How to enforce function signatures in a function declaration set in C?

Question

I want to make sure that the feature set has the same signature in some C code. Ideally, I could define a new type that describes the return value and arguments of the function, and then declare my set of functions using this new type.

Also, is there a way to specify default values ​​for the arguments to this typedef function?

Answer 1

/* define a typedef for function_t - functions that return void */ /* and take an int and char parameter */ typedef void function_t( int param1, char param2); /* declare some functions that use that signature */ function_t foo; function_t bar; 

Now, when you define functions, there will be an error if they do not use the same signature as in typedef.

 void foo( int x, char c) { /* do some stuff */ return; } /* this will result in a compiler error */ int bar( int x, char c) { /* do some stuff */ return 1; } 

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As for your new question (added October 20, 08): “Also, is there a way to specify default values ​​for the arguments to this typedef function?”

No, there is no way to add default parameters to typedef. Of course, not in C, which does not support default options at all. Even in C ++ you cannot do this because the default value for a parameter is not part of this type. In fact, a class that overrides a virtual method from the base class may specify a different value for the default parameter (or even delete the default value altogether) - however, this is something that should not be done at all, as this will just cause confusion ( http: // www.gotw.ca/gotw/005.htm ).

If you use C ++, you can get the desired behavior using one of (or a combination) of the following:

• abstract base classes
• overload
• template functions
• macros

But it would be difficult to offer something without knowing more details about what exactly you are trying to accomplish. And I think that, probably, the result may well be quite hacked.

Answer 2

This is similar to how a function pointer works:

  // Declaration of function with int arg returning int typedef int (*CALLBACK)(int); //Definition int myFunc(int arg) { return 0; } // Function pointer usage CALLBACK pFunc = myFunc; 

Answer 3

I don’t think you can do it directly, but you can declare a pointer type of a function and then collect all your functions into an array. The compiler will tell you which of them do not match.

 typedef void (*MethodSig)(int, int); static MethodSig x[] = { fnA, fnB, ... }; 

Or use a macro to declare a function signature

 #define MyFunc(X) void X(int a, int b) 

Thus, they will all be the same.

Answer 4

You cannot prevent anyone from creating a function that has a signature. But you can control what you will call. Therefore, I assume that this is what you want.

A function that calls arbitrary functions will have a pointer to the function as a parameter. Since you mentioned typedef, you can define typedef earlier in the program, for example:

Suppose you need the signature functions "unsigned short id_for_allowed_functions (int x, char * y)":

 typedef unsigned short (*id_for_allowed_functions)(int, char*); 

Then your calling function:

 void calling_function (id_for_allowed_function x) { (*x)(3, "bla"); } 

And to feed the foo function to it:

 unsigned short foo(int x, char* y) { /* ... */ } calling_function(&foo); 

Answer 5

Here is an example of creating a list of functions mapped to a simple string.

First enter typedefs:

 typedef GenList *(*DBLISTLOADER)(Database *pDB, char *quellCD, char *profilName); typedef ObjDescription *(*DBCOLUMNLOADER)(); typedef struct dbinfo { char *dbName; DBLISTLOADER dbListLoader; DBCOLUMNLOADER dbColumnLoader; char *options; } DBINFO; 

Then the mapping table:

 DBINFO dbInfoList[] = { { "SRCDOC", loadSRCDOC, colSRCDOC, "q" }, { "PRF_CD", loadPRF_CD, colPRF_CD, "" }, { "MEDIA", loadMEDIA, colMEDIA, "" }, { NULL, NULL, NULL } }; 

Now, to find the function from this table and call it:

 while (dbInfoList[i].dbName != NULL) { if (strcmp(dbInfoList[i].dbName, szDatabase) == 0) { return (dbInfoList[i].dbListLoader)(pDB, quellCD, profilName); } i++; } 

Sorry if it's a little raw. Glued directly from our code; -)