空白BSTR和空BSTR之间应该有区别吗？ [英] Should there be a difference between an empty BSTR and a NULL BSTR?

问题描述

当维护 COM 接口时，应将空 BSTR 视为 NULL ？
换句话说，这两个函数调用是否产生相同的结果？

When maintaining a COM interface should an empty BSTR be treated the same way as NULL? In other words should these two function calls produce the same result?

 // Empty BSTR
 CComBSTR empty(L""); // Or SysAllocString(L"")
 someObj->Foo(empty);

 // NULL BSTR
 someObj->Foo(NULL);     

推荐答案

是 - 空BSTR与空一。我记得当我们从VS6切换到2003时，我们遇到了各种各样的错误 - CComBSTR类改变了默认的构造函数，它使用NULL而不是空字符串来分配它。这种情况发生在你例如把一个BSTR当作一个常规的C风格字符串，并把它传递给某些函数，如 strlen ，或者尝试初始化一个 std：

Yes - a NULL BSTR is the same as an empty one. I remember we had all sorts of bugs that were uncovered when we switched from VS6 to 2003 - the CComBSTR class had a change to the default constructor that allocated it using NULL rather than an empty string. This happens when you for example treat a BSTR as a regular C style string and pass it to some function like strlen, or try to initialise a std::string with it.

Eric Lippert在 Eric的BSTR语义完整指南：

Eric Lippert discusses BSTR's in great detail in Eric's Complete Guide To BSTR Semantics:

让我先列出差异，然后讨论
，然后讨论
中每个点的细节。

Let me list the differences first and then discuss each point in excruciating detail.

1）BSTR必须有相同的
NULL和的语义。

1) A BSTR must have identical semantics for NULL and for "". A PWSZ frequently has different semantics for those.

2）BSTR必须被分配并释放
与SysAlloc *系列的
函数。 PWSZ可以是
栈中的
自动存储缓冲区，或者分配有malloc，new，
LocalAlloc或任何其他内存
分配器。

2) A BSTR must be allocated and freed with the SysAlloc* family of functions. A PWSZ can be an automatic-storage buffer from the stack or allocated with malloc, new, LocalAlloc or any other memory allocator.

3）BSTR是固定长度的。 PWSZ
可以是任何长度，只受
其
缓冲区中的有效内存量的限制。

3) A BSTR is of fixed length. A PWSZ may be of any length, limited only by the amount of valid memory in its buffer.

4）A BSTR总是指向缓冲区中的第一个
有效字符。

4) A BSTR always points to the first valid character in the buffer. A PWSZ may be a pointer to the middle or end of a string buffer.

5）当分配一个n字节的BSTR时，
可以是一个指向字符串缓冲区的中间或结束
的指针。 b $ b有n / 2个宽字符的空间。
当为PWSZ
分配n个字节时，您可以存储n / 2 - 1个字符 -
，您必须为null留出空间。

5) When allocating an n-byte BSTR you have room for n/2 wide characters. When you allocate n bytes for a PWSZ you can store n / 2 - 1 characters -- you have to leave room for the null.

6）BSTR可以包含任何Unicode数据
，包括零字符。 PWSZ
除非作为字符串结尾标记，否则不包含零字符
。
BSTR和PWSZ在最后一个有效的
字符之后总是有一个
零字符，但在BSTR中，有效的
字符可能是零字符。

6) A BSTR may contain any Unicode data including the zero character. A PWSZ never contains the zero character except as an end-of-string marker. Both a BSTR and a PWSZ always have a zero character after their last valid character, but in a BSTR a valid character may be a zero character.

7）BSTR实际上可能包含奇数
字节数 - 它可用于
移动二进制数据。 PWSZ是
，几乎总是偶数字节
，并且仅用于存储Unicode
字符串。

7) A BSTR may actually contain an odd number of bytes -- it may be used for moving binary data around. A PWSZ is almost always an even number of bytes and used only for storing Unicode strings.

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