1. Basic #define
We can use it as a way of text replacement.
#define POPULATION 1200000
When we are compiling it, we will replace every POPULATION to 1200000 in the following code.
2. Function Definition
#define can receive parameters as Funcitons.
#define max(x,y) (x)>(y)?(x):(y);
This function returns the bigger number. However, since without type detectation, it's more like funciton template. But there is still some fault.
For example:
#define ADD(a,b) a+b;
When you encounter code c*ADD(a,b)*d, the factual calculating process becomes c*a+b*d, not c*(a+b)*d. That's a big problem
Another emaple:
#define pin(int*);
pin a, b;
When you call the macro, you mean to indicate both a and b are int pointers. But factually, it is int*a, b, which means a is an int pointer, but b is int
type. Therefore, you should use another way typedef to replace define.
3. Macro Single line Definition
#define A(x) T_##x
if x = 1, then A(1) ======== T_1
You must be surprised by the result. Here "##"means combination. When you call this macro, the result is to combine T_ and x. Here x=1, so the result is T_1.
4.Macro Multilines Definintion
Many lines of #define is available. In MFC, this kind of method is constantly used.
#define MACRO(arg1, arg2) do { \
/* declarations */ \
stmt1; \
stmt2; \
/*........*/ \
} while(0) /* (no trailing ; ) */
Do not forget to add "/" after every line.
5.Condition Compiling
#ifdef WINDOWS
...............
...............
#endif
#ifdef LINUX
..............
..............
#endif
You can #define to setup compiling enviroment.
5. How to define Macro and cancel Macro
//定义宏
#define [MacroName] [MacroValue]
//取消宏
#undef [MacroName]
//普通宏
#define PI (3.1415926)
//带参数的宏
#define max(a,b) ((a)>(b)? (a),(b))
6.Macro combination
## is to combine two macros
# is to replace the name with string
For example
#define s5(a) supper_ ## a
#include
void supper_printf(const char* p )
{
printf("this is supper printf:\n%s\n",p);
}
int main()
{
s5(printf)("hello owrld");
return 0;
}
Result:
This is supper printf:
hello world
Example
#include
#define s(p) #p
int main()
{
printf(s(p)"\n");
return 0;
}
Result:
p
# can be uesd to constrain it a parameter to become a string.
