覆盖#define值 [英] override #define value

问题描述

如标题所述，我们可以覆盖#define的值



喜欢

Hi,

as title said, can we override #define's value

like

#include<stdio.h>
#define LIM 5

// in main function
// scanf x's value and assign it to LIM
// LIM = x;

我们能做到吗？

can we do that ??

推荐答案

你做不到。



#define在编译器的预编译阶段被替换（文本）。

You cannot do that.

The #define are replaced (textually) in by the pre-compilation phase of the compiler.

不，你不能用v覆盖#define常量在运行时可以运行。原因很简单：#define语句由编译器处理 - 在编译时，即在程序运行之前很久。并且编译器将这些值作为常量插入到程序代码中。



但是你可以执行以下操作：

No, you cannot override a #define constant with variable at run-time. The reason is simple: #define statements are processed by the compiler -- at compile-time, i.e. long before your program runs. And the compiler inserts those values as constants in your program code.

But you can do the following:

#define LIM 5

void MyFunction ()
{
    int x;

    if (!scanf ("%d", &x) != 1)
        x = LIM;
    ...    
}

正如其他解决方案所示，您无法在运行时更改值，但您可以在编译过程中更改宏的编译时定义。例如：

As the other solutions have indicated, you can't change a value at runtime, but you can change the compile time definition of a macro during the compile process. For example:

void foo()
{
#define LIM 5
    int zork[LIM]; // defined as size 5

#undef LIM
#define LIM 42
    int blork[LIM];  // defined as size 42

#undef LIM
#define LIM 123

    // At this point, LIM is 123, but zork is still size 5 and blork is still
    // size 42. This does not change during the compile process or during runtime.
#undef LIM
    // At this point, LIM is undefined.
}

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