C++ Primer 学习笔记_50_STL剖析（五）：变动性算法源代码分析与使用示例（for_each、copy_backward、transform、replace_copy_if 等）
一、变动性算法for_each
我们把for_each 即是非变动性算法，也可以算是变动性算法，取决于传入的第三个参数，即函数
指针。如果在函数内对容器元素做了修改，那么就属于变动性算法。
示例1：
#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
void print_element(int n)
{
cout << n << ' ';
}
void add_3(int &n)
{
n += 3;
}
int main(void)
{
int a[] = { 1, 2, 3, 4, 5 };
vector<int> v(a, a + 5);
for_each(v.begin(), v.end(), print_element);
cout << endl;
for_each(v.begin(), v.end(), add_3);
for_each(v.begin(), v.end(), print_element);
cout << endl;
return 0;
}
运行结果：
1 2 3 4 5
4 5 6 7 8
二、变动性算法源代码分析与使用示例：copy、copy_backward
1、示例1：
#include <iostream>
#include <vector>
#include <list>
#include <algorithm>
using namespace std;
void print_element(int n)
{
cout << n << ' ';
}
void add_3(int &n)
{
n += 3;
}
int main(void)
{
int a[] = { 1, 2, 3, 4, 5 };
vector<int> v(a, a + 5);
list<int> l(15); //copy必须事先分配空间
for_each(v.begin(), v.end(), print_element);
cout << endl;
for_each(v.begin(), v.end(), add_3);
for_each(v.begin(), v.end(), print_element);
cout << endl;
for_each(l.begin(), l.end(), print_element);
cout << endl;
copy(v.begin(), v.end(), l.begin());
for_each(l.begin(), l.end(), print_element);
cout << endl;
copy_backward(v.begin(), v.end(), l.end());
for_each(l.begin(), l.end(), print_element);
cout << endl;
return 0;
}
2、源代码分析
// TEMPLATE FUNCTION copy
template<class _InIt, class _OutIt, class _InOutItCat>
inline
_OutIt __CLRCALL_OR_CDECL _Copy_opt(_InIt _First, _InIt _Last, _OutIt _Dest,
_InOutItCat, _Nonscalar_ptr_iterator_tag, _Range_checked_iterator_tag)
{
// copy [_First, _Last) to [_Dest, ...), arbitrary iterators
_DEBUG_RANGE(_First, _Last);
for (; _First != _Last; ++_Dest, ++_First)
*_Dest = *_First;
return (_Dest);
}
template<class _InIt, class _OutIt>
inline
_IF_CHK(_OutIt) __CLRCALL_OR_CDECL copy(_InIt _First, _InIt _Last, _OutIt _Dest)
{
// copy [_First, _Last) to [_Dest, ...)
return (_Copy_opt(_CHECKED_BASE(_First), _CHECKED_BASE(_Last), _Dest,
_Iter_random(_First, _Dest), _Ptr_cat(_First, _Dest), _Range_checked_iterator_tag()));
}
// TEMPLATE FUNCTION copy_backward
template<class _BidIt1, class _BidIt2, class _InOutItCat>
inline
_BidIt2 __CLRCALL_OR_CDECL _Copy_backward_opt(_BidIt1 _First, _BidIt1 _Last, _BidIt2 _Dest,
_InOutItCat, _Nonscalar_ptr_iterator_tag, _Range_checked_iterator_tag)
{
// copy [_First, _Last) backwards to [..., _Dest), arbitrary iterators
_DEBUG_RANGE(_First, _Last);
while (_First != _Last)
*--_Dest = *--_Last;
return (_Dest);
}
template < class _BidIt1,
class _BidIt2 > inline
_IF_CHK(_BidIt2) __CLRCALL_OR_CDECL copy_backward(_BidIt1 _First, _BidIt1 _Last, _BidIt2 _Dest)
{
// copy [_First, _Last) backwards to [..., _Dest)
return _Copy_backward_opt(_CHECKED_BASE(_First), _CHECKED_BASE(_Last), _Dest,
_Iter_random(_First, _Dest), _Ptr_cat(_First, _Dest), _STD _Range_checked_iterator_tag());
}
（1）copy 调用了_Copy_opt，在此函数内递增迭代器，从前两个参数指定的区间内取出元素并拷贝到对应_Dest位置上。
for (; _First != _Last; ++_Dest, ++_First)
*_Dest = *_First;
（2）copy_backward 调用了_Copy_backward_opt，与copy 不同的是实现反向拷贝，即从尾端开始拷贝，所以是递减迭代器。
while (_First != _Last)
*--_Dest = *--_Last;
三、transfrom
1、示例1：
#include <iostream>
#include <vector>
#include <list>
#include <algorithm>
using namespace std;
void print_element(int n)
{
