错误：没有用于调用'std :: reference_wrapper <medium> :: reference_wrapper（）的匹配函数

Question

Book和Article是来自Medium的派生类。

为什么在尝试在参考书目中插入Medium / Book / Article时出现此错误？

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

main.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;
}

Bibliography.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

Bibliography.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

您不应在reference_wrapper中使用vector，因为vector可以保留具有默认构造函数的对象，reference_wrapper没有它，请查看reference_wrapper的这些构造函数{1}}：

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); 

您想要创建m_Size reference_wrapper个对象，但reference_wrapper的默认构造函数不存在，并且无法编译代码。

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