为什么当我尝试向向量添加元素时会调用我的复制构造函数？

Why is my copy constructor being called when I try to add an element to my vector?

提问人：A6423 提问时间：8/24/2020 更新时间：8/24/2020 访问量：276

问：

我正在尝试将客户添加到我的向量中，当我运行我的程序时，将调用复制构造函数。我正在执行一项任务，其中我需要一个客户向量，并且必须能够添加客户、显示客户、查找客户并加载/存储数据。我创建错了向量吗？我只是 c++ 的新手，真的不确定向量。

Main.cpp代码：

#include <iostream>
#include <fstream>
#include <string>
#include <sstream>
#include <vector>
#include "Customer.h"
using namespace std;


void OutputFileStream();
void parseLine(const string& str);
void InputFileStream();
void save(vector<Customer> customers);
void load(vector<Customer>& customers);
void addCustomer(vector<Customer>& vect);
void displayAll(vector<Customer>& customers);
//void printActions();
vector<Customer> customers;

void OutputFileStream()
{
    cout << "Creating and writing to file: Customer.txt" << endl;

    ofstream outStream("customers.txt");  // write mode (overwrites existing data)

    if (outStream.good())
    {
        int customerID = 150033;
        outStream << "This is a line of text.\n";
        outStream << "This is another line of text.\n";
        outStream << "This is a line of text.\n";
        int numOfPurchases = 4;
        int purchases = 0;
        outStream << customerID << "Mr" << "Jack" << "New" << numOfPurchases << purchases << endl;
        //int *purchases = customers[0].getPurchases();
        outStream.close(); //  close file
        cout << "File written.\n" << endl;
    }
    else
        cout << "Unable to open file";
}

void parseLine(const string& str) {

    stringstream strStream(str); //create string stream from the string
   // int customerID;
    string title;
    string name;
    string type;
    //int numOfPurchases;
    //int purchases;
    string s;
   
    int customerID = 150033;
    getline(strStream, s, ';');
    customerID = stoi(s);

    getline(strStream, title, ';');
    getline(strStream, name, ';');
    getline(strStream, type, ';');

    int numOfPurchases = 4;
    getline(strStream, s, ';');
    numOfPurchases = stoi(s);

    int purchases = 0;
    getline(strStream, s, ';');
    purchases = stoi(s);
    int* purchasesArray = new int[3];
    purchasesArray[0] = (purchases & (255 << 16)) >> 16;
    purchasesArray[1] = (purchases & (255 << 8)) >> 8;
    purchasesArray[2] = purchases & 255;
    for (int i = 0; i < 3; i++)
    {
        cout << purchasesArray[i];
    }

    cout << " CustomerID: " << customerID << "Title:" << title << " Name: " << name << " Type:" << type << " Number of Purchases: " << numOfPurchases << "Purchases: " << purchases << endl;
}

void InputFileStream() {
    cout << "Reading from a semi-colon delimited txt file" << endl;

    string line;
    ifstream inStream("customers.txt"); //opens file as an input file stream
    if (inStream.good()) //if the file is opened successfully and not empty
    {
        while (getline(inStream, line)) //reads line until false return
        {
            parseLine(line);

        }
        inStream.close();
    }
    else
        cout << "unable to open file or the file is empty!";
}

void save(vector<Customer> customers)
{
    ofstream out("customers.txt");
    if(out)
    {
        for (Customer& c : customers)
        {
            out << c.save();
        }
        out.flush();
        out.close();
    }
    else
    {
        cout << "Error Writing to File" << endl;
    }
}

void load(vector<Customer>& customers)
{
    ifstream in("customers.txt");
    if (in) {
        string line;
        while (!in.eof())
        {
            getline(in, line);
            if (line != "")
            {
                Customer c;
                c.parse(line);
                customers.push_back(c);
            }
        }
    }
}

void addCustomer(vector<Customer>& customers) {
    Customer customer;
    cin >> customer;
    customers.push_back(customer);
    
