std::for_each
template <class InputIterator, class Function>
Function for_each (InputIterator first, InputIterator last, Function fn);
Apply function to range
Applies function fn to each of the elements in the range [first,last).
The behavior of this template function is equivalent to:
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Parameters
- first, last
-
Input iterators to the initial and final positions in a sequence. The range used is
[first,last), which contains all the elements between first and last, including the element pointed by first but not the element pointed by last. - fn
-
Unary function that accepts an element in the range as argument.
This can either be a function pointer or a move constructible function object.
Its return value, if any, is ignored.
Applies a specified function object to each element in a forward order within a range and returns the function object.
template<class InputIterator, class Function>
Function for_each(
InputIterator _First,
InputIterator _Last,
Function _Func
);
A copy of the function object after it has been applied to all of the elements in the range.
The algorithm for_each is very flexible, allowing the modification of each element within a range in different, user-specified ways. Templatized functions may be reused in a modified form by passing different parameters. User-defined functions may accumulate information within an internal state that the algorithm may return after processing all of the elements in the range.
The range referenced must be valid; all pointers must be dereferenceable and, within the sequence, the last position must be reachable from the first by incrementation.
The complexity is linear with at most (_Last – _First) comparisons.
头文件：
<algorithm>
说明：
for_each 算法范围 [_First, _ Last) 中的每个元素调用函数_Fn1，并返回输入的参数 _Func。此函数不会修改序列中的任何元素。
// for_each example
#include <iostream> // std::cout
#include <algorithm> // std::for_each
#include <vector> // std::vector
#include <list>
void myfunction (int i)
{ // function:
std::cout << ' ' << i;
}
struct myclass
{ // function object type:
void operator() (int i) {std::cout << ' ' << i;}
} myobject;
void test_my()
{
std::vector<int> myvector;
myvector.push_back(10);
myvector.push_back(20);
myvector.push_back(30);
std::cout << "myvector contains:";
for_each (myvector.begin(), myvector.end(), myfunction);
std::cout << '\n';
// or:
std::cout << "myvector contains:";
for_each (myvector.begin(), myvector.end(), myobject);
std::cout << '\n';
std::cout << "myvector contains:";
for_each (myvector.begin(), myvector.end(), myclass());
std::cout << '\n';
}
/******
运行结果：
myvector contains: 10 20 30
myvector contains: 10 20 30
myvector contains: 10 20 30
******/
//print为仿函数
struct print
{
int count;
print(){count = 0;}
void operator()(int x)
{
std::cout << x << std::endl;
++count;
}
};
void test_print()
{
using namespace std;
list<int> testlist;
//初始化
for ( size_t i = 1991; i <2013 ; ++i)
{
testlist.push_back(i);
}
//遍历testlist元素并打印
print p = for_each(testlist.begin(), testlist.end(), print());
//打印testlist元素个数
cout << "元素个数是："<<p.count << endl;
}
/******
运行结果：
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
元素个数是：22
******/
class Multi_By_x
{
private:
int multiplier;
public:
Multi_By_x(const int& k) : multiplier(k) { }
void operator()(const int& x) { std::cout << x * multiplier << " " << std::endl; }
};
void test_Multi_By_x()
{
int sequence[6] = {2007,2010,2011,2012,2013,2014};
using std::vector;
vector<int> v(sequence, sequence+6);
Multi_By_x Multi2(2);
for_each(v.begin(),v.end(),Multi2); // Apply function
}
/*************
运行结果：
4014
4020
4022
4024
4026
4028
*************/
int main ()
{
test_my();
std::cout<<"\n***************************************************\n";
test_print();
std::cout<<"\n***************************************************\n";
test_Multi_By_x();
std::cout<<"\n***************************************************\n";
return 0;
}
/***************
myvector contains: 10 20 30
myvector contains: 10 20 30
myvector contains: 10 20 30
***************************************************
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
元素个数是：22
***************************************************
4014
4020
4022
4024
4026
4028
***************************************************
Process returned 0 (0x0) execution time : 0.039 s
Press any key to continue.
****************/
再看一下MSDN上边给出的例子程序
：
#include <vector>
#include <algorithm>
#include <iostream>
// The function object multiplies an element by a Factor
template <class Type>
class MultValue
{
private:
Type Factor; // The value to multiply by
public:
// Constructor initializes the value to multiply by
MultValue ( const Type& _Val ) : Factor ( _Val )
{
}
// The function call for the element to be multiplied
void operator ( ) ( Type& elem ) const
{
elem *= Factor;
}
};
// The function object to determine the average
class Average
{
private:
long num; // The number of elements
long sum; // The sum of the elements
public:
// Constructor initializes the value to multiply by
Average ( ) : num ( 0 ) , sum ( 0 )
{
}
// The function call to process the next elment
void operator ( ) ( int elem )
{
num++; // Increment the element count
sum += elem; // Add the value to the partial sum
}
// return Average
operator double ( )
{
return static_cast <double> (sum)/static_cast <double> (num);
}
};
int main( )
{
using namespace std;
vector <int> v1;
vector <int>::iterator Iter1;
// Constructing vector v1
int i;
for ( i = -4 ; i <= 2 ; i++ )
{
v1.push_back( i );
}
cout << "Original vector v1 = ( " ;
for ( Iter1 = v1.begin( ) ; Iter1 != v1.end( ) ; Iter1++ )
cout << *Iter1 << " ";
cout << ")." << endl;
// Using for_each to multiply each element by a Factor
for_each ( v1.begin ( ) , v1.end ( ) , MultValue<int> ( -2 ) );
cout << "Multiplying the elements of the vector v1\n "
<< "by the factor -2 gives:\n v1mod1 = ( " ;
for ( Iter1 = v1.begin( ) ; Iter1 != v1.end( ) ; Iter1++ )
cout << *Iter1 << " ";
cout << ")." << endl;
// The function object is templatized and so can be
// used again on the elements with a different Factor
for_each (v1.begin ( ) , v1.end ( ) , MultValue<int> (5 ) );
cout << "Multiplying the elements of the vector v1mod\n "
<< "by the factor 5 gives:\n v1mod2 = ( " ;
for ( Iter1 = v1.begin( ) ; Iter1 != v1.end( ) ; Iter1++ )
cout << *Iter1 << " ";
cout << ")." << endl;
// The local state of a function object can accumulate
// information about a sequence of actions that the
// return value can make available, here the Average
double avemod2 = for_each ( v1.begin ( ) , v1.end ( ) ,Average ( ) );
cout << "The average of the elements of v1 is:\n Average ( v1mod2 ) = "
<< avemod2 << "." << endl;
}
/*****************
运行结果如下：
Original vector v1 = ( -4 -3 -2 -1 0 1 2 ).
Multiplying the elements of the vector v1
by the factor -2 gives:
v1mod1 = ( 8 6 4 2 0 -2 -4 ).
Multiplying the elements of the vector v1mod
by the factor 5 gives:
v1mod2 = ( 40 30 20 10 0 -10 -20 ).
The average of the elements of v1 is:
Average ( v1mod2 ) = 10.
Process returned 0 (0x0) execution time : 0.122 s
Press any key to continue.
******************/