STL copy详解

     前面十二个算法所展现的都属于非变易算法（Non-mutating algorithms）系列，现在我们来看看变易算法。所谓变易算法(Mutating algorithms)就是一组能够修改容器元素数据的模板函数，可进行序列数据的复制，变换等。

       我们现在来看看第一个变易算法：元素复制算法copy。该算法主要用于容器之间元素的拷贝，即将迭代器区间[first，last)的元素复制到由复制目标result给定的区间[result，result+(last-first))中。下面我们来看看它的函数原型：

函数原形:

template<class InputIterator, class OutputIterator>   OutputIterator copy(      InputIterator _First,       InputIterator _Last,       OutputIterator _DestBeg   );

参数

_First, _Last

指出被复制的元素的区间范围[ _First，_Last).

_DestBeg 

指出复制到的目标区间起始位置

返回值

返回一个迭代器，指出已被复制元素区间的最后一个位置

程序示例：

首先我们来一个简单的示例，定义一个简单的整形数组myints，将其所有元素复制到容器myvector中，并将数组向左移动一位。

#include <iostream>  #include <algorithm>  #include <vector>    using namespace std;    int main ()   {      int myints[] = {10, 20, 30, 40, 50, 60, 70};      vector<int> myvector;      vector<int>::iterator it;            myvector.resize(7);   // 为容器myvector分配空间            //copy用法一：      //将数组myints中的七个元素复制到myvector容器中      copy ( myints, myints+7, myvector.begin() );            cout << "myvector contains: ";      for ( it = myvector.begin();  it != myvector.end();  ++it )      {          cout << " " << *it;      }      cout << endl;        //copy用法二:      //将数组myints中的元素向左移动一位      copy(myints + 1, myints + 7, myints);        cout << "myints contains: ";      for ( size_t i = 0; i < 7; ++i )      {          cout << " " << myints[i];      }      cout << endl;        return 0;  }  

从上例中我们看出copy算法可以很简单地将一个容器里面的元素复制至另一个目标容器中，上例中代码特别要注意一点就是myvector.resize(7);这行代码，在这里一定要先为vector分配空间，否则程序会崩，这是初学者经常犯的一个错误。其实copy函数最大的威力是结合标准输入输出迭代器的时候，我们通过下面这个示例就可以看出它的威力了。

#include <iostream>  #include <algorithm>  #include <vector>  #include <iterator>  #include <string>    using namespace std;    int main ()   {       typedef vector<int> IntVector;       typedef istream_iterator<int> IstreamItr;       typedef ostream_iterator<int> OstreamItr;       typedef back_insert_iterator< IntVector > BackInsItr;          IntVector myvector;         // 从标准输入设备读入整数       // 直到输入的是非整型数据为止 请输入整数序列，按任意非数字键并回车结束输入       cout << "Please input element：" << endl;       copy(IstreamItr(cin), IstreamItr(), BackInsItr(myvector));         //输出容器里的所有元素，元素之间用空格隔开       cout << "Output : " << endl;       copy(myvector.begin(), myvector.end(), OstreamItr(cout, " "));        cout << endl;        return 0;  }  

#include <iostream>#include <algorithm>#include <vector>#include <string>#include <list>#include <iterator>#include <fstream>#include <utility>using namespace std;void print(const string& s){cout<<s<<endl;}int main(){vector<string> vector1, vector2, vector3, modvector1, modvector2;vector1.resize(100);vector2.resize(100);vector3.resize(100);modvector1.resize(100);modvector2.resize(100);fstream fin1, fin2, fin3, fout1, fout2, fout3;fin1.open( "file1.txt" );copy( istream_iterator<string>(fin1), istream_iterator<string>(),vector1.begin());fin2.open( "file2.txt" );copy( istream_iterator<string>(fin2), istream_iterator<string>(),vector2.begin());fin3.open( "file3.txt" );copy( istream_iterator<string>(fin3), istream_iterator<string>(),vector3.begin());fout1.open( "outfile1.txt" );fout2.open( "outfile2.txt" );cout << "The size of the vector1 is "<< vector1.size() <<endl;cout << "The size of the vector2 is "<< vector2.size() <<endl;cout << "The size of the vector3 is "<< vector3.size() <<endl;cout << "The size of the modvector1 is "<< modvector1.size() <<endl;cout << "The size of the modvector2 is "<< modvector2.size() <<endl;modvector1 = vector1;modvector2 = vector2;cout << "The size of the vector1 is "<< vector1.size() <<endl;cout << "The size of the vector2 is "<< vector2.size() <<endl;cout << "The size of the vector3 is "<< vector3.size() <<endl;cout << "The size of the modvector1 is "<< modvector1.size() <<endl;cout << "The size of the modvector2 is "<< modvector2.size() <<endl;modvector1.erase( vector2.begin(), vector2.end() );modvector1.erase( vector3.begin(), vector3.end() );modvector2.erase( vector1.begin(), vector1.end() );modvector2.erase( vector3.begin(), vector3.end() );cout << "The size of the vector1 is "<< vector1.size() <<endl;cout << "The size of the vector2 is "<< vector2.size() <<endl;cout << "The size of the vector3 is "<< vector3.size() <<endl;cout << "The size of the modvector1 is "<< modvector1.size() <<endl;cout << "The size of the modvector2 is "<< modvector2.size() <<endl;ostream_iterator<string> output(cout," ");copy( modvector1.begin(), modvector1.end(), output );copy( modvector2.begin(), modvector2.end(), output );for_each( modvector1.begin( ), modvector1.end( ), print);for_each( modvector2.begin( ), modvector2.end( ), print);copy( modvector1.begin(), modvector1.end(), ostream_iterator<string>(fout1," ") );copy( modvector2.begin(), modvector2.end(), ostream_iterator<string>(fout2," ") );//copy( modvector1.begin(), modvector1.end(), fout1 );//copy( modvector2.begin(), modvector2.end(), fout2 );return 0;}