Here we talk about some misunderstandings of #define, because the last piece has already talked its main role, this is mainly a common macro trap. First of all. A classic example as follows.
#define private public # include
Using namespace std;
Class C
{
Private:
INT I;
}
INT main () {C C1; C1.I = 1; COUT <
<
}
#define actually let Private are so fragile, but reveal the root of #define traps, it is just a code replacement mechanism, in addition to this, it is nothing.
Let's take a look at the topic to see what kind of error will the definition below?
#DEFINE F (X) ((x) -1)
If this is a function, there is no problem INT f (int x) {returnx x-1;}
But here is the world of Define, f (x) only see a terrible space makes it makes the program if it appears.
f (10)
This code will eventually become
(10) (10-1) (10) Such a strange thing, of course, this code cannot be compiled, or can be checked. It is not so lucky to escape again. Let us continue.
#define abs (x) x> 0? x: -x (reference self C language traps and defects)
What is the problem with this code? Maybe everyone also noticed, I have been using countless () to write #define, not because I like () this thing, but when I am making the following call.
Z = ABS (A-B) // 呜, what will this be?
the answer is:
A-B> 0? A-B: -A-B
This is obviously not what we want, because when A-B <0 will return a -A-B, we must solve this problem, we will use () to solve it.
#define abs (x)> 0? (x) :-( x)
Now this code can work normally. As long as we keep the #define is a mechanism for code replacement, don't have any extravagance, you will avoid the above problems. In addition, because the macro is not a type, there is no data security check, there will be obstacles when debugging, so C has always advocated the use of const and inline to replace #define, maybe, # define really will disappear on the historical stage. But Define has not been forgotten in the C language era.
From http://community.9cbs.net/expert/topic/3195/3195102.xml?temp=.3936731