}

void displayAll(vector<Customer>& customers)
{
    cout << "\nvector contains:\n";
    for (Customer c : customers)
    cout << c.getCustomerID() << " " << c.getTitle() << c.getName() << c.getNumOfPurchases() << c.getPurchases() << c.getType() << endl;
    cout << endl;; //same as calling customer
}
//
//int getCustomerByPurchaseNumber(int numOfPurchases) {
//    vector<Customer> customers;
//    int pos = -1;
//    for (int i = 0; i < customers.size(); i++) {
//        if (customers.at(i).getNumOfPurchases() == numOfPurchases) {
//            return i;
//        }
//    }
//    return pos;
//}
//
//void findCustomerByPurchaseNumber(vector<Customer>& customers) {
//    vector<Customer> customers;
//    int numOfPurchases;
//    cout << "Please Enter Your Purchase Number:" << endl;
//    cin >> numOfPurchases;
//    int pos = customers.get(pos);
//    getCustomerByPurchaseNumber(numOfPurchases);
//    if (pos == -1) {
//        cout << "Number of Purchase Not Found! "  << endl;
//          return customers;
//    }
//    else {
//        cout << "Number Of Purchase Found! " << endl;
//
//        return customers* = customers;
//    }
//    
//}


int main()
{
    vector<Customer> customers;
    Customer c1 = { 150031, "Mr", "John", 5, 333,362,341, "New" };
    customers.push_back(c1);
   
    //InputFileStream();
    

   /* Customer customer;

    customer.setCustomerID(150032);
    customer.setTitle("Mr");
    customer.setName("Joey");
    customer.setNumOfPurchases(3);
    customer.setPurchases(366, 352, 334);
    customer.setType("New");

    cout << customer.getCustomerID() << endl;
    cout << customer.getTitle() << endl;
    cout << customer.getName() << endl;
    cout << customer.getNumOfPurchases() << endl;
    cout << customer.getPurchases() << endl;
    cout << customer.getType() << endl;
   */
  
    return 0;
   
     
}

来自Customer.cpp的代码：

#include "Customer.h"
#include <iostream>
#include <string>
#include <fstream>
#include <sstream>
#include<utility>
using namespace std;

//default constructor
Customer::Customer() {
}


//Full constructor
Customer::Customer(int customerID, string title, string name, int numOfPurchases, int purchase1, int purchase2, int purchase3, string type)
{
    this->customerID = customerID;
    this->title = title;
    this->name = name;
    this->numOfPurchases = numOfPurchases;
    purchases = new int[3];
    purchases[0] = purchase1;
    purchases[1] = purchase2;
    purchases[2] = purchase3;
    this->type = type;
}

Customer::Customer(const Customer& source) //copy constructor
{
    cout << "copy constructor called" << endl;
    this->customerID = source.customerID;
    this->title = source.title;
    this->name = source.name;
    this->numOfPurchases = source.numOfPurchases;
    this->purchases = new int[3];
    purchases[0] = source.purchases[0];
    purchases[1] = source.purchases[1];
    purchases[2] = source.purchases[2];
    this->type = source.type;
}

//overloaded assignment operator=
Customer& Customer::operator= (Customer& otherCustomer)
{
    cout << "Overloaded assignment operator= called" << endl;

    //self-assignment guard
    if (this == &otherCustomer)
        return *this;  //refernce to the same object

   // copy data from the source (rhs) to this object (the destination)
    name = otherCustomer.name;

    //must make a new scores object to store a copy of the other student 
    if (purchases != nullptr)
        delete[] purchases;

    purchases = new int[3];
    for (int i = 0; i < 3; i++) {
        purchases[i] = otherCustomer.purchases[i];
    }

    //return this existing object so we can chain this operator 
    return *this;
}

string Customer::save()
{
    stringstream out;
    out << this->customerID << ";";
    out << this->title << ";";
    out << this->name << ";";
    out << this->type << ";\n";
    out << this->numOfPurchases  << ";";
    int* purchases = 0;
    
    out.flush();
    return out.str();
}

void Customer::parse(string line)
{
    stringstream in(line);
    string customerIDLine;
    getline(in, customerIDLine, ';');
    customerID = stoi(customerIDLine);
    getline(in, title, ';');
    getline(in, name, ';');
    getline(in, type, ';');

    string numOfPurchases;
    getline(in, numOfPurchases, ';');
    int s = stoi(numOfPurchases);

    int* purchasesArray = new int[3];
    purchasesArray[0] = (s & (255 << 16)) >> 16;
    purchasesArray[1] = (s & (255 << 8)) >> 8;
    purchasesArray[2] = s & 255;
}  

