无法使用带有 const 引用的 lambda 对 std::vector 进行排序

Question

我想根据自定义数据类型对向量进行排序。我跟着Sorting a vector of custom objects 回答。我正在使用 lambda 函数来比较对象。但是我在排序时遇到如下编译器错误：

/usr/include/c++/7/bits/stl_algo.h:1852：错误：无法将“MyData&”类型的非常量左值引用绑定到“std::remove_reference::type {aka MyData”类型的右值}' *__first = _GLIBCXX_MOVE(__val); ^

ma​​in.cpp

#include "mydata.h"
#include <vector>

int main()
{
    std::vector<MyData> tv {MyData(2,21), MyData(3,20), MyData(10,100), MyData(9,20)};

    std::sort(tv.begin(), tv.end(), []( MyData const& lhs, MyData const& rhs ){
         return lhs.get_size() < rhs.get_size();
    });

    return 0;
}

mydata.cpp

#ifndef MYDATA_H
#define MYDATA_H
#include <iostream>
#include <algorithm>

class MyData
{
private:

    int *m_data;
    int m_x;
    size_t m_size;

public:
    MyData(const size_t &size,int const &x):
        m_data(new int[size]),
        m_x(x),
        m_size(size)
    {

        std::fill_n(m_data,m_size, m_x);
        std::cout << *m_data << " ctor" << m_size << std::endl;
    }

    MyData(const MyData& other):
        m_data(new int[other.m_size]),
        m_x(other.m_x),
        m_size(other.m_size)
    {
        std::fill_n(m_data,m_size, m_x);
        std::cout << *m_data << " cctor" << m_size << std::endl;
    }

    MyData& operator=(MyData& other)
    {
        std::cout << *m_data << " cbctor" << m_size << std::endl;
        swap(*this,other);
        std::cout << *m_data << " cactor" << m_size << std::endl;
        return *this;
    }

    ~MyData(){
        std::cout << *m_data << " dtor" << m_size << std::endl;
        delete[] m_data;
    }

    size_t get_size() const{
        return m_size;
    }

    friend void swap(MyData& first, MyData& second){    // (1)
        std::swap(first.m_size, second.m_size);
        std::swap(first.m_x, second.m_x);
        std::swap(first.m_data, second.m_data);
    }

    friend std::ostream& operator<< (std::ostream& stream, const MyData& mydata) {
        stream << *(mydata.m_data) << " " << mydata.m_size << " "<< mydata.m_x;
        return stream;

    }

};

#endif // MYDATA_H

我不明白这个错误。我没有更改引用的值，为什么会出现此错误。 我还阅读了this，但不明白为什么会在这里发生。 谢谢。

Answer 1

可以有某种类型的声明复制赋值运算符。

1. 当可以使用复制和交换习语时，这是复制赋值运算符的典型声明：

   MyData& operator=(MyData other);
   

2. 这是复制赋值运算符的典型声明，当 无法使用复制和交换习语（不可交换类型或降级 性能）：

   MyData& operator=(const MyData& other);
   

所以要在您的实现中使用交换，您可以将复制赋值运算符声明为MyData& operator=(MyData other);

Answer 2

你应该像这样修改你的代码：

#include <iostream>
#include <fstream>
#include <thread>
#include <atomic>
#include <algorithm>
#include <vector>

using namespace std;
class MyData
{
private:

    int *m_data;
    int m_x;
    size_t m_size;

public:
    MyData(const size_t &size, int const &x) :
        m_data(new int[size]),
        m_x(x),
        m_size(size)
    {

        std::fill_n(m_data, m_size, m_x);
        std::cout << *m_data << " ctor" << m_size << std::endl;
    }

    MyData(const MyData& other) :
        m_data(new int[other.m_size]),
        m_x(other.m_x),
        m_size(other.m_size)
    {
        std::fill_n(m_data, m_size, m_x);
        std::cout << *m_data << " cctor" << m_size << std::endl;
    }

    MyData& operator=(const MyData& other)
    {
        std::cout << *m_data << " cbctor" << m_size << std::endl;
        //swap(*this, other);

        if (this != &other)
        {
            this->m_data = new int[other.m_size];
            for (size_t i = 0; i < other.m_size; ++i)
            {
                this->m_data[i] = other.m_data[i];
            }
            this->m_x = other.m_x;
            this->m_size = other.m_size;
        }
        std::cout << *m_data << " cactor" << m_size << std::endl;
        return *this;
    }

    ~MyData() {
        std::cout << *m_data << " dtor" << m_size << std::endl;
        delete[] m_data;
    }

    size_t get_size() const {
        return m_size;
    }

    friend void swap(MyData& first, MyData& second) {    // (1)
        std::swap(first.m_size, second.m_size);
        std::swap(first.m_x, second.m_x);
        std::swap(first.m_data, second.m_data);
    }

    friend std::ostream& operator<< (std::ostream& stream, const MyData& mydata) {
        stream << *(mydata.m_data) << " " << mydata.m_size << " " << mydata.m_x;
        return stream;

    }

};

int main()
{
    std::vector<MyData> tv{ MyData(2,21), MyData(3,20), MyData(10,100), MyData(9,20) };

    std::sort(tv.begin(), tv.end(), [](MyData const& lhs, MyData const& rhs) {
        return lhs.get_size() < rhs.get_size();
    });
    std::system("pause");
    return 0;
}