çpreprocessor基于一个参数宏观专业化

问题描述：

是否有可能有一个不同的宏观扩展了一个具体的参数值，并以不同的方式，所有其它参数？

Is it possible to have one macro expanded differently for one specific argument value and differently for all other arguments?

说我定义当前用户：

#define CURRENT_USER john_smith

我希望能够做的是有一个宏，将的扩展的不同，如果用户通过匹配 CURRENT_USER 。你要知道，我不知道所有可能的用户的先验。最基本的情况：

What I want to be able to do is to have a macro that will be expanded differently if user passed matches CURRENT_USER. Mind you that I don't know all possible user a priori. The most basic case:

#define IS_CURRENT_USER(user)                   \
    /* this is not valid preprocessor macro */  \
    #if user == CURRENT_USER                    \
        1                                       \
    #else                                       \
        0                                       \
    #endif

有了这样所有其他宏观依靠用户名可以做下面的方式宏：

With macro like that every other macro relying on the username could be done following way:

#define SOME_USER_SPECIFIC_MACRO(user) SOME_USER_SPECIFIC_MACRO_SWITCH_1(IS_CURRENT_USER(user))

#define SOME_USER_SPECIFIC_MACRO_SWITCH_1(switch)   SOME_USER_SPECIFIC_MACRO_SWITCH_2(switch) // expand switch ...
#define SOME_USER_SPECIFIC_MACRO_SWITCH_2(switch)   SOME_USER_SPECIFIC_MACRO_##switch         // ... and select specific case

#define SOME_USER_SPECIFIC_MACRO_0  ... // not current user
#define SOME_USER_SPECIFIC_MACRO_1  ... // current user

这可能吗？

编辑：让我澄清一下。说每个程序员定义自己的配置定义不同 CURRENT_USER 。我想用户特定的宏exand为有意义当且仅当它们的用户参数匹配 CURRENT_USER 。因为我想这些宏包含 _Pragma 的IT不能运行时检查。

Let me clarify. Say each programmer defines different CURRENT_USER in their config header. I want user specific macros to exand to something meaningful if and only if their user argument matches CURRENT_USER. As I would like those macros to contain _pragmas it can't be runtime check (as proposed in some anwsers below).

编辑：再次澄清。说有宏禁用code部分路段的优化：

Again, clarification. Say there's macro to disable optimisation of some sections of code:

#define TURN_OPTIMISATION_OFF __pragma optimize("", off)

一些程序员要关闭优化功能为code的不同部分，但不是全部在同一时间。我想是有一个宏：

Some programmers want to turn optimisation off for different sections of code but not all at one time. What I'd like is to have a macro:

#define TURN_OPTIMISATION_OFF(user) /* magic */

这将匹配 CURRENT_USER 宏，每程序员配置文件采取用户参数。如果用户匹配宏被扩展成编译。如果没有，不了了之。

That will match user argument against CURRENT_USER macro, taken from per-programmer config file. If the user matches macro is expanded into pragma. If not, to nothing.

答

大桶出来的可以。此anwser基于帕兹宏，但更简单的，并且不需要定义为每个用户。

Tuns out it is possible. This anwser is based on Pauls macros, but much simpler and does not need definition for each user.

#define CAT(a, ...) PRIMITIVE_CAT(a, __VA_ARGS__)
#define PRIMITIVE_CAT(a, ...) a ## __VA_ARGS__

#define IIF(c) PRIMITIVE_CAT(IIF_, c)
#define IIF_0(t, ...) __VA_ARGS__
#define IIF_1(t, ...) t

#define PROBE(x) x, 1

现在，因为MSVC错误我不得不修改检查宏位。

Now, because of the MSVC bug I had to modify CHECK macro a bit.

#define MSVC_VA_ARGS_WORKAROUND(define, args) define args
#define CHECK(...) MSVC_VA_ARGS_WORKAROUND(CHECK_N, (__VA_ARGS__, 0))
#define CHECK_N(x, n, ...) n

而不是定义 CURRENT_USER 的

我切换到下列宏。

Instead of defining CURRENT_USER I switched to following macros.

#define ENABLE_USER_gwiazdorrr () // gwiazdorrr is now enabled
#define ENABLE_USER_foo ()        // foo is also enabled
// #define ENABLE_USER_bar ()     // bar is NOT enabled

它实际上提供了更多的灵活性，因为可以同时使多个用户。括号是必需的。下面居然宏检测，是否 ENABLE_USER_＆lt;使用者方式＆gt; 扩展成括号与否

#define USER_ENABLED_PROBE(user)            USER_ENABLED_PROBE_PROXY( ENABLE_USER_##user ) // concatenate prefix with user name
#define USER_ENABLED_PROBE_PROXY(...)       USER_ENABLED_PROBE_PRIMIVIE(__VA_ARGS__)       // expand arguments
#define USER_ENABLED_PROBE_PRIMIVIE(x)      USER_ENABLED_PROBE_COMBINE_##x                 // merge
#define USER_ENABLED_PROBE_COMBINE_(...)    PROBE(~)                                       // if merge successful, expand to probe

USER_ENABLED_PROBE(gwiazdorrr) // expands to ~, 1
USER_ENABLED_PROBE(bar)        // expands to USER_ENABLED_PROBE_COMBINE_bar

从现在它是一个孩子的戏：

From now it is a childs play:

#define IS_USER_ENABLED(user) CHECK(USER_ENABLED_PROBE(user))

IS_USER_ENABLED(gwiazdorrr)   // expands to 1
IS_USER_ENABLED(bar)          // expands to 0

有了这个宏观和 IIF （感谢保罗！）我决定执行原来的问题中提到的优化宏：

Having this macro and IIF (thanks Paul!) I decided to implement the optimisation macro mentioned in the original question:

#define TURN_OPTIMISATION_OFF(user) IIF( IS_USER_ENABLED(user) ) \
    (\
        __pragma optimize("", off),\
        /* nothing */ \
    )

TURN_OPTIMISATION_OFF(gwiazdorrr) // expands into __pragma optimize("", off)
TURN_OPTIMISATION_OFF(foo)        // expands into __pragma optimize("", off)
TURN_OPTIMISATION_OFF(bar)        // nothing emitted

感谢您的输入！

编辑：这里的GCC版本： http://ideone.com/129eo

here's the GCC version: http://ideone.com/129eo

çpreprocessor基于一个参数宏观专业化

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