测试空宏定义
我在tracing.hh 中有一组调试宏。是否生成代码并输出是由真实源代码中的宏标志控制的：
// File: foo.cc
#define TRACING 0
#include "tracing.hh"
// Away we go . . .
TRACEF("debug message");
标志TRACING应该有一个值；我通常在 0 和 1 之间切换。
在tracing.h 中，
#ifdef TRACING会告诉我跟踪已定义。#if TRACING控制诸如TRACEF()之类的函数宏的定义
，但是如果 TRACING 没有值怎么办？然后#if TRACING 会产生错误：
In file included from foo.c:3:
tracing.hh:73:12: error: #if with no expression
How can I test if TRACING is Defined but has no value？
I've got a set of debug macros in tracing.hh. Whether it generates code and output is controlled by a macro flag in the real source code:
// File: foo.cc
#define TRACING 0
#include "tracing.hh"
// Away we go . . .
TRACEF("debug message");
The flag TRACING should have a value; I usually toggle between 0 and 1.
Within tracing.h,
#ifdef TRACINGwill tell me that tracing was defined.#if TRACINGcontrols the definition of functional macros likeTRACEF()
But what if TRACING has no value? Then #if TRACING produces an error:
In file included from foo.c:3:
tracing.hh:73:12: error: #if with no expression
How can I test if TRACING is defined but has no value?
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根据 Matti 的建议和更多的探索，我认为问题是这样的：如果
TRACING没有值，我们需要在测试#if ...中使用有效的预处理器表达式。 Gnu cpp 手册说它必须计算为整数表达式，因此我们需要一个即使缺少其中一个参数也有效的表达式。我最后想到的是：
TRACING有一个数值（如#define TRACING 2 \n），cpp 有一个有效的表达式，并且我们没有改变值。TRACING没有值（如#define TRACING \n），预处理器将#if (+0)计算为false唯一不处理的情况是
TRACING具有非数字值（ie、ON）。 cpp 手册说“非宏的标识符...都被认为是数字零，”，其计算结果为false。然而，在这种情况下，将其视为true值更有意义。唯一正确的做法是布尔文字true和false。With Matti's suggestion and some more poking, I think the issue is this: if
TRACINGhas no value, we need a valid preprocessor expression in the test#if .... The Gnu cpp manualsays it has to evaluate to an integer expression, so we need an expression that is valid even if one of the arguments is missing. What I finally hit on is:
TRACINGhas a numerical value (as in#define TRACING 2 \n), cpp has a valid expression, and we haven't changed the value.TRACINGhas no value (as in#define TRACING \n), the preprocessor evaluates#if (+0)tofalseThe only case this doesn't handle is
TRACINGhas a non-numerical value (i.e.,ON). The cpp manual says "Identifiers that are not macros . . . are all considered to be the number zero," which evaluates tofalse. In this case, however, it would make more sense to consider this atruevalue. The only ones that do the right thing are the boolean literalstrueandfalse.虽然迟到了，但我发现了一个很好的技巧来区分
：
如果
TRACING为空，则表达式的计算结果为0--1=> <代码>1。如果
TRACING为 0，则表达式的计算结果为0-0-1=>-1我在
TRACING==-2情况下添加了额外的检查，这将使第一个测试通过。当然，这不适用于字符串文字。
Late to the party but I found a nice trick to distinguish
from
like this:
If
TRACINGis empty, the expression evaluates to0--1=>1.If
TRACINGis 0, the expression evaluates to0-0-1=>-1I added an extra check in case
TRACING==-2which would make the first test pass.This doesn't work for string literals of course.
会
成功吗？
Would
do the trick?