Can I use sizeof() or a #define for precision in sprintf of a string?

Question

Lookign at:
http://www.cplusplus.com/reference/clibrary/cstdio/sprintf/

for string (%.Ns) precision.

When I use sizeof or a #define length in the precsion it reads it as actual text.

Why is this? What are the rules of this? Does it have to be an integer value only?

i.e. -

buffer[50];

sprintf (buffer, "%.sizeof(buffer)s", string);  

or

#define MAX_LEN

sprintf (buffer, "%.MAX_LENs", string);  

Thanks

Answer 1

Anything inside quotation marks is part of the string, and the compiler won't even think of touching it. Instead, you can use a '*' to let sprintf know your precision is an extra argument it can read. Also, you need the '.' before your precision, or otherwise it will be a pad-width instead.

sprintf(buffer, "%.*s", (int) sizeof(buffer), string);

Answer 2

That's the way strings work; anything inside the "s will be interpreted as a string. You can use #defined numeric constants if you want like this:

#define MAX_LEN 50
buffer[50];
sprintf (buffer, "%" #MAX_LEN "s", string);

That uses compile-time string concatenation.

Answer 3

Not relevant to your example but important in similar situations if you a define like this:

#define SOMEONES_MIDDLE_NAME "Ray"

You can insert it into strings like this:

sprintf(buffer, "Billy " SOMEONES_MIDDLE_NAME " Cyrus", sizeof(buffer));

The compiler will automagically collect string literals into single blocks.