C++中的对象创建和销毁顺序

Question

我写了一个简单的程序来学习更多关于在 C++ 中创建和销毁对象的顺序（使用 Visual Studio 2015）。在这里：

#include <iostream>
#include <string>

using namespace std;

class A
{
public:
    A(string name)
        : name(name)
    {
        cout << "A(" << name << ")::constructor()" << endl;
    }
    ~A()
    {
        cout << "A(" << name << ")::destructor()" << endl;
    }
private:
    string name;
};

class C
{
public:
    C(string name, A a)
        : name(name), a(a)
    {
        cout << "C(" << name << ")::constructor()" << endl;
    }
    ~C()
    {
        cout << "C(" << name << ")::destructor()" << endl;
    }
private:
    string name;
    A a;
};

class B
{
public:
    B(string name)
        : name(name)
    {
        cout << "B(" << name << ")::constructor()" << endl;
    }
    ~B()
    {
        cout << "B(" << name << ")::destructor()" << endl;
    }
private:
    string name;
    A a1{"a1"};
    A a2{"a2"};
    C c1{"c1", a1};
    A a3{"a3"};
};

int main()
{
    B b("b1");
    return 0;
}

输出让我有点吃惊（a1s）：

A(a1)::constructor()
A(a2)::constructor()
C(c1)::constructor()
A(a1)::destructor()
A(a3)::constructor()
B(b1)::constructor()
B(b1)::destructor()
A(a3)::destructor()
C(c1)::destructor()
A(a1)::destructor()
A(a2)::destructor()
A(a1)::destructor()

为了详细了解发生了什么，我添加了有关对象实例的信息：

    A(string name)
        : name(name)
    {
        cout << "A(" << name << ")::constructor(), this = " << this << endl;
    }
    ~A()
    {
        cout << "A(" << name << ")::destructor(), this = " << this << endl;
    }

结果更让人意外：

A(a1)::constructor(), this = 0039FB28
A(a2)::constructor(), this = 0039FB44
C(c1)::constructor()
A(a1)::destructor(), this = 0039F8A8
A(a3)::constructor(), this = 0039FB98
B(b1)::constructor()
B(b1)::destructor()
A(a3)::destructor(), this = 0039FB98
C(c1)::destructor()
A(a1)::destructor(), this = 0039FB7C
A(a2)::destructor(), this = 0039FB44
A(a1)::destructor(), this = 0039FB28

也就是说，为什么a1的构造函数只被调用了一次而析构函数只被调用了3次？我按值传递 a 所以显然至少创建了 1 个临时对象，但请向我解释 when 和 how many A实例被创建和销毁？

Answer 1

正如评论中已经指出的那样，当您按值将它们作为参数传递时，A 类型的对象也是通过复制构造构造的。为了看到这一点，您可以自己添加一个复制构造函数：

A(const A& other)
: name(other.name)
{
    cout << "A(" << name << ")::copy-constructor(), this = " << this << endl;
}

示例输出：

A(a1)::constructor(), this = 0xbff3512c
A(a2)::constructor(), this = 0xbff35130
A(a1)::copy-constructor(), this = 0xbff350e8
A(a1)::copy-constructor(), this = 0xbff35138
C(c1)::constructor()
A(a1)::destructor(), this = 0xbff350e8
A(a3)::constructor(), this = 0xbff3513c
B(b1)::constructor()
B(b1)::destructor()
A(a3)::destructor(), this = 0xbff3513c
C(c1)::destructor()
A(a1)::destructor(), this = 0xbff35138
A(a2)::destructor(), this = 0xbff35130
A(a1)::destructor(), this = 0xbff3512c

Try it online

如您所见，当您将 a1 作为参数传递给 c1 的构造函数时会发生一次复制构造，而当此构造函数初始化其成员 a 时会发生第二次复制构造。临时副本随即被销毁，而成员在 c 被销毁时被销毁。

编辑：
Here 您可以在创建复制构造函数时阅读确切的规则。
为了不创建默认的复制构造函数，提供任何用户定义的构造函数是不够的，它需要是 copy/move-constructor.

编辑2：

取自 C++14 标准（12.8 复制和移动 class 对象）：

7 If the class definition does not explicitly declare a copy constructor, one is declared implicitly. If the class definition declares a move constructor or move assignment operator, the implicitly declared copy constructor is defined as deleted; otherwise, it is defined as defaulted (8.4). The latter case is deprecated if the class has a user-declared copy assignment operator or a user-declared destructor.

C++中的对象创建和销毁顺序

Object creation and destruction order in C++

c++

temporary-objects

object-construction

object-destruction