make_shared是否为每个成员变量执行默认的初始化（零初始化） [英] Does make_shared do a default initialization (zero-init) for each member variable

问题描述

采用普通的旧数据类型和对象作为成员的普通结构（或类）。注意，没有定义默认构造函数。

Take an ordinary struct (or class) with Plain Old Data types and objects as members. Note that there is no default constructor defined.

struct Foo
{
    int x;
    int y;
    double z;
    string str;
};

现在，如果我在堆栈上声明实例f并尝试打印其内容：

Now if I declare an instance f on the stack and attempt to print its contents:

{
    Foo f;
    std::cout << f.x << " " << f.y << " " << f.z << f.str << std::endl;
}

结果是为x，y和z打印的垃圾数据。并且该字符串默认初始化为空。 符合预期。

The result is garbage data printed for x, y, and z. And the string is default initialized to be empty. As expected.

如果我使用以下方式创建 shared_ptr< Foo> 的实例 make_shared 并打印：

If I create an instance of a shared_ptr<Foo> using make_shared and print:

{
    shared_ptr<Foo> spFoo = make_shared<Foo>();
    cout << spFoo->x << " " << spFoo->y << " " << spFoo->z << spFoo->str << endl;
}

然后，x，y和z均 0 。这样，在构造对象实例之后， shared_ptr 似乎在每个成员上执行默认初始化（零初始化）。至少我在Visual Studio的编译器中观察到了这一点。

Then, x, y, and z are all 0. Which makes it appear that shared_ptr performs a default initialization (zero init) on each member after the object instance is constructed. At least that's what I observe with Visual Studio's compiler.

这是C ++的标准吗？还是在实例化之后有必要使用显式构造函数或显式 = {} 语句来保证所有编译器的零初始化行为？

Is this standard for C++? Or would it be necessary to have an explicit constructor or explicit ={} statement after instantiation to guarantee zero-init behavior across all compilers?

推荐答案

如果看到例如此 std :: make_shared 参考您会看到

If you see e.g. this std::make_shared reference you will see that

该对象的构造就像由表达式 :: new（pv ）T（std :: forward< Args>（args）...），其中 pv 是内部 void * 指向适合保存类型为 T 的对象的存储的指针。

The object is constructed as if by the expression ::new (pv) T(std::forward<Args>(args)...), where pv is an internal void* pointer to storage suitable to hold an object of type T.

这意味着 std :: make_shared< Foo>（）基本上是 new Foo（）。也就是说，它 值初始化 归零非类成员变量。

That means std::make_shared<Foo>() basically does new Foo(). That is, it value initializes the structure which leads to the zeroing of the non-class member variables.

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