C++ Object allocation and deallocation

Question

Following program creates Objects in one loop and store the reference in vector for future deletion.

I am seeing an unusual behavior, even though the objects are deleting in the second iteration, the getrusage gives a resident memory higher compared to object creation.

Execution environment is in Linux Kernel 3.2.0-49-generic.

#include <iostream>
#include <vector>
#include <stdio.h>

#include <mcheck.h>
#include <sys/time.h>
#include <sys/resource.h>

using namespace std;

void printUsage(string tag)
{
    struct rusage usage;
    getrusage(RUSAGE_SELF, &usage);
    printf("%s -- Max RSS - %ld\n", tag.c_str() ,usage.ru_maxrss);
}

class MyObject
{
    public:
        char array[1024 * 1024];
        MyObject() {};
        ~MyObject() {};

};


int main()
{
    printUsage("Starting");

    vector<MyObject *> *v = new vector<MyObject *>();

    for(int i = 0; i < 10000; i++)
    {
        MyObject * h = new MyObject();

        v->push_back(h);

        // The max resident value is same. usual behavior.
        // delete h;
    }

    printUsage("After Object creation");

    for(size_t i = 0; i < v->size(); i++)
    {
        MyObject * h =  v->at(i);
        delete h;
    }

    v->clear();

    delete v;

    printUsage("After Object deletion");

    return 0;

}

g++ test/test.cpp  -Wall -O2 -g

Output

Starting -- Max RSS - 3060
After Object creation -- Max RSS - 41192
**After Object deletion -- Max RSS - 41380**

try adding http://www.cplusplus.com/reference/vector/vector/shrink_to_fit/ — firda, Aug 04 '14 at 18:14
@πάνταῥεῖ Then how can I create object in heap memory with out new. — Harikrishnan R, Aug 04 '14 at 18:19
`delete` returns memory to heap. It doesn't necessarily return memory to the OS. — T.C., Aug 04 '14 at 18:22
@HarikrishnanR I don't see any point, why it's necessary to create these objects on _the heap_. `std::vector` will manage the necessary heap allocations correctly for you. — πάντα ῥεῖ, Aug 04 '14 at 18:23
@πάνταῥεῖ MyObject can be any thing in the world. For some operations if want to pass myobject reference to other functions/classes/threads I need to make use of heap memory. — Harikrishnan R, Aug 04 '14 at 18:31
@HarikrishnanR If it's in a vector, it's on the heap (assuming your implementation uses a heap for dynamic allocation). But resizing the vector would be cheaper with pointers than with whole objects. — dlf, Aug 04 '14 at 18:37

score 1 · Accepted Answer · answered Aug 04 '14 at 18:21

I'm not up on the specifics of getrusage but from a quick google, it seems to be reports OS resources used. Typically, the C++ Run-time library which manages the memory used by malloc/new will request a large block of memory from the OS when it needs it, make malloc requests out of that block, and then hold onto the block even after all the allocations are freed, so it has some avaiable to handle the next request without having to ask the OS again.

C++ Object allocation and deallocation

Output

1 Answers1