void Customer::addCustomer(vector<Customer>& customers )
{
    //after read data 
    int customerID;
    cout << "Please Enter Customer ID: " << endl;
    cin >> customerID; 
    string title;
    cout << "Please Enter Title: " << endl;
    getline(cin, title);
    string name;
    cout << "Please Enter Name: " << endl;
    getline(cin, name);
    string type;
    cout << "Please Enter Type: " << endl;
    getline(cin, type);
    int numOfPurchases;
    cout << "Please Enter Number of Purchases: " << endl;
    cin >> numOfPurchases;
    int purchase1;
    cout << "Please Enter First Purchase: " << endl;
    cin >> purchase1;
    int purchase2;
    cout << "Please Enter Second Purchase: " << endl;
    cin >> purchase2;
    int purchase3;
    cout << "Please Enter Third Purchase: " << endl;
    cin >> purchase3;
    Customer c;
    customers.push_back(c);
    //Customer c();
}


Customer::~Customer() {
    cout << "Destructor ~Customer called" << endl;
    delete[] purchases;
}

// Overloaded insertion operator  (Outputs Character object data as an output stream)
// Defined in header file as a  "friend" function, as it is not a member function
//
ostream& operator<<(ostream& out, Customer& customer)
{
    cout << "Customer details ( output by insertion operator<< )" << endl;
    cout << "Customer ID: " << customer.customerID << endl;
    cout << "Title: " << customer.title << endl;
    cout << "Name: " << customer.name << endl;
    cout << "Number of purchases: " << customer.numOfPurchases << endl;
    cout << "Purchases: ";
    for (int i = 0; i < 3; i++)
    {
        if (i > 0) cout << ",";
        cout << customer.purchases[i];
    }
    cout << "Type: " << customer.type << endl;
    
    return out;
}


istream& operator>> (istream& in, Customer& customer)
{
    cout << "Enter Customer details ( using the extraction operator>> )" << endl;
    cout << "Enter Customer ID: " << endl;
    cin >> customer.customerID;
    cout << "Enter Title: " << endl;
    getline(cin, customer.title);
    cout << "Enter Name: " << endl;
    getline(cin, customer.name);
    cout << "Enter Number of Purchases: ";
    cin >> customer.numOfPurchases; 
    cout << "Enter Purchases: ";
    cin >> customer.purchases[0];
    cin >> customer.purchases[1];
    cin >> customer.purchases[2];
    cout << "Enter Type";
    getline(cin, customer.type);
    cout << endl;

    return in;
}

int Customer::getCustomerID()
{
    return customerID;
}

string Customer::getTitle()
{
    return title;
}

string Customer::getName()
{
    return name;
}

int Customer::getNumOfPurchases()
{
    return numOfPurchases;
}

int* Customer::getPurchases()
{
    return purchases;
}

string Customer::getType()
{
    return type;
}

void Customer::setCustomerID(int customerID)
{
    if (customerID < 1) {
        cout << "Customer ID has to be equal to 1 or more" << endl; //Changed all the "throw invalid_argument" messages to cout as they were causing an issue with my main.cpp file and an abort message kept appearing every time I ran my main.cpp file.
    }
    this->customerID = customerID;
}

void Customer::setTitle(string title)
{
    if (title.length() < 2) {
        cout << "Title has to be more than or equal to 2 characters" << endl;
    }
    this->title = title;
}

void Customer::setName(string name)
{
    if (name.length() < 4) {
        cout << "Length of name should be more than or equal to 4 characters" << endl;
    }
    this->name = name;
}

//Got help ith this on stack overflow as I was using "&&" instead of using "||" for the if statement
void Customer::setNumOfPurchases(int numOfPurchases)
{
    if(numOfPurchases <0 || numOfPurchases > 10000){
        cout << "Number of purchases should be between 0 to 10000" << endl;
    }
    this->numOfPurchases = numOfPurchases;
}

void Customer::setPurchases(int purchase1, int purchase2, int purchase3)
{
    if (purchase1 < 0 || purchase2 < 0 || purchase3 < 0) {
        cout << "Purchases must be more than or equal to zero" << endl;
    }
}

//Got help from stack overflow on comparing strings as I originally didnt use "type.compare"
void Customer::setType(string type) {
    if (type.compare("New") !=0 || type.compare("Either") !=0) {
        cout << "Type of purchase has to be New or Either" << endl;
    }
}

来自 Customer.h 的代码：

#pragma once
#include<iostream>
using namespace std;
#include<string>
#include <vector>

class Customer
{
private:
    int customerID;
    string title;
    string name;
    int numOfPurchases;
    int* purchases;
    string type;

public:
    Customer(); // default constructor
    Customer(int customerID, string title, string name, int numOfPurchases, int purchase1, int purchase2, int purchase3, string type);
    
