CODEWITHSUNDEEP
c++functionparameterspass-by-reference
Nicholas Barczak
Nicholas Barczak

Reputation: 67

C++ Update Function Argument

I have a function defined with several parameters passed by value. Both the function and inputs for the parameters depend on a common global variable. I need some way to get my function to re-evaluate the inputs of its parameters while executing within its own scope. Here is a simplified version of the code.

#include <iostream>

using namespace std;

int Sum(int arg1, int from, int to);
int i;
int func();

int main(int argc, char *argv[])
{

    Sum(func(), 0, 10);

    return 0;
}

int Sum(int arg1, int from, int to)
{
    int out = 0;
    for (i = from; i <= to; i++)
    {
        out += arg1;
        cout << "arg1 = " << arg1 << ", out = " << out << endl;
    }
    return out;
}

int func()
{
    return i;
}

Some highlights:
* Here I am trying to update the input values for parameter arg1 on function Sum().

Is there some way to define a temporary unnamed function inline with the inputs for Sum? Then I could pass parameters by reference and solve my troubles. Or any other ideas for how to make this work?

Thanks!

Upvotes: 1

Views: 2991

Answers (1)

Joe Z
Joe Z

Reputation: 17946

This is a place you could use a function pointer. Instead of passing func(), you pass simply func, and call it from within your function:

int Sum(int (*func_arg1)(void), int from, int to)
{
    int out = 0;
    for (i = from; i <= to; i++)
    {
        int arg1 = func_arg1();
        out += arg1;
        cout << "arg1 = " << arg1 << ", out = " << out << endl;
    }
    return out;
}

The syntax for function pointers is a bit unusual in C and C++. The declaration int (*func_arg1)(void) declares a symbol named func_arg1 that is a pointer to a function taking no arguments, but returning int. In this case, that symbol is also the first argument of Sum.

The only other changes you need to make to your program are the prototype for Sum to match the function above, and to call Sum as follows:

Sum(func, 0, 10);

Upvotes: 1

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