C/C++ Function Call

[Neerad]

I wonder if it is possible to call a function whose name is stored in a character variable.
i.e. if Str1[4] holds "Sum" Then is it possible to call Sum by refering it through Str1? Inline Assembly, Macro Substitution, parameterized classes or any **** solution is most welcome.

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[mtrosman]

typedef void (*MyFuncPtr) (void);
typedef struct __vTableEntry
{
const char* pszFunctionName;
MyFuncPtr fptr;

} VTENTRY;

const VTENTRY g_VTable[] =
{
{"Diff", Diff},
{"Sum", Sum},
...
};

void execute (const char *pszFunctionName)
{
// search for the vtable entry, witht the specified function name
VTENTRY* pEntry = search_vtable (g_VTable, pszFunctionName);

if (pEntry != NULL) // if the entry was found
pEntry->fptr (); // execute function
}




You can presort your vtable array and use _bsearch to find the function
pointer, associated with the string or use a map:


#include <map>
#include <string>


typedef void (*MyFuncPtr) (void);
typedef std::map <std::string, MyFuncPtr> MyVTableType;

MyVTableType g_VTable;

void main (void)
{
// preallocate vtable
g_VTable["Diff"] = Diff;
g_VTable["Sum"] = Sum;

...
// Execute function
MyVTableType::iterator i_fptr = g_VTable.find ("Sum");
if (i_fptr != g_VTable.end())
(*i_fptr)();
}



P.S.
In my examples I was assuming that the functions, stored in the vTable have the same
signatures: return the same type and use the same arguments. If this is not the case,
you can pass va_list for the arguments and use template to handle different return values.

[ankursaxena]

why and where would you use this? dont people use array of function pointers for similar calling a desired function(never used them myself, just heard)? i know it is a little different, but i want to know why would you want to call a function stored in a string?

a little explaination would be awesome..

thanx
Ankur

[Neerad]

It's about really making things flexible. U See if this were possible the way i want, I would be able to define the rules (applied to the incomming data) into the ini file and thus making things quite generalized. The Scene is - Data comming from switches (telecom) and the flags often modify the meaning of the field. And on top of it the modification rules are just different for different packages.

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[NMTop40]

mtrosman gave you two possible solutions, of which his second (using std::map) is the best approach. His actual code was for illustration purpose. Your job is to adapt the idea for your specific requirements.

If that is not what you are looking for, I suggest you give a clearer specification.


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[Neerad]

Your solution really helped me thinking in new direction. But may be i wasn't very clear in putting up my requirements. I needed something like an & operator in Foxpro or a function like "CallByName" in Visual Basic. The idea of mapping is not what i required. Thanks really.

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[Neerad]

I wish to read an ini file whereupon i wish to write something like this.
ValueModifier = "Sum F1 F2"
When i have the above string "Sum F1 F2" read in some character array, i should be able to call 'Sum' passing F1 F2 as parameters to Sum.
mtrosman solution will surely fit into this requirement but i was just hoping that there is some way similer to & of Foxpro or CallbyName in visual Basic........I wanted to be sure that there is no such possiblity in c/c++. mtrosman's solution has given me an idea of changing my requirements to suit this implementation of mapping function nams with the corresponding function pointers.

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[Alexis Moshinsky]

Hi Neerad,

templates, macro, function names etc. are compilation stage instruments.
Your wish is to interpret .ini file at run time.
So, whatever You do, it should be probably association SymbolicInfo-->FunctionPointer.
There are a couple of solutions to do this job.
In addition to using std::map You can use Command design pattern and RTTI mechanism
to determine witch Command-derived object to build.
Better variant of this solution is to use Builder class.
Another solution - to put Sum, Sub etc. functions into dll and use
its export table to associate symbolic name with function pointer. (Microsoft specific solution