为什么一些const变量引用一些导出的const变量得到的值为0？ [英] Why do some const variables referring to some exported const variables get the value 0?

问题描述

请考虑以下内容。我有两个导出的常数如下：

Consider the following. I have two exported constants as follows:

// somefile.h
extern const double cMyConstDouble;
extern const double cMyConstDouble2;

和

// somefile.cpp
const double cMyConstDouble = 3.14;
const double cMyConstDouble2 = 2.5*cMyConstDouble;

这些常量现在被引用一些地方以定义两个静态（局部可见）常量：

These constants are now referenced some place else to define two static (locally visible) constants:

// someotherfile.cpp
#include "somefile.h"
static const double cAnotherDouble = 1.1*cMyConstDouble;
static const double cAnotherDouble2 = 1.1*cMyConstDouble2;
printf("cAnotherDouble = %g, cAnotherDouble2 = %g\n",
       cAnotherDouble, cAnotherDouble2);

这将产生以下输出：

cAnotherDouble = 3.454, cAnotherDouble2 = 0

0？我使用.NET 2003 C ++编译器（13.10.3077）。

Why is the second double 0? I'm using .NET 2003 C++ compiler (13.10.3077).

推荐答案

因为cMyConstDouble声明为extern，能够承担其值并且不为cMyConstDouble2生成编译时初始化。由于cMyConstDouble2不是编译时初始化的，因此其相对于cAnotherDouble2的初始化顺序是随机的（未定义的）。有关详细信息，请参见静态初始化fiasco 。

Because cMyConstDouble is declared as extern, compiler is not able to assume its value and does not generate a compile time initialization for cMyConstDouble2. As the cMyConstDouble2 is not compile time initialized, its order of initialization relative to cAnotherDouble2 is random (undefined). See static initialization fiasco for more information.

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