Function returning function pointer from table as a parameter - c++

Question

I have been reading for a while, but today I can't figure someting out and find a solution.
How to return a function pointer from a function table as parameter? All similair solutions don't work for this one and end up not compiling.
I have tried a lot of methods but the compiler always returns with errors like:
function returning function is not allowed solution (when using typedef void (*func)();)
As NO parameters have to be passed into the final routine it should be possible.
My simplified example:
void PrintOne(void) { printf("One")};
void PrintTwo(void) { printf("Two")};
struct ScanListStruct
{
int Value;
void (*Routine)(void);
}
const ScanListStruct DoList[] =
{
{1, PrintOne},
{2, PrintTwo}
}
bool GetRoutine(void *Ptr, int Nr)
{
for (int x =0; x<=1; x++)
{
if (DoList[x].Value = Nr)
{
Ptr = DoList[(x)].Routine;
//((*DoList[(x)].Routine)()); // Original Working and executing version!
return true;
}
}
return false;
}
void main(void)
{
int y = 1;
void (*RoutineInMain)(); // Define
if (GetRoutine( RoutineInMain, y) == true) // get the address
{
RoutineInMain(); // Execute the function
}
}

Answer 1

There a few things wrong with the code;
Syntax errors (missing ; etc.)
main must return int
GetRoutine should accept the function pointer by reference, not just a void* pointer to anything
if condition should contain an equality test, not an assignment
As follows, works as expected;
void PrintOne(void) { printf("One"); };
void PrintTwo(void) { printf("Two"); };
struct ScanListStruct
{
int Value;
void (*Routine)(void);
};
const ScanListStruct DoList[] =
{
{1, &PrintOne},
{2, &PrintTwo}
};
bool GetRoutine(void (*&Ptr)(), int Nr)
{
for (int x =0; x<=1; x++)
{
if (DoList[x].Value == Nr)
{
Ptr = *DoList[(x)].Routine;
//((*DoList[(x)].Routine)()); // Original Working and executing version!
return true;
}
}
return false;
}
int main(void)
{
int y = 1;
void (*RoutineInMain)(); // Define
if (GetRoutine( RoutineInMain, y) == true) // get the address
{
RoutineInMain(); // Execute the function
}
}
Prints One.

Answer 2

You have lots of errors in your code. Like here you put the comas at the wrong place:
void PrintOne(void) { printf("One")};
void PrintTwo(void) { printf("Two")};
It should be
void PrintOne(void) { printf("One");}
void PrintTwo(void) { printf("Two");}
And here you are using the wrong operator, = instead of ==.
if (DoList[x].Value = Nr)
When the argument Ptr is a pointer, and that is passed by value, so the value assigned in the function will not be available when the function returns.
This is how your code should be:
void PrintOne(void) { printf("One"); }
void PrintTwo(void) { printf("Two"); }
typedef void(*prototype)();
struct ScanListStruct
{
int Value;
prototype Routine;
};
const ScanListStruct DoList[] =
{
{ 1, PrintOne },
{ 2, PrintTwo }
};
bool GetRoutine(prototype &Ptr, int Nr)
{
for (int x = 0; x <= 1; x++)
{
if (DoList[x].Value == Nr)
{
Ptr = DoList[(x)].Routine;
return true;
}
}
return false;
}
int main()
{
int y = 1;
prototype RoutineInMain; // Define
if (GetRoutine(RoutineInMain, y) == true) // get the address
{
RoutineInMain(); // Execute the function
}
return 0;
}