    //copy overload assignment
    Customer& operator=(Customer& otherCustomer);
    Customer(const Customer& source);

    string save();
    void parse(string line);
    void addCustomer(vector<Customer>& customers);
    ~Customer(); //destructor


    //Getters and Setters
    void setCustomerID(int customerID);
    void setTitle(string title);
    void setName(string name);
    void setNumOfPurchases(int numOfPurchases);
    void setPurchases(int purchase1, int purchase2, int purchase3);
    void setType(string type);

    int getCustomerID();
    string getTitle();
    string getName();
    int getNumOfPurchases();
    int* getPurchases();
    string getType();

    void printCustomer() {
        cout << customerID << "," << title << "," << name << "," << numOfPurchases << "," << purchases << "," << type << endl;
    }

    

    friend std::ostream& operator<<(std::ostream& out, Customer& customer); // overloaded operator<<
    friend istream& operator>> (istream& in, Customer& customer); // overloaded operator >>
};
C++ Vector 复制构造函数

评论

2赞 user253751 8/24/2020
否则元素将如何进入向量？
2赞 463035818_is_not_an_ai 8/24/2020
你为什么认为这是一个问题？
0赞 A6423 8/24/2020
我很困惑，复制构造函数是否要被调用，然后我打印了矢量？如果这是一个愚蠢的问题，@user253751真的很抱歉，但我是 C++ 的新手。
0赞 Giogre 8/24/2020
您关心自己的自定义类的复制构造函数。你不应该关心类的内部机制，比如标准库 （STL） 提供的 。后者开箱即用，一旦您在文件的开头。vector#include <vector>
1赞 user253751 8/24/2020
无法移动对象。当您将对象添加到向量时，它会在向量中创建一个新对象，该对象是原始对象的副本。

答：

1赞 Niclas Larsson 8/24/2020 #1

按值保存元素，因此只需使用即可将本地对象复制到向量中。std::vector<Customer>std::vector::push_back

为了缓解这种情况，您可以实现移动语义并像这样移动对象：Customer

    #include <utility>
    ....
    Customer c;
    c.parse(line);
    customers.push_back(std::move(c));

或就地构造对象：

    // Post c++17 you could use `push_back()` here as well as 
    // c++17 has mandatory copy elision.
    customers.emplace_back(Customer{}); // or customers.emplace_back(); only
    customers.back().parse(line);

这两种解决方案都需要 c++11。

您还可以存储引用/指针并在堆上分配对象。

    std::vector<std::unique_ptr<Customer>> customerVector;
    customer_vector.push_back(std::make_unique<Customer>());
    // or customer_vector.push_back(std::make_shared<Customer>());

或

    std::vector<Customer*> customerVector;
    // Beware for memory leaks, needs to be deleted.
    // Basically never use code like this post c++11).
    customer_vector.push_back(new Customer{});

评论

0赞 user253751 8/24/2020
这并不能回答这个问题。
0赞 Asteroids With Wings 8/24/2020
@user253751 是的，确实如此。
0赞 A6423 8/24/2020
感谢您的帮助！我现在要试试这个！:)
2赞 463035818_is_not_an_ai 8/24/2020 #2

向量保存对象。如果在向量之外有一个对象，则无法在向量中拥有相同的对象。您可以在向量中从一个实例移动到一个实例（这将移动实例内部部分，但仍有两个实例）。 当无法移动其参数时，确实会复制其参数。push_back

由于您有一个采用所有必要参数的构造函数，因此您可以使用来避免复制并在向量中就地构造实例：emplace_back

customers.emplace_back( customerID, title, name, numOfPurchases, purchase1, purchase2, purchase3, type);

评论

0赞 A6423 8/24/2020
这摆脱了被调用的复制构造函数！非常感谢您的帮助！:)
1赞 Nikola 8/24/2020 #3

如果选中对 std：：vector<T 的引用，则可以通过复制（选项 1）或移动（选项 2）将 Allocator>：:p ush_back 对象添加到 vector 中。

因此，您可以为客户类创建移动构造函数：

class_name ( class_name && );

然后在向量上使用该移动构造器（选项 2）调用push_back：

void push_back( T&& value );

上一个：在复制构造函数中做指针的深度复制向量是否更改了两个向量成员？

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