C++ 推回之间的差异

Question

下面是具有不同类型的推回的代码。 我的意思是用不同的输入反击。 一个例子是accounts.push_back(five) 。 我不确定为什么每种推回的构造函数打印输出都不同。 不仅顺序不同，而且使用的深度复制构造函数的次数也不同。

#include <iostream>
#include <vector>

class account
{
private:

    std::string *name_ptr;
    double *total_ptr;

public:
    
    std::string get_name() { return *name_ptr; }
    void set_name(std::string new_name) { *name_ptr = new_name; }
    
    double get_total() { return *total_ptr; }
    void set_total(double new_total) { *total_ptr = new_total; }
    
    void deposit(double amount) { *total_ptr += amount; }
    void withdraw(double amount) { *total_ptr -= amount; }

    account(std::string name, double total = 0)
    {
        name_ptr = new std::string {name};
        total_ptr = new double {total};
        std::cout << "Constructor for " << *name_ptr << std::endl;
    }
    
    account(const account &source)
    : account {*(source.name_ptr), *(source.total_ptr)}
    { std::cout << "Deep copy constructor for " << *name_ptr << std::endl; }
    
    account(account &&source) noexcept
    : name_ptr(source.name_ptr), total_ptr(source.total_ptr)
    {
        source.name_ptr = nullptr;
        source.total_ptr = nullptr;
        std::cout << "Move constructor for " << *name_ptr << std::endl;
    }
    
    ~account()
    {
        if(name_ptr == nullptr) std::cout << "Destructor for nullptr" << std::endl;
        else std::cout << "Destructor for " << *name_ptr << std::endl;
        delete name_ptr;
        delete total_ptr;
    }

};

int main()
{
    std::vector<account> accounts;
    
    account one {"Jonathon Stevenson"};
    accounts.push_back( account(one) );
    account two {"Cliffard Stevenson"};
    accounts.push_back( account(two) );
    account three {"Tom Stevenson"};
    accounts.push_back( account(three) );
    std::cout << std::endl;
    
    account four {"Jonathon Richardson"};
    accounts.push_back(four);
    account five {"Cliffard Richardson"};
    accounts.push_back(five);
    account six {"Tom Richardson"};
    accounts.push_back(six);
    std::cout << std::endl;
    
    accounts.push_back( account {"Jonathon Watson"} );
    accounts.push_back( account {"Cliffard Watson"} );
    accounts.push_back( account {"Tom Watson"} );
    std::cout << std::endl;
    
    accounts.push_back( account("Jonathon Sullivanson") );
    accounts.push_back( account("Cliffard Sullivanson") );
    accounts.push_back( account("Tom Sullivanson") );
    std::cout << std::endl;

    return 0;
}

Output：

Constructor for Jonathon Stevenson
Constructor for Jonathon Stevenson
Deep copy constructor for Jonathon Stevenson
Move constructor for Jonathon Stevenson
Destructor for nullptr
Constructor for Cliffard Stevenson
Constructor for Cliffard Stevenson
Deep copy constructor for Cliffard Stevenson
Move constructor for Cliffard Stevenson
Move constructor for Jonathon Stevenson
Destructor for nullptr
Destructor for nullptr
Constructor for Tom Stevenson
Constructor for Tom Stevenson
Deep copy constructor for Tom Stevenson
Move constructor for Tom Stevenson
Move constructor for Jonathon Stevenson
Move constructor for Cliffard Stevenson
Destructor for nullptr
Destructor for nullptr
Destructor for nullptr

Constructor for Jonathon Richardson
Constructor for Jonathon Richardson
Deep copy constructor for Jonathon Richardson
Constructor for Cliffard Richardson
Constructor for Cliffard Richardson
Deep copy constructor for Cliffard Richardson
Move constructor for Jonathon Stevenson
Move constructor for Cliffard Stevenson
Move constructor for Tom Stevenson
Move constructor for Jonathon Richardson
Destructor for nullptr
Destructor for nullptr
Destructor for nullptr
Destructor for nullptr
Constructor for Tom Richardson
Constructor for Tom Richardson
Deep copy constructor for Tom Richardson

Constructor for Jonathon Watson
Move constructor for Jonathon Watson
Destructor for nullptr
Constructor for Cliffard Watson
Move constructor for Cliffard Watson
Destructor for nullptr
Constructor for Tom Watson
Move constructor for Tom Watson
Move constructor for Jonathon Stevenson
Move constructor for Cliffard Stevenson
Move constructor for Tom Stevenson
Move constructor for Jonathon Richardson
Move constructor for Cliffard Richardson
Move constructor for Tom Richardson
Move constructor for Jonathon Watson
Move constructor for Cliffard Watson
Destructor for nullptr
Destructor for nullptr
Destructor for nullptr
Destructor for nullptr
Destructor for nullptr
Destructor for nullptr
Destructor for nullptr
Destructor for nullptr
Destructor for nullptr

Constructor for Jonathon Sullivanson
Move constructor for Jonathon Sullivanson
Destructor for nullptr
Constructor for Cliffard Sullivanson
Move constructor for Cliffard Sullivanson
Destructor for nullptr
Constructor for Tom Sullivanson
Move constructor for Tom Sullivanson
Destructor for nullptr

Destructor for Tom Richardson
Destructor for Cliffard Richardson
Destructor for Jonathon Richardson
Destructor for Tom Stevenson
Destructor for Cliffard Stevenson
Destructor for Jonathon Stevenson
Destructor for Jonathon Stevenson
Destructor for Cliffard Stevenson
Destructor for Tom Stevenson
Destructor for Jonathon Richardson
Destructor for Cliffard Richardson
Destructor for Tom Richardson
Destructor for Jonathon Watson
Destructor for Cliffard Watson
Destructor for Tom Watson
Destructor for Jonathon Sullivanson
Destructor for Cliffard Sullivanson
Destructor for Tom Sullivanson

Answer 1

这就是std::vector如何工作的全部内容。

每当它用完 memory 时，在下一个std::vector::push_back上它只是分配一个新块，复制（或移动，如果可能的话）所有数据，然后删除旧数据。 由于它无法真正知道您需要多少空间，因此自动重新分配依赖于某种策略，其中最常见的是分配N * 2 ，其中N是当前容量。 现在让我们看看main中发生了什么。

std::vector<account> accounts; - 这会创建一个空向量（无分配）。

account one {"Jonathon Stevenson"}; - 这会通过调用您的 class 创建一个 object，我们称之为字符串构造函数，这就是您将第一个 output 放入控制台的地方。

accounts.push_back(account(one)); - 这个负责接下来的三条消息（ account(one)使用复制构造函数创建一个临时的，它通过调用字符串构造函数来工作；之后，所说的临时被移动到向量中）。 这里accounts的容量是1；

创建和推送的下一个“迭代”以相同的方式工作，除了您获得额外的移动：即vector将数据移动到新位置。 这里accounts的容量是2；

下一个也一样。 这里accounts的容量是4；

然后，在accounts.push_back(four); - 您不再创建过多的临时文件，因此看不到 move-constructor 工作。 这里accounts的容量是4；

在下一次push_back发生另一个重新分配，您可以看到vector将所有内容（4 个元素）移动到新位置。

最后，当你尝试accounts.push_back( account("Tom Sullivanson") ); 您的容量是 8，并且accounts中的元素数量也是 8，因此您可以看到vector再次将其内容移动到新位置 - 正好 8 次。 现在容量为 16，并且此程序中不会再发生重新分配。