << " " << *it;
cout << endl;
return 0;
}

17行比较好理解，19行怎么解释呢？如果想在比较函数中加入参数又应如何解决呢？

请看下面这个例子，在C++中class与struct在这方面的应用没有太大差别，我这里用struct来实现Predicate pred与Compare comp。

[cpp] view plaincopyprint?
//! /brief 检索得分映射结构体
struct score_doc
{
double score;
string doc_id;
score_doc(string doc = "", double score = 0.0):doc_id(doc), score(score){}
};
typedef vector<struct score_doc *> SCORE_VCT;
//! /brief 用于比较score_doc结构体实例
struct score_doc_compare
{
//! 用于排序算法
bool operator ()(const struct score_doc *sd1, const struct score_doc *sd2) const
{
return sd1->score > sd2->score;
}
//! 用于查找算法
bool operator ()(const struct score_doc *sd) const
{
return sd->doc_id == m_doc_id;
}
//! 用于排序算法
score_doc_compare(){}
//! 用于查找算法
score_doc_compare(string doc_id):m_doc_id(doc_id){}
private:
string m_doc_id;
};
//! 查找用法
it_vct = find_if(r_rslt_ptr->score_doc_vct.begin(),r_rslt_ptr->score_doc_vct.end(), score_doc_compare("文档1"));
if(r_rslt_ptr->score_doc_vct.end() != it_vct)
{
cout << "存在文档1" << endl;
}
//! 排序用法
sort(r_rslt_ptr->score_doc_vct.begin(),r_rslt_ptr->score_doc_vct.end(), score_doc_compare());
//! /brief 检索得分映射结构体
struct score_doc
{
double score;
string doc_id;
score_doc(string doc = "", double score = 0.0):doc_id(doc), score(score){}
};
typedef vector<struct score_doc *> SCORE_VCT;
//! /brief 用于比较score_doc结构体实例
struct score_doc_compare
{
//! 用于排序算法
bool operator ()(const struct score_doc *sd1, const struct score_doc *sd2) const
{
return sd1->score > sd2->score;
}
//! 用于查找算法
bool operator ()(const struct score_doc *sd) const
{
return sd->doc_id == m_doc_id;
}
//! 用于排序算法
score_doc_compare(){}
//! 用于查找算法
score_doc_compare(string doc_id):m_doc_id(doc_id){}
private:
string m_doc_id;
};
//! 查找用法
it_vct = find_if(r_rslt_ptr->score_doc_vct.begin(),r_rslt_ptr->score_doc_vct.end(), score_doc_compare("文档1"));
if(r_rslt_ptr->score_doc_vct.end() != it_vct)
{
cout << "存在文档1" << endl;
}
//! 排序用法
sort(r_rslt_ptr->score_doc_vct.begin(),r_rslt_ptr->score_doc_vct.end(), score_doc_compare());

由此可见find_if()调用的实际是score_doc_compare("文档1")()，同理sort()调用的是score_doc_compare()()。
在C++中，score_doc_compare("文档1")或score_doc_compare()创建了一个结构体实例，然后由find_if()或sort()调用了()操作符。
如果想加入参数，如上例中find_if的用法，只需在Compare comp中添加载入对应参数的构造函数和成员变量，然后重载()操作符时调取这个成员变量当参数就OK了。