CODEWITHSUNDEEP
c++pointersmemory
ivory
ivory

Reputation: 253

pointer being freed was not allocated(delete one element in an array)

I am a beginner of oop, here is my question

int main(){
        int *p = new int[3];
        p[0] = 1;
        p[1] = 2;
        p[2] = 3;
        int* q = p;
        p++;  //here!
        delete p;
}

and 

int main(){
        int *p = new int[3];
        p[0] = 1;
        p[1] = 2;
        p[2] = 3;
        int* q = p + 1;
        p++;  //here!
        delete p;
}

got bugs: pointer being freed was not allocated but and 

int main(){
        int *p = new int[3];
        p[0] = 1;
        p[1] = 2;
        p[2] = 3;
        int* q = p + 1;
        delete p;
}

seems fine! I want to delete one element in this array. Can someone tell me the reason for the bug?

Upvotes: 4

Views: 3703

Answers (2)

kfsone
kfsone

Reputation: 24249

Memory is allocated in contiguous blocks, and must be returned in the same way. It is best to assume that your memory block comes with a small prefix that the allocator uses to determine where it came from and how large it was.

Also, when you "new" an array, you must use "delete []" to release it.

thing* thingArray = new thing[64];

...

delete [] thingArray;

If you are trying to actively manage the array of elements, you may want to investigate a std::vector instead.

#include <vector>

std::vector<int> v;
v.push_back(10);
v.push_back(20);
v.push_back(30);
// v.size() is now 3 (num elements)
v.erase(v.begin()); // remove the first integer.
// v now "looks" like { 20, 30 }
// v.size() is now 2, not 3.

Upvotes: 9

Igor Popov
Igor Popov

Reputation: 2620

delete is used for deallocation of dynamically allocated memory (by new). delete is not used for deleting elements. The bug is the following: firstly you allocated memory so after int *p = new int[3]; in p pointer is memory address of the allocated space. Then you add 1 to this address in the p++ line, which means that p is pointing now to different place in memory. Then your delete fails because you try to deallocate memory at the modified address.

Upvotes: 0

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