Function pointers...I don't know what they are

In my program, someone enters a value for x, and this value goes into a function and the answer is returned. Now, I have a main.cpp, functions.h, and functions.cpp

There are 2 functions f(x) and g(x).

In main.cpp:

The user enters a value for x
The user is prompted to enter 1 to use f(x) or 2 for g(x).
functions(x) is called and finds f(x) or g(x)

Now, in my assignment it says:

"All functions and the main program must be defined in separate c++ source files and all functions must have separate header files). There is a way in C++ to pass a pointer to a function as an argument in a function call. Use this method to avoid writing two versions of the source code and run the code once to obtain the results."

I have read a lot of tutorials on function pointers and I still do not know how to implement this. From what I read, I would need two functions, one for f(x) and one for g(x). But, according to the assignment I would have to put them in seperate .cpp and .h files. But, this would be writing two versions of the source code.

I'm not sure how I am supposed to implement a function pointer without having seperate functions (one for f(x) and one for g(x)). I could easily just use if statements, but its not allowed.


Any ideas?

Thanks

The problem with function pointers is that in toy cases they seem a bit pointless. You only realize how useful they are once you get into more complex stuff.

In generic terms:
1) Define a function pointer func that matches the signatures of functions f and g.
2) Read input. Depending on input, assign either f or g to func.
3) Call whichever function func points to by dereferencing func.

Let's say you have 10 places where f or g need to be called. The "normal" way, you'd need 10 if statements. With a function pointer, you only need 1 (to set the pointer).

Quote:

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From what I read, I would need two functions, one for f(x) and one for g(x). But, according to the assignment I would have to put them in seperate .cpp and .h files. But, this would be writing two versions of the source code.

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The bottom line is that every function you call has to be defined. You cannot call a function that is not defined. Your assignment starts off by saying you need two functions: one called f and one called g. That means you have to have two function definitions: one defining f and one defining g.

I have no idea what the admonition "do not duplicate the source code" means.

Quote:

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Originally Posted by Lindley

The problem with function pointers is that in toy cases they seem a bit pointless. You only realize how useful they are once you get into more complex stuff.

In generic terms:
1) Define a function pointer func that matches the signatures of functions f and g.
2) Read input. Depending on input, assign either f or g to func.
3) Call whichever function func points to by dereferencing func.

Let's say you have 10 places where f or g need to be called. The "normal" way, you'd need 10 if statements. With a function pointer, you only need 1 (to set the pointer).

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Uhhmm...how about this:

Quote:

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A) A function name is a pointer to the function.

In generic terms:
1) Define a function pointer pfunc that matches the signatures of functions f and g.
2) Read input. Depending on input, assign either f or g to pfunc.
3) Call the function pfunc by supplying the correct arguments.

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Here is an example:

Code:

---

#include <iostream>
using namespace std;

void some_func(int n)
{
        cout<<n<<endl;
}


int main () 
{
        void(*pfunc)(int);
        pfunc = some_func;
       
        pfunc(10);
               
        cout<<">>>Hit Enter to Quit";
        cin.get();
        return 0;
}

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My own knowledge of function pointers is limited, but I'm fairly certain that in some cases you'd have to write:

Code:

---

(*pfunc)(10);

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I'm not sure if that's a compiler thing, or a C-vs-C++ thing, or what. But I definitely recall seeing that as the preferred way.

The above is certainly cleaner, though. Almost what you'd expect from a language that supports functions as first-class objects, like one of the ML variants.

Quote:

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Originally Posted by Lindley

My own knowledge of function pointers is limited, but I'm fairly certain that in some cases you'd have to write:

Code:

---

(*pfunc)(10);

---

I'm not sure if that's a compiler thing, or a C-vs-C++ thing, or what.

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Me neither. This tutorial says both the &(to get the address of a function) and *(to dereference a function pointer) are optional:

http://www.cprogramming.com/tutorial...-pointers.html

And the Function Pointer Tutorial recommends using & because some compilers require it, and the tutorial implies dereferencing is optional:

http://www.newty.de/fpt/fpt.html#callconv

Hello spetsacdc,

I'm a beginner here, but like 7stud said, can't it be that you're leaving something out of the context?

because, you said if statement is now allowed (and I assume neither is switch), but you said a user is prompted to enter a choice, how is this possible without a condition?

Here's my best guess on what must be done.
1. both functions must give(not necessairly return) same End Result.
2. we need to create 5 seperate files. one for main.cpp, 2 for g.h/g.cpp, and 2 for f.h/f.cpp.
3. Let's assume "avoid two versions of the source code" as "One definition in one function is not seen in the other.
4. One of either function must accept a pointer as an ARGUMENT.
5. No conditional statements are allowed.

given all these, I believe (hopefully) either of the functions must call the other.
for instance; g.cpp would be something like

Code:

---

#include "g.h"
#include "f.h"

// pass by pointer
int g(int* y)
{
    return f(*y);
    }

---

and f.cpp

Code:

---

// pass by value
int f(int y)
{
    int twice(0);
    twice = y * 2;
       
    return twice;
    }

---

as you can see, we did not write two versions of the same source code(#3)
and both functions give the same result(#1) and satisfies #4 and the rest with the remainding header files.

but somehow,
I have a feeling (as usual...) what 7stud and Lindley said is more likely to be your answer.

see ya!

Quote:

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My own knowledge of function pointers is limited, but I'm fairly certain that in some cases you'd have to write:

I'm not sure if that's a compiler thing, or a C-vs-C++ thing, or what.

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For both C and C++, both (*pfunc)(10) and pfunc(10) are legal.

Quote:

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But I definitely recall seeing that as the preferred way.

The above is certainly cleaner, though. Almost what you'd expect from a language that supports functions as first-class objects, like one of the ML variants.

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I prefer the pfunc(10) version: if you substitute the function pointer with a function object, the code need not change.

Quote:

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Originally Posted by potatoCode

Hello spetsacdc,

I'm a beginner here, but like 7stud said, can't it be that you're leaving something out of the context?

because, you said if statement is now allowed (and I assume neither is switch), but you said a user is prompted to enter a choice, how is this possible without a condition?

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Maybe, I was trying to summarize what the assignment was. The only thing mentioned about function pointers in the assignment is the following:

"There is a way in C++ to pass a pointer to a function as an argument in a function call. Use this method to avoid writing two versions of the source code and run the code once to obtain the results."

I guess I was wrong in assuming the user will enter 1 or 2 to choose which function to use.

If you are not allowed to use if keyword, maybe something like this ...

Code:

---

#include <stdio.h>

void fn_zero()
{
        printf("zero\n");
}

void fn_one()
{
        printf("one\n");
}


void function(void (*p_fn) ())
{
        p_fn();
}


int main()
{
        void (*p_fn[]) () = {fn_zero, fn_one};

        function(p_fn[0]);
        function(p_fn[1]);

        return 0;
}

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:)