Question

I’ve been banging my head hard over this…I create a pointer in main(), which I pass on to another function. Inside that function, the pointer is used to create a new array (using the new operator), the array is filled, and the function ends.

If I then try to access elements in the new array in the caller, I get a segfault. Since the new operator was used, I expect the new array to be on the heap and thus not cleared by it going out of scope…I really don’t get this. What am I overlooking? I also don’t know precisely what to google for, so no luck there yet.

I can get it to work if I let the function return the pointer (instead of passing it), but I don’t want to do that because eventually I’d like my function to create a few such newly created arrays. So what am I missing here?

Here is a minimal working example:

#include <iostream>
#include <stdio.h>

bool getData(double *myData)
{
    myData = new double[2];
    if (!myData)
        return false;

    myData[0] = +4.53;
    myData[1] = -3.25;    
    return true;
}


int main()
{
    double *myData = NULL;
    if (!getData(myData))
        fprintf(stderr, "Could not get data.\n");

    std::cout << myData[0] << std::endl;
    std::cout << myData[1] << std::endl;

    delete [] myData;    
}

Answer 1

Root Cause of the Crash:
When you pass a pointer to the function by value. An copy of the pointer gets passed to the function. Further You allocate memory to the copy of pointer passed through main, this pointer is not same as the one you access in main, it is an copy. The pointer myData in main was never allocated any memory, so eventually you are dereferencing a NULL pointer which results in a Undefined Behavior and an crash.

Suggested Solution 1:

Pass the pointer by Reference:

bool getData(double *&myData)
                     ^

And you are good to go.This is the C++ way of doing it.

Another Solution:

You could also do:

bool getData(double **myData) 
                    ^
{
     *myData = new double[2];
      //so on
}

while calling it as:

getData(&myData); 
        ^

A word of caution:
new does not return NULL in case of failure to allocate memory. It throws a std::bad_alloc exception. So you need to handle that exception or in case you want to check for null you should use the nothrow version of new.