问题描述
一些C ++编译器允许匿名联合和结构作为标准C ++的扩展。
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.
有什么理由阻止这种做法成为标准的一部分?是否有技术障碍?哲学的?
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?
这里是我所说的一个例子:
Here's a sample of what I'm talking about:
struct vector3 {
union {
struct {
float x;
float y;
float z;
};
float v[3];
};
};
我的编译器会接受这个,但它警告,。
推荐答案
由于其他人指出,标准C ++中允许使用匿名联合,但匿名结构不允许。
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支持匿名联合而不是匿名structs *,所以C ++支持前者兼容性,但不支持后者,因为它不需要兼容性。
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 ++中的匿名结构没有太多用处。使用你演示,有一个结构包含三个浮动可以引用 .v [i]
或 .x
, .y
和 .z
,我相信结果在C ++中未定义的行为。 C ++不允许您写入联合的一个成员,例如 .v [1]
,然后从另一个成员读取,例如 .y
。虽然这样做的代码并不罕见,但实际上并没有很好定义。
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 ++的用户定义类型的设施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]; }
};
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