Invalid Lvalue, pointer to function also, whats the use of this? Its much simpler to call the function

Question

So I'm practicing pointers to functions, and tried out making this simple program, here's a snippet of it. It still gives me an error "invalid lvalue" when it comes to assigning the address. funcptr = &addnum for example. Also I can't help but wonder what's the use of this? Isn't it much simpler to call the function? Or am I misunderstanding something

#include <stdio.h>
int arithnum(int base);
int addnum(int base,int new);
int subnum(int base,int new);
int mulnum(int base,int new);
int divnum(int base,int new);
typedef int *ptrdef(int,int);
int arithnum(int base)
{
    char operator;
    int operand;
    ptrdef funcptr;
    printf("Enter operator: ");
    scanf("\n%c",&operator);
    printf("Enter second operand: ");
    scanf("%d",&operand);
    switch(operator)
    {
        case '+':
            funcptr = &addnum;
            break;
        case '-':
            funcptr = &subnum;
            break;
        case '*':
            funcptr = &mulnum;
            break;
        case '/':
            funcptr = &divnum;
            break;
    }
    return funcptr(base,operand);
}

[What are function pointers used for...](http://stackoverflow.com/questions/1758564/what-are-function-pointers-used-for-and-how-would-i-use-them). — DCoder, Sep 12 '12 at 07:28

James · Answer 1 · 2012-09-12T07:26:11.067

1

Change your typdef.

Change:

typedef int *ptrdef(int,int);

to

typedef int (*ptrdef)(int,int);

To answer your other question/statement: "Function pointers seem useless": In your example their use is trivial, but a more useful example is vtables in C++. Function pointers allow a base class to define the signature of a function and then subclasses can replace those function pointers with their own implementation changing how the object responds to a function.

You can also use them in a COM model API where the main application dynamically links with a plugin and they plugin returns structure of function pointers for the requested interface.

edited Sep 12 '12 at 07:26

answered Sep 12 '12 at 07:19

James

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I'm pretty sure you have to, well tutorials in the internet say so. And K&R – jantristanmilan Sep 12 '12 at 07:20
It is not useful to change the thing on the right side if you have an invalid lvalue. In this case, something with `funcptr` is wrong. See my answer. – glglgl Sep 12 '12 at 07:21
You do not have to take the address or deference the funcptr either. `funcptr = divnum` and `funcptr(base,operand)` compile just fine. – James Sep 12 '12 at 07:31

score 1 · Accepted Answer · answered Sep 12 '12 at 07:20

1

ITYM

typedef int (*ptrdef)(int,int);

as your version is a function which returns an int * while you want a function pointer which returns an int.

Just a hint: I know that the following is not common sense, but I prefer to typedef the function itself and then do

typedef int myfunc(int,int);
myfunc therealfunction; // bites me if I do a mistake
int therealfunction(int a, int b)
{
    // do stuff and
    return 42;
}
myfunc * funcptr = &therealfunction;

in order to get bitten by an error instead of a warning if I accidentally change the declaration of therealfunction.

answered Sep 12 '12 at 07:20

glglgl

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Thanks man!! I got it now, but how about my 2nd question? What's the use of this? Its much more simpler to call the function.. – jantristanmilan Sep 12 '12 at 07:27
The use is, commonly spoken, to decide what to do at one time and really do it later. Doing so, you can e. g. create a job queue system, or a table of function in an array, mapping an index (essentially an int) to a function to be executed. – glglgl Sep 12 '12 at 08:23

Invalid Lvalue, pointer to function also, whats the use of this? Its much simpler to call the function

2 Answers2