Using pointers in c++

Question

I'm trying to write a function that converts int to byte with the following manner:

int * key = convertTo8bits(255);
for(int i = 0; i<8; i++)
cout<<key[i]<<endl;

This returns unexpected output. The array that it prints out is made of absurdly large numbers, whereas this works perfectly fine:

int * convertTo8bits(int x)
{
int total = 0;
int key[8];
for(int i = 0; i<8; i++)
    key[i] = 0;
if(total + 128 <= x)
{
    key[7] = 1;
    total += 128;
}
if(total + 64 <= x)
{
    key[6] = 1;
    total += 64;
}
if(total + 32 <= x)
{
    key[5] = 1;
    total += 32;
}
if(total + 16 <= x)
{
    key[4] = 1;
    total += 16;
}
if(total + 8 <= x)
{
    key[3] = 1;
    total += 8;
}
if(total + 4 <= x)
{
    key[2] = 1;
    total += 4;
}
if(total + 2 <= x)
{
    key[1] = 1;
    total += 2;
}
if(total + 1 <= x)
{
    key[0] = 1;
    total += 1;
}

for(int i = 0; i<8; i++)
    cout<<key[i]<<endl;

return key;
}

Can you point out my mistake ? Thx.

Oliver Charlesworth · Accepted Answer · 2011-12-17 17:45:33Z

8

You are returning a pointer to a local variable (the array int key[8]). This is undefined behaviour, because the local variable goes out of scope (i.e. its lifetime ends) when the function completes.

In C++, you could use a std::vector<int> instead of a raw array, because you can return it by value, rather than via a pointer.

_{I initially thought this was a C question, in which case my initial answer would have been appropriate:

You have a number of possible solutions:

Dynamically allocate the array with malloc. (This isn't great, because you'll have to remember to free it at some point.)
Pass a pointer to an array in as a function argument, and write the results to that array.
Declare typedef struct { int x[8]; } key;; you can then return a struct by value rather than via a pointer.}

edited Dec 17, 2011 at 17:45

answered Dec 17, 2011 at 16:43

Oliver Charlesworth

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Comments

David Heffernan · Accepted Answer · 2011-12-17 17:13:45Z

2

Oli has correctly identified the cause of your problem. I think it is also worth pointing out that your code is needlessly complex. You can write it more easily with a loop and bitwise shifting:

void extractbits(int x, int key[], int len)
{
    for (int i = 0; i<len; i++)
    {
        key[i] = x & 1;
        x >>= 1;
    }
}

edited Dec 17, 2011 at 17:13

answered Dec 17, 2011 at 17:00

David Heffernan

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