c++ pointer after delete still take memory [duplicate] - c++

Question

This question already has answers here:
Freeing Memory From An Array
(2 answers)
Closed 4 years ago.
I have some problem with c++ pointer.
Here is my code:
#include <iostream>
#include <vector>
int main() {
std::vector<int*> * data = new std::vector<int*>;
for (int i = 0; i < 1000; i++) {
data->push_back(new int[100000]);
}
for (int i = 0; i < 100; i++) {
delete data->at(i);
}
data->clear();
delete data;
data = nullptr;
return 0;
}
After
std::vector<int*> * data = new std::vector<int*>;
for (int i = 0; i < 1000; i++) {
data->push_back(new int[100000]);
}
It takes 384Mb (I found it in task manager)
But after
for (int i = 0; i < 100; i++) {
delete data->at(i);
}
It still takes 346Mb
After
delete data;
data = nullptr;
It doesn't change anything
My problem is, what can I do to completely delete a pointer and free memory?

Answer 1

First, you probably aren't actually deleting everything. You loop only goes to 100, and you're pushing on 1000 items. Secondly, your use of data->clear() is largely pointless and irrelevant. Vectors frequently do not ever shrink themselves no matter what you do. And even if they do, you're just deleting it anyway.
Lastly, don't use new and delete, and use raw pointers sparingly. If you hadn't used them in the first place, you wouldn't have made the mistakes you did. You should've done this:
#include <iostream>
#include <vector>
#include <memory>
int main() {
using ::std::make_unique;
using ::std::unique_ptr;
typedef unique_ptr<int []> ary_el_t;
auto data = make_unique<::std::vector<ary_el_t>>();
for (int i = 0; i < 1000; i++)
{
data->push_back(make_unique<int[]>(100000));
}
data.reset();
return 0;
}
And, even if you did, you might still not get the memory back. Allocators will reuse freed space when asked for more space, but they often don't return it to the operating system.
The above code does require C++14 to work. But recent versions of Visual Studio should support that. I tested this with g++ on my Linux box, and my allocator does return memory to the OS, so I was able to verify that indeed, all the de-allocation works.