Define a macro off of the content of a macro

Is it possible to define a macro off of the content of a macro?

For example:

#define SET(key,开发者_如何学JAVAvalue) #define key value

SET(myKey,"value")

int main(){
   char str[] = myKey;
   printf("%s",str);
}

would result in

int main(){
   char str[] = "value";
   printf("%s",str);
}

after being preprocessed.

Why would I do this? Because I'm curious ;)

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No, its not possible to define a macro within another macro.

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The preprocessor only iterates once before the compiler. What you're suggesting would require an undetermined amount of iterations.

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No you can't - # in a replacment list of a macro means QUOTE NEXT TOKEN. It's more of a spelling issue, than any logical puzzle :)

(If you require this kind of solution in your code, than there are ways and tricks of using macro's, but you need to be specific about the use cases you need - as your example can be achieved by defining: #define mykey "value")

Here it is from the ansi C99 standard

6.10.3.2 The # operator

Constraints

1 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 2 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 composing 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), except that it is implementation-defined whether a \ character is inserted before the \ character beginning a universal character name. If the replacement that results is not a valid character string literal, the behavior is undefined. The character string literal corresponding to an empty argument is "". The order of evaluation of # and ## operators is unspecified.

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Macros are a simple text substitution. Generating new preprocessor directives from a macro would require the preprocessor to continue preprocessing from the beginning of the substitution. However, the standard defined preprocessing to continue behind the substitution.

This makes sense from a streaming point of view, viewing the unprocessed code as the input stream and the processed (and substituted) code as the output stream. Macro substitutions can have an arbitrary length, which means for the preprocessing from the beginning that an arbitrary number of characters must be inserted at the beginning of the input stream to be processed again.

When the processing continues behind the substitution, then the input simply is handled in one single run without any insertion or buffering, because everything directly goes to the output.

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whilst it is not possible to use a macro to define another macro, depending on what you are seeking to achieve, you can use macros to effectively achieve the same thing by having them define constants. for example, i have an extensive library of c macros i use to define objective C constant strings and key values.

here are some snippets of code from some of my headers.

// use defineStringsIn_X_File to define a NSString constant to a literal value.
// usage (direct)   : defineStringsIn_X_File(constname,value);

#define defineStringsIn_h_File(constname,value)   extern NSString * const constname; 
#define defineStringsIn_m_File(constname,value)   NSString * const constname = value; 


// use defineKeysIn_X_File when the value is the same as the key. 
// eg myKeyname has the value @"myKeyname"
// usage (direct)   : defineKeysIn_X_File(keyname);
// usage (indirect) : myKeyDefiner(defineKeysIn_X_File);
#define defineKeysIn_h_File(key)  defineStringsIn_h_File(key,key) 
#define defineKeysIn_m_File(key)  defineStringsIn_m_File(key,@#key) 



// use defineKeyValuesIn_X_File when the value is completely unrelated to the key - ie you supply a quoted value.
// eg myKeyname has the value @"keyvalue"
// usage: defineKeyValuesIn_X_File(keyname,@"keyvalue");
// usage (indirect) : myKeyDefiner(defineKeyValuesIn_X_File);
#define defineKeyValuesIn_h_File(key,value)   defineStringsIn_h_File(key,value)  
#define defineKeyValuesIn_m_File(key,value)   defineStringsIn_m_File(key,value) 



// use definePrefixedKeys_in_X_File when the last part of the keyname is the same as the value.
// eg myPrefixed_keyname has the value @"keyname"
// usage (direct)   : definePrefixedKeys_in_X_File(prefix_,keyname);
// usage (indirect) : myKeyDefiner(definePrefixedKeys_in_X_File);

#define definePrefixedKeys_in_h_File_2(prefix,key) defineKeyValuesIn_h_File(prefix##key,@#key) 
#define definePrefixedKeys_in_m_File_2(prefix,key) defineKeyValuesIn_m_File(prefix##key,@#key) 

#define definePrefixedKeys_in_h_File_3(prefix,key,NSObject) definePrefixedKeys_in_h_File_2(prefix,key)
#define definePrefixedKeys_in_m_File_3(prefix,key,NSObject) definePrefixedKeys_in_m_File_2(prefix,key)

#define definePrefixedKeys_in_h_File(...) VARARG(definePrefixedKeys_in_h_File_, __VA_ARGS__)
#define definePrefixedKeys_in_m_File(...) VARARG(definePrefixedKeys_in_m_File_, __VA_ARGS__)




// use definePrefixedKeyValues_in_X_File when the value has no relation to the keyname, but the keyname has a common prefixe
// eg myPrefixed_keyname has the value @"bollocks"
// usage: definePrefixedKeyValues_in_X_File(prefix_,keyname,@"bollocks");
// usage (indirect) : myKeyDefiner(definePrefixedKeyValues_in_X_File);
#define definePrefixedKeyValues_in_h_File(prefix,key,value) defineKeyValuesIn_h_File(prefix##key,value) 
#define definePrefixedKeyValues_in_m_File(prefix,key,value) defineKeyValuesIn_m_File(prefix##key,value) 







#define VA_NARGS_IMPL(_1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, N, ...) N
#define VA_NARGS(...) VA_NARGS_IMPL(X,##__VA_ARGS__, 11, 10,9, 8, 7, 6, 5, 4, 3, 2, 1, 0)
#define VARARG_IMPL2(base, count, ...) base##count(__VA_ARGS__)
#define VARARG_IMPL(base, count, ...) VARARG_IMPL2(base, count, __VA_ARGS__) 
#define VARARG(base, ...) VARARG_IMPL(base, VA_NARGS(__VA_ARGS__), __VA_ARGS__)

and a usage example that invokes it:

#define sw_Logging_defineKeys(defineKeyValue) \
/** start of key list for sw_Logging_ **/\
/**/defineKeyValue(sw_Logging_,log)\
/**/defineKeyValue(sw_Logging_,time)\
/**/defineKeyValue(sw_Logging_,message)\
/**/defineKeyValue(sw_Logging_,object)\
/**/defineKeyValue(sw_Logging_,findCallStack)\
/**/defineKeyValue(sw_Logging_,debugging)\
/**/defineKeyValue(sw_Logging_,callStackSymbols)\
/**/defineKeyValue(sw_Logging_,callStackReturnAddresses)\
/** end of key list for sw_Logging_ **/
sw_Logging_defineKeys(definePrefixedKeys_in_h_File);

the last part may be a little difficult to get your head around. the sw_Logging_defineKeys() macro defines a list that takes the name of a macro as it's parameter (defineKeyValue) this is then used to invoke the macro that does the actual definition process. ie, for each item in the list, the macro name passed in is used to define the context ( "header", or "implementation", eg either "h" or "m" file, if you understand the objective c file extensions) whilst this is used for objective c, it is simply plain old c macros, used for a "higher purpose" than possibly Kernighan and Richie ever envisaged. :-)