我在将 std::for_each 和其他算法与多映射一起使用时遇到困难,并且想知道是否有人可以帮助我开发一个可以将适当参数传递给“通用”函数的函子。
我对 map/multimap 的具体问题是它们的迭代器评估为 std::pair 而不是我需要使用的包含值(我的意思是 mapped_type)。所以,我的问题是,有没有办法将正确的值传递给设计用于使用其中一种包含类型的地图/多地图的函数?
这是我的示例代码:
// the key type for the multimap
typedef enum { ... } Module;
// my class as the mapped type for the multimap
struct Patch
{
void Apply(bool enable);
}
// I have some functors designed to work directly with Patch pointers
// because in other places I use set<Patch*> or other containers
struct ApplyOnCondtion: public unary_function<Patch*, void>
{
void operator() (Patch* patch)
{
if(some_condition) patch->Apply(enable_or_not);
}
}
// somewhere I have a collection of patches attributed to some module
multimap<Module, Patch*> patches;
// the problem is that using for_each with a map or a multimap results in
// a `pair<Module,Patch*>` as argument to the functor, not the Patch* as desired.
for_each(patches.begin(), patches.end(), ApplyOnCondition(...));
我认为也许 bind1st 或 bind2nd 与 mem_fun 结合可以解决这个问题,或者我能想到的另一种方法是创建一个新的仿函数来存储原始仿函数并传递对的正确成员,但我不是设法得到任何好的结果。有STL经验的人可以给点建议吗?
EDIT 1
好的,在不使用 boost 或额外的临时容器的情况下,我能得到的最好的结果如下:
#include <functional>
#include <utility>
using namespace std;
//////////////////////////////////////////////////////////////////////////
// any functor to be called must be derived from unary_function or
// have defined result_type and argument_type.
// template 'First' should be set to pair::first_type
template<typename First, typename Func>
class passSecond_t: public unary_function<
pair<First,typename Func::argument_type>,
typename Func::result_type>
{
Func* func;
public:
passSecond_t(Func &functor): func(&functor) {}
result_type operator()(argument_type value)
{
return (*func)(value.second);
}
};
// construction helper, unfortunately 'First' must be explicitly specified
template <typename First, typename Func>
passSecond_t<First, Func> passSecond(Func& functor)
{
return passSecond_t<First, Func> (functor);
}
// the following is a sample
#include <map>
#include <algorithm>
#include <iostream>
struct SampleClass
{
void execute(char* text)
{
cout << "this: " << this << ", text: " << text << endl;
}
};
struct SampleFunctor: public unary_function<SampleClass*,void>
{
char* text;
SampleFunctor(char* text_): text(text_) {}
result_type operator() (argument_type argu)
{
argu->execute(text);
}
};
void main()
{
map<int,SampleClass*> mymap;
SampleClass s1, s2;
mymap[0] = &s1;
mymap[1] = &s2;
SampleFunctor myfunctor("my text");
for_each(mymap.begin(), mymap.end(), passSecond<int>(myfunctor));
}