Return Class 函数指针 c++

Question

为了解释我的项目，我试图让一个容器包含从基础 class 派生的计划对象。重点是启动具有特定功能的未指定数量的对象，并将它们运行按一定顺序排列。为此，我需要从对象中获取此特定功能。

所以...我在处理函数指针和多态性时遇到问题。我想从 class 方法 return 我的 Class 的函数指针。

我有以下基础class：

#ifndef PLUGIN_POLYGON_BASE_H_
#define PLUGIN_POLYGON_BASE_H_

#include <iostream>
#include <functional>

typedef std::function<int(void*,void*)> return_function;


namespace polygon_base
{
  class RegularPolygon
  {
    public:
      virtual int area(void* in, void* out) = 0;
      virtual return_function getFunction(void) = 0;
      virtual ~RegularPolygon(){}


    protected:
      RegularPolygon(){}
  };  
};
#endif

我有这个派生 class:

#include "polygon_base.h"
#include <cmath>

namespace polygon_plugin
{
  class Triangle : public polygon_base::RegularPolygon
  {
    public:
      Triangle(){}

      int area(void* in, void* out)
      {
        double* in_temp = (double*)in;
        double* out_temp = (double *)out;
        *out_temp = 0.5 *  *in_temp * getHeight(*in_temp);
        return 0;
      }

      return_function getFunction()
      {
        return std::bind(&Triangle::area, this);
      }

    private:
      double getHeight(double side_length_)
      {
        return sqrt((side_length_ * side_length_) - ((side_length_ / 2) * (side_length_ / 2)));
      }
    };

如您所见，我正在尝试 return 功能区。我尝试在没有 std::function 和 std::bind 的情况下执行此操作，但无法设法 return 函数指针。

现在我在使用 g++ 和 c++ 11 标准时遇到这个编译器错误。

/usr/include/c++/5/functional: In instantiation of ‘struct std::_Bind_check_arity<int (polygon_plugin::Triangle::*)(void*, void*), polygon_plugin::Triangle*>’:
/usr/include/c++/5/functional:1439:12:   required from ‘struct std::_Bind_helper<false, int (polygon_plugin::Triangle::*)(void*, void*), polygon_plugin::Triangle*>’
/usr/include/c++/5/functional:1462:5:   required by substitution of ‘template<class _Func, class ... _BoundArgs> typename std::_Bind_helper<std::__is_socketlike<_Func>::value, _Func, _BoundArgs ...>::type std::bind(_Func&&, _BoundArgs&& ...) [with _Func = int (polygon_plugin::Triangle::*)(void*, void*); _BoundArgs = {polygon_plugin::Triangle*}]’
polygon_plugin.h:27:47:   required from here
/usr/include/c++/5/functional:1426:7: error: static assertion failed: Wrong number of arguments for pointer-to-member
       static_assert(_Varargs::value
       ^
polygon_plugin.h: In member function ‘virtual return_function polygon_plugin::Triangle::getFunction()’:
polygon_plugin.h:27:25: error: could not convert ‘std::bind(_Func&&, _BoundArgs&& ...) [with _Func = int (polygon_plugin::Triangle::*)(void*, void*); _BoundArgs = {polygon_plugin::Triangle*}; typename std::_Bind_helper<std::__is_socketlike<_Func>::value, _Func, _BoundArgs ...>::type = std::_Bind<std::_Mem_fn<int (polygon_plugin::Triangle::*)(void*, void*)>(polygon_plugin::Triangle*)>](((polygon_plugin::Triangle*)this))’ from ‘std::_Bind_helper<false, int (polygon_plugin::Triangle::*)(void*, void*), polygon_plugin::Triangle*>::type {aka std::_Bind<std::_Mem_fn<int (polygon_plugin::Triangle::*)(void*, void*)>(polygon_plugin::Triangle*)>}’ to ‘return_function {aka std::function<int(void*, void*)>}’
         return std::bind(&Triangle::area, this);

bind 似乎没有正确数量的参数，我不明白为什么，我也不明白第二个错误。

谁能帮我解决我的问题？

谢谢

Answer 1

我认为您需要占位符来表示未来的参数（即 return_function 的参数）

using namespace std::placeholders;  // for _1, _2, _3...

std::bind(&Triangle::area, this, _1, _2);

Answer 2

您可以使用 lambda 表达式代替绑定

替换

std::bind(&Triangle::area, this);

来自

[this](void* in, void *out){return area(in, out);};

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