Question

I have following problem:

When I use std::vector with built-ins i don’t get a memory but if I use classes I get memory leak. To illustrate:

//No leak
std::vector<double>* vecPtr1=new std::vector<double>();
//add some elements
delete vecPtr1;

//Leaks some memory but not all
std::vector<SomeClass>* vecPtr2=new std::vector<SomeClass>();
//add some elements with vecPtr2->push_back(SomeClass());
delete vecPtr2;

As far as I understand it delete should call the destructor of std::vector which should in turn call the destructor of SomeClass -> no leak. I have invested some thought and testing into this and the same behaviour happens if I use std::vector in a scope such as here:

{
  std::vector<SomeClass> vector;
  //add elements as before
}
//memory is still used here

I’m using gcc 4.6.1 under Ubuntu 11.10. Is something amiss in my library or do I have a misconception how std::vector destructs elements?

For clarification my complete code with SomeClass replaced with std::pair (yes I know some parts are hacked but it’s just an example):

#include <iostream>
#include <vector>
#include <utility>

int main()
{
    std::string inString;
    std::cout<<"Started"<<std::endl;
    //wait
    std::cin>>inString;
    {
        //assign vector
        std::vector<std::pair<std::string,unsigned int> > vec=std::vector<std::pair<std::string,unsigned int> >();
        //push elements
        for(unsigned int i=0;i<1e7;++i)
        {
            vec.push_back(std::pair<std::string,unsigned int>("something",i));
        }
        std::cout<<"Created vector with capacity: "<<vec.capacity()<<std::endl;
        //wait
        std::cin>>inString;
    }
    //vec should go out of scope but not all memory gets freed
    std::cout<<"Deleted vector"<<std::endl;
    //wait
    std::cin>>inString;
    std::cout<<"Shutting down"<<std::endl;

    return 0;
}

Answer 1

First of all, you should not create object of vector dynamically. That is simply a bad idea. That is, use automatic vector:

std::vector<SomeClass> classes; //automatic object

Anyway, in your case, I guess the problem lies with the class SomeClass.

I guess this class manages memory and has not implemented at least one of the following properly:

• Copy-constructor
• Copy-assignment
• Destructor

If you’re using C++11, then there are two more members:

• Move-constructor
• Move-assignment

I would suggest you to read these:

• What is The Rule of Three?
• Rule-of-Three becomes Rule-of-Five with C++11?