CODEWITHSUNDEEP
c++pointersarraysscopesubscript-operator
awshepard
awshepard

Reputation: 2737

C++ arrays as parameters, EDIT: now includes variable scoping

Alright, I'm guessing this is an easy question, so I'll take the knocks, but I'm not finding what I need on google or SO. I'd like to create an array in one place, and populate it inside a different function.

I define a function:

void someFunction(double results[])
{
    for (int i = 0; i<100; ++i)
    {
       for (int n = 0; n<16; ++n) //note this iteration limit
       {
           results[n] += i * n;
       }
    }
}

That's an approximation to what my code is doing, but regardless, shouldn't be running into any overflow or out of bounds issues or anything. I generate an array:

double result[16];
for(int i = 0; i<16; i++)
{
    result[i] = -1;
}

then I want to pass it to someFunction

someFunction(result);

When I set breakpoints and step through the code, upon entering someFunction, results is set to the same address as result, and the value there is -1.000000 as expected. However, when I start iterating through the loop, results[n] doesn't seem to resolve to *(results+n) or *(results+n*sizeof(double)), it just seems to resolve to *(results). What I end up with is that instead of populating my result array, I just get one value. What am I doing wrong?

EDIT Oh fun, I have a typo: it wasn't void someFunction(double results[]). It was:

void someFunction(double result[])...

So perhaps this is turning into a scoping question. If my double result[16] array is defined in a main.cpp, and someFunction is defined in a Utils.h file that's included by the main.cpp, does the result variable in someFunction then wreak havoc on the result array in main?

EDIT 2:

@gf, in the process of trying to reproduce this problem with a fresh project, the original project "magically" started working.

I don't know how to explain it, as nothing changed, but I'm pretty sure of what I saw - my original description of the issue was pretty clear, so I don't think I was hallucinating. I appreciate the time and answers...sorry for wasting your time. I'll update again if it happens again, but for the meantime, I think I'm in the clear. Thanks again.

Upvotes: 2

Views: 167

Answers (5)

foxwoods
foxwoods

Reputation: 500

Just a point about the variable scope part of the question - there is no issue of variable scope here. result/results in your someFunction definition is a parameter -> it will take on the value passed in. There is no relation between variables in a called function and it's caller -> the variables in the caller function are unknown to the called function unless passed in. Also, variable scoping issues do not occur between routines in C++ because there are no nested routines. The following pieces of code would demonstrate scoping issues:

int i = 0;  
{  
    int i = 0;  
    i = 5; //changes the second i, not the first. 
    //The first is aliased by the second i defined first.  
}  
i = 5; //now changes the first i; the inner block is gone and so is its local i

so if C++ did have nested routines, this would cause variable scoping

void main()  
{  
    double results[16];  
    double blah[16];  
    doSomething(blah);  
    void doSomething(double * results)  
    {  
         //blah doing something here uses our parameter results, 
         //which refers to blah, but not to the results in the higher scope. 
         //The results in the higher scope is hidden.  
     }  
}

Upvotes: 1

Puppy
Puppy

Reputation: 146940

To ensure this problem doesn't occur, you should never use global variables like that. If you absolutely must have one, put it in a namespace for clarity.

Upvotes: 1

Michael Dorgan
Michael Dorgan

Reputation: 12515

Have you perhaps double defined your results array in a couple places and then accidently refered to one copy in one place and another copy elsewhere? Perhaps the second is a pointer and not an array and that is why the debugger is confused?

Upvotes: 1

baol
baol

Reputation: 4358

To me it seems that your code should simply work.

I just tried this in g++ and worked fine. I guess your problem is elsewhere? have you tried the snipped you posted?

#include <iostream>

void someFunction(double results[])
{
    for (int i = 0; i<100; ++i)
    {
       for (int n = 0; n<16; ++n) //note this iteration limit
       {
           results[n] += i * n;
       }
    }
}

int main() 
{
  double result[16];
  for(int i = 0; i<16; i++)
  {
    result[i] = -1;
  }
  someFunction(result);
  for(int i = 0; i<16; i++)
    std::cerr << result[i] << " ";
  std::cerr << std::endl;  
}

Upvotes: 1

Potatoswatter
Potatoswatter

Reputation: 137810

void someFunction(double results[])

should be exactly equivalent to

void someFunction(double *results)

Try using the alternative declaration and see if the problem persists.

Upvotes: 1

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