为什么C ++不允许匿名结构? [英] Why does C++ disallow anonymous structs?
问题描述
某些C ++编译器允许匿名联合和结构作为标准C ++的扩展。这是一种语法糖,有时非常有帮助。
Some C++ compilers permit anonymous unions and structs as an extension to standard C++. It's a bit of syntactic sugar that's occasionally very helpful.
阻止该标准成为标准的原因是什么?有技术障碍吗?哲学的?还是只是没有足够的理由来证明这一点?
What's the rationale that prevents this from being part of the standard? Is there a technical roadblock? A philosophical one? Or just not enough of a need to justify it?
这里是我正在谈论的示例:
Here's a sample of what I'm talking about:
struct vector3 {
union {
struct {
float x;
float y;
float z;
};
float v[3];
};
};
我的编译器会接受此操作,但是它会警告无名结构/联合是对C ++的非标准扩展。。
My compiler will accept this, but it warns that "nameless struct/union" is a non-standard extension to C++.
推荐答案
正如其他人指出的那样,标准C ++中允许匿名联合,但匿名结构则不允许。
As others have pointed out anonymous unions are permitted in standard C++, but anonymous structs are not.
这样做的原因是C支持匿名联合,但不支持匿名结构*,因此C ++支持前者以实现兼容性,但不支持后者,因为不需要兼容性。
The reason for this is that C supports anonymous unions but not anonymous structs*, so C++ supports the former for compatibility but not the latter because it's not needed for compatibility.
此外,C ++中的匿名结构用处不大。您演示的用法具有一个包含三个浮点数的结构,可以用 .v [i]
或 .x $来引用c $ c>,
.y
和 .z
,我相信会导致C ++中的行为不确定。 C ++不允许您写一个联合成员,例如 .v [1]
,然后再读另一个成员,例如 .y
。尽管执行此操作的代码并不罕见,但实际上定义不正确。
Furthermore, there's not much use to anonymous structs in C++. The use you demonstrate, to have a struct containing three floats which can be referred to either by .v[i]
, or .x
, .y
, and .z
, I believe results in undefined behavior in C++. C++ does not allow you to write to one member of a union, say .v[1]
, and then read from another member, say .y
. Although code that does this is not uncommon it is not actually well defined.
C ++的用户定义类型工具提供了替代解决方案。例如:
C++'s facilities for user-defined types provide alternative solutions. For example:
struct vector3 {
float v[3];
float &operator[] (int i) { return v[i]; }
float &x() { return v[0]; }
float &y() { return v[1]; }
float &z() { return v[2]; }
};
* C11显然添加了匿名结构,因此将来对C ++的修订可能会添加它们。 / sub>
* C11 apparently adds anonymous structs, so a future revision to C++ may add them.
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