Array Creation in C++

Question

What I'm Essentially trying to do is write a function that can take an array of length n, and make an array of say, length n-2. here is the code i have so far:

float* comp_arr(float vals[], int arr_size, float scale, float dim){

    int arr_dim = (int)(arr_size+1-2*scale);
    float curvs[arr_dim];

    for(int i = scale; i < sizeof(vals)-scale+1; i++){
            float cur = comp_cur((i-scale)*dim, vals[i-1], i*dim, vals[i], (i+scale)*dim, vals[i+1]);
            int new_index = (int)(i-scale);
            curvs[new_index] = cur;
            printf("%f\n",cur);
    }
    return curvs;
}

Ive been calling it in the main function like this:

main(){
    float vals [] = {2,3,6,1,7};
    float *curvs = comp_arr(vals,5,1.0,1.0);
}

but i get this error:

comp.cpp: In function ‘float* comp_arr(float*, int, float, float)’:
comp.cpp:35:8: warning: address of local variable ‘curvs’ returned [enabled by default]
/tmp/ccrDJjYq.o: In function `comp_arr(float*, int, float, float)':
comp.cpp:(.text+0x590): undefined reference to `__cxa_end_cleanup'
/tmp/ccrDJjYq.o:(.ARM.extab+0xc): undefined reference to `__gxx_personality_v0'
collect2: ld returned 1 exit status

I'm pretty new to C++, what am i doing wrong?????

You are returning a pointer to a piece of memory on the stack (in automatic storage to use the language from the standard). That memory can be re-used at anytime. Very bad. You want to dynamically allocate memory for this purpose. Consider new[]. There are many duplicate questions on the site, but I'm too lazy to go find one. — dmckee --- ex-moderator kitten
– dmckee --- ex-moderator kitten, Commented Feb 12, 2013 at 18:58

ryanbwork · Accepted Answer · 2013-02-12 18:59:04Z

1

The curvs array is a local variable within the comp_arr function. The first warning is being thrown because as soon as this function returns, the memory that it was using (which includes the curvs array) will go out of scope. Referencing the returned array in your main will cause undefined behavior; if you'd like to return an array from a function, you'll have to dynamically allocate it via new/malloc.

answered Feb 12, 2013 at 18:59

ryanbwork

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2 Comments

tmathmeyer Over a year ago

i changed it to say: float curvs[arr_dim] = new float[arr_dim]; but now i get an error telling me that it "may have not been initialized"

ryanbwork Over a year ago

Now you've reserved space for your array, but the elements have not been initialized. You can do this via a for loop, or may also be able to utilize syntax like curvs[arr_dim] = {0}.

JBentley · Accepted Answer · 2013-02-12 18:58:50Z

0

You're returning a pointer to a local variable. As soon as the closing brace of your comp_arr function is reached, curvs is out of scope and no longer exists, yet you are returning it's address. The program might even behave correctly if the data is still present in memory, but it could be overwritten at any time.

answered Feb 12, 2013 at 18:58

JBentley

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