Question

Given the code below, how would you create/implement SR.h so that it produces the correct output WITHOUT any asterisks in your solution?

I got bummed by this question. I would like to know some of the different approaches that people use for this problem.

#include <cstdio>
#include "SR.h"

int main()
{
    int j = 5;
    int a[] = {10, 15};
    {
        SR x(j), y(a[0]), z(a[1]);

        j = a[0];
        a[0] = a[1];
        a[1] = j;

        printf("j = %d, a = {%d, %d}\n", j, a[0], a[1]);
    }

    printf("j = %d, a = {%d, %d}\n", j, a[0], a[1]);
}

Output:

j = 10, a = {15, 10}
j = 5, a = {10, 15}

Second one:

#include <cstdio>
#include "SR.h"
int main()
{
    int sum = 0;
    for (int i = 1; i < 100; i++) {
        SR ii(i);
        while (i--)
            sum += i;
    }
    printf("sum = %d\n", sum);
}

//The output is "sum = 161700".

Answer 1

SR is acting as a captured-variable-restorer. When it goes out of scope it restores some value that it previously captured.

The constructor will do two things: capture a reference, and capture the value of that reference. The destructor will restore the original value to that reference.

class SR
{
public:
  SR(int& var) : capture(var), value(var) {}
  ~SR() { capture = value; }

private:
  int& capture;
  int value;
};

Edit: Just a guess, but I assume SR is supposed to stand for ScopeRestorer?