为什么我的简单的C宏观调控工作？ [英] Why doesn't my simple C macro work?

问题描述

我想打一个简单的宏调用的printf（）两次这样的

I want to make a simple macro that calls printf() twice like this

#ifdef ENABLE_DEBUGPRINTF
#define DEBUGPRINTF(msg) printf("At sim_time = %f:", sim_time); printf(msg);
#else
#define DEBUGPRINTF(msg)  //evalutes to nothing
#endif

现在，当我打电话

DEBUGPRINTF("Processed event type: %d with value %f\n", id, data)

它打印出的第一部分在sime_time = ...正确，但它说：处理的事件......后期打印的ID和数据不正确的值。

It prints the first part "At sime_time = ... " correctly but the latter part where it says "Processed events ... " prints the value for id and data incorrectly.

同时

printf("Processed event type: %d with value %f\n", id, data);

正确打印值。

当我试图通过准确写出了我认为宏观想要得到的，我有执行它。

When I try executing it by writing exactly out what I thought the macro would evaluate to, I have.

printf("At sim_time = %f:", sim_time); printf("Processed event type: %d with value %f\n", id, data);

这正常打印的一切！那么，为什么不是我的宏观评估这样做呢？

This prints everything correctly! So why isn't my macro evaluating to this?

推荐答案

由于你想和使用的常规的printf ，你想要的是一个宏观的充分的灵活性以可变参数参数：

Because you want and are using the full flexibility of a regular printf, what you want is a macro with a variadic argument:

#ifdef ENABLE_DEBUGPRINTF
#define DEBUGPRINTF(msg...) \
    printf("At sim_time = %f:", sim_time); printf(msg);
#else
#define DEBUGPRINTF(msg...)  /*evalutes to nothing*/
#endif

---

我以前做过很多次，我建议用封装做{}而（0）：

#ifdef ENABLE_DEBUGPRINTF
#define DEBUGPRINTF(msg...) \
    do { \
        printf("At sim_time = %f:", sim_time); \
        printf(msg); \
    } while (0)
#else
#define DEBUGPRINTF(msg...)  //evalutes to nothing
#endif

这允许你做的是这样的：

This allows you to do something like:

if (showit)
    DEBUGPRINTF("hit the showit point -- showit=%d\n",showit);

因此​​，使用宏code不必知道它实际上是两个语句[或无]

Thus, the code that uses the macro doesn't have to know that it's actually two statements [or none]

更新：

DEBUGPRINTF（MSG ...）不符合标准，但一些旧的编译器扩展。您的省略号之前错过了一个逗号。

DEBUGPRINTF(msg...) is not standard compliant, but some legacy compiler extension. You missed a comma before the ellipsis.

也许吧，但是，我个人还是preFER它，并用它在生产中code了10多年，一直

Perhaps, but, personally, I still prefer it, and have been using it in production code for 10+ years.

然而，这里有一些资源用于那些可能希望使用其他方法：

However, here are some resources for those that might wish to use the alternative ways:

1. https://gcc.gnu.org/onlinedocs/cpp/Variadic- Macros.html


2. https://en.wikipedia.org/wiki/Variadic_macro

1. https://gcc.gnu.org/onlinedocs/cpp/Variadic-Macros.html
2. https://en.wikipedia.org/wiki/Variadic_macro

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