什么是 int() 调用?

问题描述

原始类型没有构造函数已经被反复强调了.例如，当我调用 Foo() 时，此 _bar 未初始化为 0:

It's been rehashed over and over that primitive types don't have constructors. For example this _bar is not initialized to 0 when I call Foo():

class Foo{
    int _bar;
};

显然 int() 不是构造函数.但是是它的名字是什么?

So obviously int() is not a constructor. But what is it's name?

在这个例子中，我会说 i 是:(构造的?初始化的?吃错的?)

In this example I would say i is: (constructed? initialized? fooed?)

for(int i{}; i < 13; ++i)

Loki Astari 提到 这里该技术有某种名称.

Loki Astari mentions here that the technique has some sort of name.

编辑以回应 Mike Seymour:

#include <iostream>

using namespace std;

class Foo{
    int _bar;
public:
    void printBar(){ cout << _bar << endl; }
};

int main()
{
    Foo foo;

    foo.printBar();

    Foo().printBar();

    return 0;
}

在 Visual Studio 2013 上运行此代码会产生:

Running this code on Visual Studio 2013 yields:

3382592
3382592

有趣的是 gcc 4.8.1 产量:

Interestingly on gcc 4.8.1 yields:

134514651
0

推荐答案

没错.

例如，当我调用 Foo()

是的.Foo() 指定值初始化，对于像这样没有用户提供构造函数的类，这意味着它在初始化其成员之前是零初始化的.所以 _bar 以零值结束.(尽管如评论中所述，一种流行的编译器没有正确地对此类类进行值初始化.)

Yes it is. Foo() specifies value-initialisation which, for class like this with no user-provided constructor, means it's zero-initialised before initialising its members. So _bar ends up with the value zero. (Although, as noted in the comments, one popular compiler doesn't correctly value-initialise such classes.)

如果你改用默认初始化，它不会被初始化.你不能用临时的来做到这一点；但是声明的变量 Foo f; 或 new F 的对象将被默认初始化.原始类型的默认初始化什么都不做，给它们留下一个不确定的值.

It would not be initialised if you were to use default-initialisation instead. You can't do that with a temporary; but a declared variable Foo f; or an object by new F will be default-initialised. Default-initialisation of primitive types does nothing, leaving them with an indeterminate value.

如果该类有一个用户提供的默认构造函数，并且该构造函数没有专门初始化 _bar，它也不会被初始化.同样，它将被默认初始化，没有任何效果.

It would also not be initialised if the class had a user-provided default constructor, and that constructor didn't specifically initialise _bar. Again, it would be default-initialised, with no effect.

显然 int() 不是构造函数.但它叫什么名字?

作为表达式，它是 int 类型的值初始化临时值.

As an expression, it's a value-initialised temporary of type int.

从句法上讲，它是显式类型转换(函数式)"的特例；但是将该术语用于除类型转换之外的任何内容都会相当混乱.

Syntactically, it's a special case of an "explicit type conversion (functional notation)"; but it would be rather confusing to use that term for anything other than a type conversion.

在这个例子中，我会说 i 是:(构造的?初始化的?吃错的?)

已初始化.如果您想更具体，列表初始化(带有空列表)、值初始化或零初始化.

Initialised. List-initialised (with an empty list), value-initialised, or zero-initialised, if you want to be more specific.

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