从命令行覆盖宏 [英] Override macro from the command line

问题描述

我想从命令行重写一个宏。在我的源代码中，有一个像这样的定义：

  #define MY_FOO 1 
  
 
 
 
我想在编译我的程式时设定这个宏的值：
 
 
  g ++ -DMY_FOO = 2 ... 
  
 ，宏由源代码重新定义为其旧值1.问题是我不拥有定义宏的源头部分。如果是我自己的代码，我可以简单地写
  #ifndef MY_FOO 
 #define MY_FOO 1 
 #endif 
  
我的问题将会消失。那么有没有办法在命令行中使用g ++指定一个宏，以便源代码无法重新定义它？ 
 
解决方案
您可以't。
 
 
 
命令行在任何源文件和头文件之前处理。
 
如果源文件定义了一个宏，那么它肯定不是定义之前，并将从现在开始获得新值。
 
更改它的唯一方法是 #undef 和 #define 在稍后的时间再次。如果您有权访问定义后包含的标题，那么您有机会。 
I would like to override a macro from the command line. Somewhere in my source, there is a definition like this:
#define MY_FOO 1
What I would like is to set the value of this macro when I compile my program:
g++ -DMY_FOO=2 ...
But then, the macro is redefined by the source code to its old value 1. The problem is that I don't own the part of the source that defines the macro in the first place. If it were my own code, I could simply write
#ifndef MY_FOO
#define MY_FOO 1
#endif
And my problem would be gone. So is there way to specify a macro on the command line using g++ so that the source code cannot redefine it?
解决方案
You can't.

The command line is handled before any source and header files.

If a source file defines a macro, then it must not be defined before, and will get the new value from now on.

The only way to change it, is to #undef and #define it again, at a later point. If you have access to a header that is included after the definition, then you have a chance.

                        
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