问题描述
People recommend #ifdef for conditional compilation by a wide margin. A search for #ifdef substantiates that its use is pervasive.
还 #ifdef名称（或等价的 #ifdefined（NAME）和相关的 #ifndef NAME （和 #if！defined（NAME））有一个严重缺陷：
Yet #ifdef NAME (or equivalently #if defined(NAME) and related #ifndef NAME (and #if !defined(NAME)) have a severe flaw:
#ifndef IS_SPECIAL
#error You're not special enough
#endif
source.cpp
source.cpp
#include "header.h"
显然会通过
#define IS_SPECIAL 1
#include "header.h"
但是，
#define IS_SPECIAL 0
#include "header.h"
错误的事情。而且一些C ++编译器传递了以 C 模式处理的文件（由于扩展名或命令行选项），从而有效地执行了 #define __cplusplus 0 。我见过当
which is quite the wrong thing to do. And some C++ compilers, passed a file processed in C mode (due to extension or command-line option) effectively do #define __cplusplus 0. I have seen things break when
#ifdef __cplusplus
extern "C" {
#endif
/* ... */
#ifdef __cplusplus
}
#endif
是在 C 模式下处理的，其中 extern C 是无效的语法，因为 __ cplusplus 实际上是自动定义为 0 。
was processed in C mode, where extern "C" is invalid syntax, because __cplusplus was in fact automatically defined to 0.
另一方面，此行为正确无误对于所有编译器：
On the other hand, this behaves correctly for all compilers:
#if __cplusplus
extern "C" {
#endif
/* ... */
#if __cplusplus
}
#endif
在这种情况下，为什么人们仍然使用 #ifdef ？他们是否只是不知道 #if 在未定义的名称上工作得很好？或者对于条件编译， #if 与 #ifdef 有实际的不利之处吗？
Why do people still use #ifdef in this scenario? Are they simply unaware that #if works perfectly fine on undefined names? Or is there an actual disadvantage to #if vs #ifdef for conditional compilation?
显然， #ifdef 确实具有有效的用法，例如提供可配置参数的默认值：
Obviously, #ifdef does have valid uses, such as providing default values for configurable parameters:
#ifndef MAX_FILES
#define MAX_FILES 64
#endif
我只讨论标志测试的情况。
I'm only discussing the case of flag testing.
推荐答案
个人意见：从命令行进行控制稍微容易一些。与 -DOPTION = 1 相比，我更喜欢 -DOPTION 。
Personal opinion: it's marginally easier to control from the command line. I prefer -DOPTION over -DOPTION=1.
此外，名称的存在显然是二进制的。我不必处理{0，非零，未定义}。
Also, existence of a name is clearly binary. I don't have to be able to handle {0, non-zero, undefined}.
我不知道。这是什么语义？未定义的名称是否假定为0？我是否要向几乎不了解预处理器的人解释一下？
I wasn't aware. What are the semantics of this? Is an undefined name assumed to be 0? Do I want to have to explain that to the guy who barely understands the preprocessor to begin with?
对我来说，＃ifdef /＃ifndef 带来了明显的好处。
另外，我对这两种构造的主要用法是用于include警卫。使用 #ifndef 这种模式最干净。
To me, the binary nature of #ifdef/#ifndef of name existence is a clarity benefit.Also, my primary usage of either construct is for include guards. That pattern is cleanest with #ifndef.
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