Are there in Arduino pointers to function?
As they are declared and defined?
As to the function accessed through a pointer?
Ogogon
1 Like
Are there in Arduino pointers to function?
As they are declared and defined?
- yes
- exactly as for C/C++
And for anyone that hasn't seen it done recently:
// Generic arithmetic funtion
typedef int (*GeneralFunction) (const int arg1, const int arg2);
int Add (const int arg1, const int arg2)
{
return arg1 + arg2;
} // end of Add
int Subtract (const int arg1, const int arg2)
{
return arg1 - arg2;
} // end of Subtract
int Divide (const int arg1, const int arg2)
{
return arg1 / arg2;
} // end of Divide
int Multiply (const int arg1, const int arg2)
{
return arg1 * arg2;
} // end of Multiply
void setup ()
{
// make pointers to functions, put them in local variables
GeneralFunction fAdd = Add;
GeneralFunction fSubtract = Subtract;
GeneralFunction fDivide = Divide;
GeneralFunction fMultiply = Multiply;
Serial.begin (115200);
Serial.println ();
// use the function pointers
Serial.println (fAdd (40, 2));
Serial.println (fSubtract (40, 2));
Serial.println (fDivide (40, 2));
Serial.println (fMultiply (40, 2));
} // end of setup
void loop () {}
Output:
42
38
20
80
Function pointers are useful for generic things (eg. do to everything in an array). Also for callbacks. That is, you might want to do on an interrupt.
2 Likes
Here is an example of where you might pass the pointer to a function to another "generic" function:
int Add (const int arg1, const int arg2)
{
return arg1 + arg2;
} // end of Add
int Subtract (const int arg1, const int arg2)
{
return arg1 - arg2;
} // end of Subtract
int Divide (const int arg1, const int arg2)
{
return arg1 / arg2;
} // end of Divide
int Multiply (const int arg1, const int arg2)
{
return arg1 * arg2;
} // end of Multiply
int DoSomething (int (*f) (const int arg1, const int arg2), const int a, const int b)
{
Serial.println (f (a, b)); // call the passed-in function
}
void setup ()
{
Serial.begin (115200);
Serial.println ();
DoSomething (Add, 40, 2);
DoSomething (Subtract, 40, 2);
DoSomething (Divide, 40, 2);
DoSomething (Multiply, 40, 2);
} // end of setup
void loop () {}
Here the DoSomething function is passsed the pointer to a function which is the function which you want it to call.
(edit) Amended to change (*f) (arg1, arg2) to f (a, b) in DoSomething.
However I believe there is a bug in the IDE preprocessor. This "easier-to-read" version does not compile:
// Generic arithmetic function
typedef int (*GeneralFunction) (const int arg1, const int arg2);
int Add (const int arg1, const int arg2)
{
return arg1 + arg2;
} // end of Add
int Subtract (const int arg1, const int arg2)
{
return arg1 - arg2;
} // end of Subtract
int Divide (const int arg1, const int arg2)
{
return arg1 / arg2;
} // end of Divide
int Multiply (const int arg1, const int arg2)
{
return arg1 * arg2;
} // end of Multiply
int DoSomething (GeneralFunction f, const int arg1, const int arg2)
{
Serial.println ((*f) (arg1, arg2)); // call the passed-in function
}
void setup ()
{
Serial.begin (115200);
Serial.println ();
DoSomething (Add, 40, 2);
DoSomething (Subtract, 40, 2);
DoSomething (Divide, 40, 2);
DoSomething (Multiply, 40, 2);
} // end of setup
void loop () {}
Error message:
sketch_apr16a:-1: error: 'GeneralFunction' was not declared in this scope
sketch_apr16a:-1: error: expected primary-expression before 'const'
sketch_apr16a:-1: error: expected primary-expression before 'const'
sketch_apr16a:-1: error: initializer expression list treated as compound expression
sketch_apr16a.cpp: In function 'int DoSomething(int (*)(int, int), int, int)':
sketch_apr16a:23: error: 'int DoSomething(int (*)(int, int), int, int)' redeclared as different kind of symbol
sketch_apr16a:-1: error: previous declaration of 'int DoSomething'
sketch_apr16a.cpp: In function 'void setup()':
sketch_apr16a:33: error: 'DoSomething' cannot be used as a function
sketch_apr16a:34: error: 'DoSomething' cannot be used as a function
sketch_apr16a:35: error: 'DoSomething' cannot be used as a function
sketch_apr16a:36: error: 'DoSomething' cannot be used as a function
But GeneralFunction is declared in this scope. It worked in the earlier example (reply #2). However this code does compile under normal g++. So I have the syntax right.
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Ah well, I worked it out. It's always satisfying to do that. 
If you split the code up into two files the cleaner-looking one compiles OK. Main file:
#include "functionheader.h"
int Add (const int arg1, const int arg2)
{
return arg1 + arg2;
} // end of Add
int Subtract (const int arg1, const int arg2)
{
return arg1 - arg2;
} // end of Subtract
int Divide (const int arg1, const int arg2)
{
return arg1 / arg2;
} // end of Divide
int Multiply (const int arg1, const int arg2)
{
return arg1 * arg2;
} // end of Multiply
int DoSomething (GeneralFunction f, const int a, const int b)
{
Serial.println (f (a, b)); // call the passed-in function
}
void setup ()
{
Serial.begin (115200);
Serial.println ();
// call the general "do something" routine passing down what function to do, and its arguments
DoSomething (Add, 40, 2);
DoSomething (Subtract, 40, 2);
DoSomething (Divide, 40, 2);
DoSomething (Multiply, 40, 2);
} // end of setup
void loop () {}
Then make another tab in the IDE called functionheader.h and put this into it:
// Generic arithmetic function
typedef int (*GeneralFunction) (const int arg1, const int arg2);
(edit) Amended to change (*f) (a, b) to simply: f (a, b) in DoSomething.
1 Like
Thank you very much. I have written in both C and everything is fine success.
Ogogon.
I found this very useful. Thanks Nick