cout << n << ' ';
}
int fun(int a)
{
return 2 * a;
}
int fun2(int a, int b)
{
return a + b;
}
int main(void)
{
int a[] = { 1, 2, 3, 4, 5 };
vector<int> v(a, a + 5);
list<int> l(5);
list<int> ll(2);
transform(v.begin(), v.end(), l.begin(), fun); //根据传入函数和初始区间，变换到目标区间
for_each(l.begin(), l.end(), print_element);
cout << endl;
transform(v.begin(), v.begin() + 2, v.begin() + 3, ll.begin(), fun2);
for_each(ll.begin(), ll.end(), print_element); //1+4, 2+5
cout << endl;
return 0;
}
输出为 ：
2 4 6 8 10
5 7
2、源代码分析
// TEMPLATE FUNCTION transform WITH UNARY OP
template<class _InIt, class _OutIt, class _Fn1, class _InOutItCat>
inline
_OutIt _Transform(_InIt _First, _InIt _Last, _OutIt _Dest, _Fn1 _Func,
_InOutItCat, _Range_checked_iterator_tag)
{
// transform [_First, _Last) with _Func
_DEBUG_RANGE(_First, _Last);
_DEBUG_POINTER(_Dest);
_DEBUG_POINTER(_Func);
for (; _First != _Last; ++_First, ++_Dest)
*_Dest = _Func(*_First);
return (_Dest);
}
template<class _InIt, class _OutIt, class _Fn1>
inline
_IF_CHK(_OutIt) transform(_InIt _First, _InIt _Last, _OutIt _Dest, _Fn1 _Func)
{
return _Transform(_CHECKED_BASE(_First), _CHECKED_BASE(_Last), _Dest, _Func,
_Iter_random(_First, _Dest), _STD _Range_checked_iterator_tag());
}
// TEMPLATE FUNCTION transform WITH BINARY OP
template<class _InIt1, class _InIt2, class _OutIt, class _Fn2, class _InItCats, class _InOutItCat>
inline
_OutIt _Transform(_InIt1 _First1, _InIt1 _Last1, _InIt2 _First2,
_OutIt _Dest, _Fn2 _Func,
_InItCats, _InOutItCat,
_Range_checked_iterator_tag, _Range_checked_iterator_tag)
{
// transform [_First1, _Last1) and [_First2, _Last2) with _Func
_DEBUG_RANGE(_First1, _Last1);
_DEBUG_POINTER(_Dest);
_DEBUG_POINTER(_Func);
for (; _First1 != _Last1; ++_First1, ++_First2, ++_Dest)
*_Dest = _Func(*_First1, *_First2);
return (_Dest);
}
template<class _InIt1, class _InIt2, class _OutIt, class _Fn2, class _InOutItCat>
inline
_OutIt _Transform(_InIt1 _First1, _InIt1 _Last1, _InIt2 _First2,
_OutIt _Dest, _Fn2 _Func,
random_access_iterator_tag, _InOutItCat,
_Range_checked_iterator_tag, _Range_checked_iterator_tag)
{
// transform [_First1, _Last1) and [_First2, _Last2) with _Func
// for range checked iterators, this will make sure there is enough space
_InIt2 _Last2 = _First2 + (_Last1 - _First1);
(_Last2);
return _Transform(_First1, _Last1, _CHECKED_BASE(_First2),
_Dest, _Func,
forward_iterator_tag(), forward_iterator_tag(),
_Range_checked_iterator_tag(), _Range_checked_iterator_tag());
}
template<class _InIt1, class _InIt2, class _OutIt, class _Fn2>
inline
_IF_CHK2_(_InIt2, _OutIt, _OutIt) transform(_InIt1 _First1, _InIt1 _Last1, _InIt2 _First2,
_OutIt _Dest, _Fn2 _Func)
{
return _Transform(_CHECKED_BASE(_First1), _CHECKED_BASE(_Last1), _First2, _Dest, _Func,
_Iter_random(_First1, _First2), _Iter_random(_First1, _Dest),
_STD _Range_checked_iterator_tag(), _STD _Range_checked_iterator_tag());
}
实际上transfrom 重载了两个版本，一个是四个参数的，即将前两个参数指定区间内的元素执行某种操作（函数内）后拷贝到第三个
参数指示的区间上。而另一个版本是五个参数的，即将两个区间的对应元素进行某种操作后拷贝到第三个区间上去。核心的代码区
别在于下面两行：
*_Dest = _Func(*_First);
*_Dest = _Func(*_First1, *_First2);
三、replace、replace_copy、replace_copy_if
1、示例1：
#include <iostream>
#include <vector>
#include <list>
#include <algorithm>
using namespace std;
void print_element(int n)
{
cout << n << ' ';
}
