错误：没有匹配函数调用 'std::reference_wrapper<Medium>::reference_wrapper()

Question

Book 和Article 是Medium 的派生类。

为什么我在尝试在参考书目中插入 Medium / Book / Article 时收到此错误？

error: no matching function for call to '**std::reference_wrapper<Medium>::reference_wrapper()**

ma​​in.cc

#include <iostream>
using namespace std;
#include "Bibliography.h"
#include "Medium.h"
#include "Book.h"
#include "Article.h"

int main()
{
    Bibliography p(1);
    Medium m1("PN","I","Pasol nah",2017);
    p.insert(m1);
    cout << p;
    return 0;
}

---

参考书目.h

#ifndef BIBLIOGRAPHY_H_
#define BIBLIOGRAPHY_H_

#include "Medium.h"
#include "Article.h"
#include "Book.h"
#include <iostream>
#include <functional>
#include <vector>

class Bibliography
{
  private:
    int m_Size;
    std::vector<std::reference_wrapper<Medium>> v;
    int index;
  public:
    Bibliography(int size);
    void insert(Medium m);
    friend std::ostream& operator<<(std::ostream& out, const Bibliography &b1);
};

#endif

---

参考书目.cc

#include "Bibliography.h"

Bibliography::Bibliography(int size)
{
    std::cout << "Bibliography created \n";
    m_Size = size;
    v.resize(m_Size);
    index = 0;
}

void Bibliography::insert(Medium m)
{
    v.push_back(m);
}

std::ostream& operator<<(std::ostream& out, const Bibliography &b1)
{
    for (Medium &Medium : b1.v)
    {
        out << Medium.toString() << std::endl;
    }
    return out;
}

Answer 1

你不应该在vector中使用reference_wrapper，因为vector可以保留有默认构造函数的对象，reference_wrapper没有，看看reference_wrapper的这些构造函数：

initialization (1)  
reference_wrapper (type& ref) noexcept;
reference_wrapper (type&&) = delete;
copy (2)    
reference_wrapper (const reference_wrapper& x) noexcept;

在这一行

v.resize(m_Size); 

您想创建 m_Size reference_wrapperobjects，但 reference_wrapper 的默认构造函数不存在，代码无法编译。

您可以将reference_wrapper 与vector 一起使用，但会出现编译错误 当调用vector的方法需要定义默认构造函数时。