Array Pointer Returns Garbage [duplicate]

Question

I am trying to subtract two integers of the same size without using an explicit loop using valarray. For this purpose I've written a function i.e subtract(int *firstarray, int *secondarray). However, the subtraction occurs correctly in function as printed out. But when returned to main() the first two values of array contain garbage. What is my mistake?

int* subtract(int* lastline, int* firstline){// takes two user defined arrays and performs subtraction

    std::valarray<int> foo (firstline, 7);  //  6 6 5 4 5 6 7
    std::valarray<int> bar (lastline,7);    //  1 8 8 8 8 8 8
    std::valarray<int> res (7);

    res=bar-foo; //subtracts two valarrays

    for (size_t i=0; i<NUMBEROFCLASSES;i++){
        cout<<res[i]<<" ";                  //prints 5 -2 -3 -4 -3 -2 -1
   }

    return &res[0];
}


int main(){

int first[7]={6,6,5,4,5,6,7};
int second[7]={1,8,8,8,8,8,8};
int *e= subtract(first, second);
cout<<endl;
for(int i=0; i<7;i++){
    cout<<e[i]<<" ";                       // prints 0 0 -3 -4 -3 -2 -1
    }
return 1;
}

The local objects will be destroyed when get ouf of the function; the returned pointer becomes dangled. — songyuanyao
– songyuanyao, Commented Dec 21, 2017 at 14:59
res is a local variable it is gone after the call to subtract(). You can't return a pointer to a local. — drescherjm
– drescherjm, Commented Dec 21, 2017 at 14:59
Please read a couple of good beginner books. Learn about scoping and life-time. — Some programmer dude
– Some programmer dude, Commented Dec 21, 2017 at 15:00
Should I declare it static? Or is there any other better method? — mibrahimy
– mibrahimy, Commented Dec 21, 2017 at 15:00

iBug · Accepted Answer · 2017-12-21 15:07:36Z

2

res is a variable with automatic storage duration, meaning that it will be destroyed right when the function exits. e is a dangling pointer, so accessing it is undefined behavior. You can return a std::valarray instead.

std::valarray<int> subtract(int* lastline, int* firstline){
    // Stuff
    return res;
}

edited Dec 21, 2017 at 15:07

answered Dec 21, 2017 at 15:02

iBug

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7 Comments

mibrahimy Over a year ago

I was returning the pointer to convert the valarray to a basic array. By returning the value, I won't be able to achieve that.

iBug Over a year ago

@IbrahimYousaf Return by value then get another pointer to it.

mibrahimy Over a year ago

Great! Thank you. Since the learned fellows have taken down my reputation for amusement, I can't upvote your comment.

iBug Over a year ago

@IbrahimYousaf But hey, they downvoted you not for amusement. It's because they thought your question was too trivial. Indeed one about returning a pointer to a local variable is trivial (I agree).

mibrahimy Over a year ago

True, but my initial problem was looking for a method to convert valarray to a basic array.

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