#define string with numeric define

Question

For example, here is a C common #define:

#define USERNAME_LEN  100
#define SCAN_FMT  "%100s"

// str is input from somewhere
char username[USERNAME_LEN + 1];
ret = sscanf(str, SCAN_FMT, username);
// check ret == 1 ?

can we have something like:

#define SCAN_FMT   "%" USERNAME_LEN "s"

of course, this syntax is not what we want, but the ultimate goal is to mix numeric #define into string #define

Note: I know we can do something like:

sprintf(SCAN_FMT, "%%ds", USERNAME_LEN); // char SCAN_FMT[10];

but this is not what I am looking for, because it requires run-time generation, the best is to base on ANSI-C or std99.

This answer might help you: stackoverflow.com/a/20642870/694576 — alk
– alk, Commented Jan 15, 2014 at 11:52

alk · Accepted Answer · 2014-01-15 12:58:08Z

2

You might like to do it like this:

#define SCAN_FMT_STRINGIFY(max) "%"#max"s"
#define SCAN_FMT(max) SCAN_FMT_STRINGIFY(max)

#define USERNAME_MAXLEN (100)

...

  char username[USERNAME_MAXLEN + 1] = ""; /* Add one for the `0`-terminator. */
  int ret = sscanf(str, SCAN_FMT(USERNAME_MAXLEN), username);

answered Jan 15, 2014 at 12:58

alk

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Comments

shadeglare · Accepted Answer · 2014-01-15 11:56:22Z

2

You can use the preprocessor directives for these kind of tasks.

1.The first directive is # allows you to do such things:

#define str(x) #x
cout << str(test);

This will be translated into:

cout << "test";

2.The second directive is ##:

#define glue(a,b) a ## b
glue(c,out) << "test";

will be translated into:

cout << "test";

Look here for more info preprocessor

answered Jan 15, 2014 at 11:56

shadeglare

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2 Comments

Klas Lindbäck Over a year ago

There is a piece missing. OP wants to stringize a defined value after it has been substuted. IIRC, doing so requires a third #define.

Jens Gustedt Over a year ago

Not that it matters much for your answer, but the question is tagged C and no C++.

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