bool fun(int a)
{
return a < 10;
}
int main(void)
{
int a[] = { 1, 2, 3, 4, 3 };
vector<int> v(a, a + 5);
list<int> l(5);
replace(v.begin(), v.end(), 3, 13); //所有的3替换为13
for_each(v.begin(), v.end(), print_element);
cout << endl;
replace_copy(v.begin(), v.end(), l.begin(), 13, 3); //原区间不替换，只对拷贝替换
for_each(v.begin(), v.end(), print_element);
cout << endl;
for_each(l.begin(), l.end(), print_element);
cout << endl;
replace_copy_if(v.begin(), v.end(), l.begin(), fun, 0); //<10的替换成0
for_each(l.begin(), l.end(), print_element);
cout << endl;
return 0;
}
输出为：
1 2 13 4 13
1 2 13 4 13
1 2 3 4 3
0 0 13 0 13
2、源代码分析
// TEMPLATE FUNCTION replace
template < class _FwdIt,
class _Ty > inline
void _Replace(_FwdIt _First, _FwdIt _Last,
const _Ty &_Oldval, const _Ty &_Newval)
{
// replace each matching _Oldval with _Newval
_DEBUG_RANGE(_First, _Last);
for (; _First != _Last; ++_First)
if (*_First == _Oldval)
*_First = _Newval;
}
template < class _FwdIt,
class _Ty > inline
void replace(_FwdIt _First, _FwdIt _Last,
const _Ty &_Oldval, const _Ty &_Newval)
{
// replace each matching _Oldval with _Newval
_Replace(_CHECKED_BASE(_First), _CHECKED_BASE(_Last), _Oldval, _Newval);
}
// TEMPLATE FUNCTION replace_copy
template<class _InIt, class _OutIt, class _Ty, class _InOutItCat>
inline
_OutIt _Replace_copy(_InIt _First, _InIt _Last, _OutIt _Dest,
const _Ty &_Oldval, const _Ty &_Newval,
_InOutItCat, _Range_checked_iterator_tag)
{
// copy replacing each matching _Oldval with _Newval
_DEBUG_RANGE(_First, _Last);
_DEBUG_POINTER(_Dest);
for (; _First != _Last; ++_First, ++_Dest)
*_Dest = *_First == _Oldval ? _Newval : *_First;
return (_Dest);
}
template < class _InIt,
class _OutIt,
class _Ty > inline
_IF_CHK(_OutIt) replace_copy(_InIt _First, _InIt _Last, _OutIt _Dest,
const _Ty &_Oldval, const _Ty &_Newval)
{
// copy replacing each matching _Oldval with _Newval
return _Replace_copy(_CHECKED_BASE(_First), _CHECKED_BASE(_Last), _Dest, _Oldval, _Newval,
_Iter_random(_First, _Dest), _STD _Range_checked_iterator_tag());
}
// TEMPLATE FUNCTION replace_copy_if
template<class _InIt, class _OutIt, class _Pr, class _Ty, class _InOutItCat>
inline
_OutIt _Replace_copy_if(_InIt _First, _InIt _Last, _OutIt _Dest,
_Pr _Pred, const _Ty &_Val, _InOutItCat, _Range_checked_iterator_tag)
{
// copy replacing each satisfying _Pred with _Val
_DEBUG_RANGE(_First, _Last);
_DEBUG_POINTER(_Dest);
_DEBUG_POINTER(_Pred);
for (; _First != _Last; ++_First, ++_Dest)
*_Dest = _Pred(*_First) ? _Val : *_First;
return (_Dest);
}
template < class _InIt,
class _OutIt,
class _Pr,
class _Ty > inline
_IF_CHK(_OutIt) replace_copy_if(_InIt _First, _InIt _Last, _OutIt _Dest,
_Pr _Pred, const _Ty &_Val)
{
// copy replacing each satisfying _Pred with _Val
return _Replace_copy_if(_CHECKED_BASE(_First), _CHECKED_BASE(_Last), _Dest, _Pred, _Val,
_Iter_random(_First, _Dest), _STD _Range_checked_iterator_tag());
}
（1）replace 带4个参数，将前两个参数指示的区间元素值为_Oldval 的替换成_Newval。
if (*_First == _Oldval)
*_First = _Newval;
（2）replace_copy 带5个参数，先判断前两个参数指示区间的元素是否是_Oldval，若是则替换成_Newval 赋值到第三个参数指示的区间上，否则直接赋值
*_Dest = *_First == _Oldval ? _Newval : *_First;
replace_copy_if 带5个参数，在每个元素拷贝时先判断是否满足条件（函数返回为真），满足则替换成_Val，否则保持不变。
*_Dest = _Pred(*_First) ? _Val : *_First;
参考：
C++ primer 第四版
Effective C++ 3rd
C++编程规范