如何在for_each中组合函数和谓词？ [英] How to combine a function and a predicate in for_each?

问题描述

如何使用 for_each（）在部分容器中调用 Function $ c>？

我创建了一个 for_each_if（）来执行

  for（i in shapes）
 if（i.color == 1）
 displayShape（i）; 
  

，呼叫看起来像

  for_each_if（shapes.begin（），shapes.end（），
 bind2nd（ptr_fun（colorEquals），0），
 ptr_fun（displayShape） 
 
 bool colorEquals（Shape& s，int color）{
 return s.color == color; 
} 
  

但是，我认为类似STL的算法不是我应该

p>

我没有想做

  for_each（shapes.begin（），shapes.end（），
 bind2nd（ptr_fun（display_shape_if_color_equals），0））; 
  

因为在一个更复杂的情况下，函子名称会误导， / p>

*有一种方法可以访问结构的成员（如 colorEquals $ c> for_each 而不必创建一个函数？ *

解决方案

如果您需要使用常规的for_each模仿if条件的函子。

  #include< algorithm> 
 #include< vector> 
 #include< functional> 
 #include< iostream> 
 #include< boost / bind.hpp> 
 
 using namespace std; 
 
 struct incr {
 typedef void result_type; 
 void operator（）（int& i）{++ i; } 
}; 
 
 struct is_odd {
 typedef bool return_type; 
 bool operator（）（const int& value）{return（value％2）== 1; } 
}; 
 
 
 template< class Fun，class Cond> 
 struct if_fun {
 typedef void result_type; 
 void operator（）（Fun fun，Cond cond，int& i）{
 if（cond（i））fun（i）; 
} 
}; 
 
 
 int main（）{
 vector< int> vec; 
 for（int i = 0; i <10; ++ i）vec.push_back（i）; 
 
 for_each（vec.begin（），vec.end（），boost :: bind（if_fun< incr，is_odd>（），incr（），is_odd（），_1） 
 for（vector< int> :: const_iterator it = vec.begin（）; it！= vec.end（）; ++ it）
 cout< * it< ; 
} 
  

不幸的是，我的模板hackery不够好用bind1st和bind2nd因为它不知何故让粘合剂返回是一个 unary_function 混淆，但它看起来不错， boost :: bind 。我的例子不是完美的，因为它不允许Func传递到if_fun返回，我想有人可以指出更多的缺陷。欢迎建议。

How can you call a Function over some part of a container, using for_each() ?

I have created a for_each_if() to do a

for( i in shapes )
    if( i.color == 1 )
        displayShape(i);

and the call looks like

for_each_if( shapes.begin(), shapes.end(),
                       bind2nd( ptr_fun(colorEquals), 0 ),
                       ptr_fun( displayShape ) );

bool colorEquals( Shape& s, int color ) {
    return s.color == color;
}

However, I feel immitating STL-like algorithms is not something that I should be doing.

1. Is there a way to use only existing STL keywords to produce this ?

   I did not want to do a

    for_each( shapes.begin(), shapes.end(),
                      bind2nd( ptr_fun(display_shape_if_color_equals), 0 ) );
   

   because, in a more complicated case, the functor name would be misleading with respect to what the functor

2. *Is there a way to access a struct's member (like colorEquals) for functions like for_each without having to create a function ? *

解决方案

To use a regular for_each with an if you would need a Functor that emulates an if condition.

#include <algorithm>
#include <vector>
#include <functional>
#include <iostream>
#include <boost/bind.hpp>

using namespace std;

struct incr {
  typedef void result_type;
  void operator()(int& i) { ++i; }
};

struct is_odd {
  typedef bool return_type;
  bool operator() (const int& value) {return (value%2)==1; }
};


template<class Fun, class Cond>
struct if_fun {
  typedef void result_type;
  void operator()(Fun fun, Cond cond, int& i) {
    if(cond(i)) fun(i);
  }
};


int main() {
  vector<int> vec;
  for(int i = 0; i < 10; ++i) vec.push_back(i);

  for_each(vec.begin(), vec.end(), boost::bind(if_fun<incr, is_odd>(), incr(), is_odd(), _1));
  for(vector<int>::const_iterator it = vec.begin(); it != vec.end(); ++it)
    cout << *it << " ";
}

Unfortunately my template hackery isn't good enough to manage this with bind1st and bind2nd as it somehow gets confusing with the binder being returned being a unary_function but it looks pretty good with boost::bind anyhow. My example is no means perfect as it doesn't allow the Func passed into if_fun to return and I guess somebody could point out more flaws. Suggestions are welcome.

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