std::emplace_back 调用向量中其他对象的构造函数？

Posted 2023-02-16

【中文标题】std::emplace_back 调用向量中其他对象的构造函数？

【英文标题】：std::emplace_back invoking constructor of other objects in vector?

【发布时间】：2020-03-07 16:35:16

【问题描述】：

我真的对std::vector::emplace_back 感到困惑。我运行以下代码：

struct Thing 
    explicit Thing(std::string name) : name_std::move(name) 
        std::cout << "Constructed a thing called " << name_ << std::endl;
    

    ~Thing() 
        std::cout << "Deconstructed a thing called " << name_ << std::endl;
    ;
    std::string name_;
;


int main() 
    std::vector<Thing> thingsThing("A");
    std::cout << "Size: " << things.size() << std::endl;
    things.emplace_back("B");
    std::cout << "Size: " << things.size() << std::endl;

并得到这个输出：

Constructed a thing called A
Deconstructed a thing called A
Size: 1
Constructed a thing called B
Deconstructed a thing called A
Size: 2
Deconstructed a thing called A
Deconstructed a thing called B

到底为什么things.emplace_back("B") 会调用 A 的解构器？

【问题讨论】：

添加第二个元素是调用向量的调整大小，将旧元素复制到新向量，以及销毁旧元素。您的对象有一个默认的复制构造函数。创建一个具有类似 cout 的显式复制构造函数（和一个移动构造函数），您将看到正在复制或移动构造的新 A 对象。

请注意，C++ 术语是析构函数，而不是解构函数。

【参考方案1】：

std::vector 将对象连续存储在分配的内存块中。当该块的大小变得太小而无法向其中添加新元素时，它必须分配一个新的更大的内存块并将向量中当前存在的元素复制/移动到该新分配中。之后，之前分配中的对象被销毁，涉及到析构函数调用。

您没有看到正在制作的副本，因为您没有定义自定义复制构造函数。如果你这样做了，你会看到A 的副本是在原始A 被破坏之前构造的副本。如果定义noexcept 移动构造函数，您将看到A 的副本在原始A 被破坏之前被移动构造。

【讨论】：

世界再次变得有意义。谢谢！

【参考方案2】：

发生的情况是，当您将 B 添加到向量时，该向量可能只有 1 个元素的容量，因此必须将其现有元素移动到另一个向量中（破坏原始存储中的那个）

让我们添加更多日志来看看会发生什么：

#include <iostream>
#include <string>
#include <vector>

struct Thing 
    explicit Thing(std::string name) : name_std::move(name) 
        std::cout << "Constructed a thing called " << name_ << " at " << this << std::endl;
    
    Thing(Thing&& t) noexcept : name_std::move(t.name_) 
        std::cout << "Moved a thing called " << name_ << " from " << &t << " to " << this << std::endl;
    
    Thing(const Thing& t) : name_t.name_ 
        std::cout << "Copied a thing called " << name_ << " from " << &t << " to " << this << std::endl;
    

    ~Thing() 
        std::cout << "Deconstructed a thing called " << name_ << " at " << this << std::endl;
    ;
    std::string name_;
;


int main() 
    std::vector<Thing> thingsThing("A");
    std::cout << "Size: " << things.size() << " Capacity: " << things.capacity() << std::endl;
    things.emplace_back("B");
    std::cout << "Size: " << things.size() << " Capacity: " << things.capacity() << std::endl;

示例输出：

Constructed a thing called A at 0x1111
Copied a thing called A from 0x1111 to 0x2222
Deconstructed a thing called A at 0x1111
Size: 1 Capacity: 1
Constructed a thing called B at 0x3333
Moved a thing called A from 0x2222 to 0x4444
Deconstructed a thing called A at 0x2222
Size: 2 Capacity: 2
Deconstructed a thing called A at 0x4444
Deconstructed a thing called B at 0x3333