How to use # and # # special symbols in C language macros
This article will explain in detail how to use # and # # special symbols in C language macros. The editor thinks it is very practical, so I share it with you as a reference. I hope you can get something after reading this article.
The usage of "#" and "#" in macros
First, use "#" to change the macro parameters into a string, and use "#" to fit the two macro parameters together.
# define STR (val) (# val) # define CONS (a, b) (int) (a##e##b)-> STR (hello) = > "hello" CONS (2,3) = > 2000 / / 2e3
Second, when the macro parameter is another macro
It is important to note that where the macro definition is useful'#'or'# 'macro parameters are no longer expanded.
1, non -'# 'and' # 'cases
# define TOW (2) # define MUL (aPowerb) (asancb) printf ("% d*%d=%d\ n", TOW,TOW, MUL (TOW,TOW)); = > printf ("% d*%d=%d\ n", (2), (2), (2) * (2)
The parameter TOW in MUL will be expanded to (2).
2, when there is a'#'or'# #'
# define A (2) # define STR (s) # s # define CONS (define STR b) int (a##e##b) printf ("int max:% s\ n", STR (INT_MAX)); / / INT_MAX # include = > printf ("int max:% s\ n", "INT_MAX"); printf ("% s\ n", CONS (A, A)); / / compile error = > printf ("% s\ n", int (AeA))
INT_MAX and A will no longer be expanded, but the solution to this problem is very simple: add an additional layer of intermediate conversion macros, which is intended to expand all the macro parameters in this layer, then the macro in the conversion macro (_ STR) can get the correct macro parameters.
# define A (2) # define _ STR (s) (# s) # define STR (s) _ STR (s) / conversion macro # define _ CONS (amemb) int (a##e##b) # define CONS (ameme b) _ CONS (amemb) / / convert macro printf ("int max:% s\ n", STR (INT_MAX)); / / INT_MAX, the maximum of int type, is a variable # include
The output is: int max: 0x7fffffff
STR (INT_MAX)-- > _ STR (0x7fffffff) and then converted to a string; printf ("% d\ n", CONS (A, A))
Output: 200
CONS (A, A)-- > _ CONS ((2), (2)-- > int ((2) e (2))
Third, some special cases of'# 'and' #'
1. Merge anonymous variable names
# define _ _ ANONYMOUS1 (type, var, line) type var##line # define _ _ ANONYMOUS0 (type, line) _ _ ANONYMOUS1 (type, _ anonymous, line) # define ANONYMOUS (type) _ ANONYMOUS0 (type, _ LINE__)
Example: ANONYMOUS (static int)
That is, static int _ anonymous70; 70 represents the line number
First floor: ANONYMOUS (static int)
-- > _ _ ANONYMOUS0 (static int, _ LINE__)
Layer 2:-- > _ ANONYMOUS1 (static int, _ anonymous, 70)
Layer 3:-- > static int _ anonymous70
That is, you can only unlock the macro of the current layer at a time, so the _ _ LINE__ can only be unlocked at the second layer.
2. Filling structure
# define FILL (a) {a, # a} enum IDD {OPEN, CLOSE}; typedef struct MSG {IDD id; const char * msg;} MSG; MSG _ msg [] = {FILL (OPEN), FILL (CLOSE)}; = > MSG _ msg [] = {{OPEN, "OPEN"}, {CLOSE, "CLOSE"}}
3. Record file name
# define _ GET_FILE_NAME (f) # f # define GET_FILE_NAME (f) _ GET_FILE_NAME (f) static char FILE_NAME [] = GET_FILE_NAME (_ _ FILE__)
4. Get the string buffer size corresponding to a numeric type
# define _ TYPE_BUF_SIZE (type) sizeof # type # define TYPE_BUF_SIZE (type) _ TYPE_BUF_SIZE (type) char buf [type _ BUF_SIZE (INT_MAX)];-- > char buf [_ TYPE_BUF_SIZE (0x7fffffff)];-- > char buf [sizeof "0x7fffffff"]
This is equivalent to: char buf [11]
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