using define label in a string

[hschew00]

Is there a way to declare a string where inside the string, a predefined label is included? I.e.,

#define MY_CODE 0x1000
char str[]="Here is MY_CODE"; //will not work I know

Any way to get around this instead of hard-coding it?

[JosAH]

Is there a way to declare a string where inside the string, a predefined label is included? I.e.,

#define MY_CODE 0x1000
char str[]="Here is MY_CODE"; //will not work I know

Any way to get around this instead of hard-coding it?

You can use a bit of preprocessor wizardry:

#define str(x) #x
#define concat(a, b) a##str(b)

#define MY_CODE 0x1000
char str[]=concat("Here is ",  MY_CODE);

kind regards,

Jos

[hschew00]

Jos,
Thanks for the quick reply. However, although it works, can you explain to me how it works in preprocessor? Intuitively I thought the above 2 defines is equivalent to sth like this
#define concat(a,b) a###b

Of course the above didn't work.

[JosAH]

Copied right from the Standard:

3.8.3.1 Argument substitution

After the arguments for the invocation of a function-like macro
have been identified, argument substitution takes place. A parameter in the replacement list, unless preceded by a # or ## preprocessing token or followed by a ## preprocessing token (see below), is replaced by the corresponding argument after all macros contained therein have been expanded. Before being substituted, each argument's preprocessing tokens are completely macro replaced as if they formed the rest of the source file; no other preprocessing tokens are available.


3.8.3.2 The # operator

Constraints

Each # preprocessing token in the replacement list for a function-like macro shall be followed by a parameter as the next preprocessing token in the replacement list.

Semantics

If, in the replacement list, a parameter is immediately preceded by a # preprocessing token, both are replaced by a single character string literal preprocessing token that contains the spelling of the preprocessing token sequence for the corresponding argument. Each occurrence of white space between the argument's preprocessing tokens becomes a single space character in the character string literal. White space before the first preprocessing token and after the last preprocessing token comprising the argument is deleted. Otherwise, the original spelling of each preprocessing token in the argument is retained in the character string literal, except for special handling for producing the spelling of string literals and character constants: a \ character is inserted before each and \ character of a character
constant or string literal (including the delimiting characters). If the replacement that results is not a valid character string literal, the behavior is undefined. The order of evaluation of # and ## operators is unspecified.


3.8.3.3 The ## operator

Constraints

A ## preprocessing token shall not occur at the beginning or at the end of a replacement list for either form of macro definition.

Semantics

If, in the replacement list, a parameter is immediately preceded or followed by a ## preprocessing token, the parameter is replaced by the corresponding argument's preprocessing token sequence. For both object-like and function-like macro invocations, before the replacement list is reexamined for more macro names to replace, each instance of a ## preprocessing token in the replacement list (not from an argument) is deleted and the preceding preprocessing token is concatenated with the following preprocessing token. If the result is not a valid preprocessing token, the behavior is undefined. The
resulting token is available for further macro replacement. The order of evaluation of ## operators is unspecified.

kind regards,

Jos