Question

为了解释我的项目，我试图为容器提供一个从基类派生的Schedule对象。关键是要启动具有特定功能的未指定数量的对象，并将它们按特定顺序运行。为此，我需要从对象中获取此特定功能。

所以...我在操纵函数指针和多态性时遇到麻烦。我想从类方法中返回我的类的函数指针。

我有以下基础类：

#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

我有这个派生类：

#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，我正在尝试返回功能区域。我试图在没有std :: function和std :: bind的情况下执行此操作，但是无法返回函数指针。

现在我在使用带有c ++ 11标准的g ++时出现此编译器错误。

/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);

似乎bind的参数个数不正确，我不知道为什么，而且我不理解第二个错误。

有人可以帮助我解决我的问题吗？

谢谢

Answer 1

我认为以后的参数（即return_function的参数）需要占位符

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bind

通过替换

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

作者

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

返回类函数指针c ++

2 个答案: