C++ 多行 DEFINE 指令？
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Multi-line DEFINE directives?
提问by Adam S
I am not an expert, so go easy on me. Are there any differences between these two code segments?
我不是专家，所以放轻松。这两个代码段之间有什么区别吗？
#define BIT3 (0x1
<
<
3)
static int a;
and
和
#define BIT3 (0x1 << 3) static int a;
Also, is there a way to write the first in one line? What is the point of this multi-line style? Is the following code good?
另外，有没有办法在一行中写第一个？这种多线样式的重点是什么？下面的代码好吗？
#define BIT3 (0x1 << 3)
static int a;
回答by Oliver Charlesworth
A multi-line macro is useful if you have a very complex macro which would be difficult to read if it were all on one line (although it's inadvisable to have very complex macros).
如果您有一个非常复杂的宏，如果它们都在一行上就会难以阅读，那么多行宏很有用（尽管不建议拥有非常复杂的宏）。
In general, you can write a multi-line define using the line-continuation character, \. So e.g.
通常，您可以使用行继续符\. 所以例如
#define MY_MACRO printf( \
"I like %d types of cheese\n", \
5 \
)
But you cannot do that with your first example. You cannot split tokens like that; the <<left-shift operator must always be written without any separating whitespace, otherwise it would be interpreted as two less-than operators. So maybe:
但是你不能用你的第一个例子做到这一点。你不能像那样拆分代币；该<<左移操作人员必须始终没有任何空格分开写，否则会被解释为两低于运营商。所以也许：
#define BIT3 (0x1 \
<< \
3) \
static int a;
which is now equivalent to your second example.
现在相当于你的第二个例子。
[Although I'm not sure how that macro would ever be useful!]
[虽然我不确定那个宏会有什么用处！]
回答by phoxis
For example:
例如：
#define fact(f,n) for (f=1; (n); (n)--) \
f*=n;
You can separate the lines with the \character. Note that it is not macro specific. You can add the \character in your code whenever you would like to break a long line.
您可以将行与\字符分开。请注意，它不是特定于宏的。您可以随时\在代码中添加该字符来打断长行。
回答by Prof. Falken contract breached
The first one should not work. Lines should be separated with backslash THEN newline. Like so:
第一个不应该工作。行应该用反斜杠 THEN 换行符分隔。像这样：
#define SOME_MACRO "whatever" \
"more" \
"yet more"
