C++ class inheritance 和成員函數

Question

所有，我很難理解為什么會出現以下錯誤。 我 promise 這不是作業問題，但我是 C++ 的新手：這是我的代碼：

#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;
}

我得到的編譯器錯誤如下：

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(); }

I don't understand why the member function get_label() wasn't inherited from the Base class to the Derived class, such that I'm not able to access it via the VecDerived class. 有沒有辦法可以解決這個問題？ 在此先感謝您的幫助！

Answer 1

Visual Studio 發出一條錯誤消息，為您提供更多提示：

錯誤 C2247：“Base::get_label”不可訪問，因為“Derived”使用“private”從“Base”繼承

因此，如果您想通過Derived的 object 訪問Base::get_label ，那么您需要將基礎 class 公開：

class Derived : public Base

或公開get_label ：

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