C++ 类继承和成员函数

Question

所有， 我很难理解为什么会出现以下错误。我保证这不是作业问题，但我是 C++ 新手！这是我的代码：

#include <iostream>
#include <string>
#include <vector>

class Base {
protected:
  std::string label;
public:
  Base(std::string _label) : label(_label) { }
  std::string get_label() { return label; }
};

class Derived : private Base {
private:
  std::string fancylabel;
public:
  Derived(std::string _label, std::string _fancylabel)
   : Base{_label}, fancylabel{_fancylabel} { }
  std::string get_fancylabel() { return fancylabel; }
};

class VecDerived {
private:
  std::vector<Derived> vec_derived;
public:
  VecDerived(int n)
  {
    vec_derived = {};
    for (int i = 0; i < n; ++i) {
      Derived newDerived(std::to_string(i), std::to_string(2*i));
      vec_derived.push_back(newDerived);
    }
  }
  std::string get_label(int n)  { return vec_derived.at(n).get_label(); }
  std::string get_fancylabel(int n) { return vec_derived.at(n).get_fancylabel(); }
};

int main (void)
{
  VecDerived obj(5);
  std::cout << obj.get_label(2) << " " << obj.get_fancylabel(2) << "\n";
  return 0;
}

我得到的编译器错误如下：

test1.cpp: In member function ‘std::__cxx11::string VecDerived::get_label(int)’:
test1.cpp:33:70: error: ‘std::__cxx11::string Base::get_label()’ is inaccessible within this context
   std::string get_label(int n)  { return vec_derived.at(n).get_label(); }
                                                                  ^
test1.cpp:9:15: note: declared here
   std::string get_label() { return label; }
               ^~~~~~~~~
test1.cpp:33:70: error: ‘Base’ is not an accessible base of ‘__gnu_cxx::__alloc_traits<std::allocator<Derived> >::value_type {aka Derived}’
   std::string get_label(int n)  { return vec_derived.at(n).get_label(); }

我不明白为什么成员函数 get_label() 没有从 Base 类继承到 Derived 类，因此我无法通过 VecDerived 类访问它。有没有办法可以解决这个问题？提前感谢您的帮助！

Answer 1

Visual Studio 发出一条错误消息，为您提供更多提示：

错误 C2247：“Base::get_label”不可访问，因为“Derived”使用“private”从“Base”继承

因此，如果您想通过Derived 对象访问Base::get_label，那么您需要将基类公开：

class Derived : public Base

或将get_label公开：

class Derived : private Base {
public:
    using Base::get_label;