vector destruction and malloc

[mmfuller]

I'm learning how to code with vectors. I wrote a few lines to try out the vector destructor. I thought it was simple enough, but the code generates a runtime error that refers to memory allocation. It seems to be saying that memory allocated to the vector had already been freed before I called the destructor.

Can you tell me what I'm doing wrong? I compiled it using gcc 4.0.1 on a MacPro running OS 10.5.3. Below is the code and the output.

Thanks!
Mike

#include <vector>

using namespace std;

int main() 
{
	vector<int> data;
	
	data.push_back(1);
	data.~vector<int>();

	return 0;
}

OUTPUT
a.out(11915) malloc: *** error for object 0x100150: double free
*** set a breakpoint in malloc_error_break to debug

[tabstop]

The variable data is destroyed when it goes out of scope. Things you didn't allocate (such as data) you shouldn't destroy.

[matsp]

tabstop tells you what is wrong, but I thought I'd try to explain what actually happens:
When you explicitly call the destructor for vector<int>, it goes and deallocates the internal data that the vector allocated to store your data. Then, when main reaches the return 0 and the end of the function, it automatically destroys the vector AGAIN. This means that the same data is now deallocated again - this is why it says "double free".

There are VERY few places where you legally call a destructor explicitly - the only place I can think of is when you write your own new/delete functions that are not actually called new and delete.

--
Mats

[Elysia]

Do not call the destructor on the vector.
It will then destruct twice, causing errors.

[mmfuller]

Okay, I see what you mean. What if data were a member of a class? Would I need to explicitly destroy it within the class destructor? For example, would I need to include a call to the vector destructor in my class destructor, like the following?

class Myclass
{
    public:
      Myclass();
      ~Myclass();
      void insert(int n);

    private:
        vector<int> data;
};

Myclass::Myclass()
{
    data.reserve(10);
}
Myclass::~Myclass()
{
     data.~vector<int>();
}
void Myclass::insert(int n) 
{
     data.push_back(n);
}

[tabstop]

The vector data will go out of scope on its own when the object is destroyed.

[Daved]

>> Would I need to explicitly destroy it within the class destructor?

No. You will likely never need to explicitly call a destructor. The compiler calls it for you.

A vector<int> is an object like an int or string. It gets destroyed when it goes out of scope. You don't call ~int, right? So you wouldn't call ~vector<int> either.

[cpjust]

Until you learn a lot more about C++, NEVER explicitly call a destructor. I've been using C++ for almost a decade and I still haven't had a need to call a destructor (but there are a few very rare cases when you do need to).

[mmfuller]

Okay, thanks for everyone's advice!
Mike

[medievalelks]

~Thread();

[mmfuller]

Have you certain books you'd like to recommend? I've been reading the following:

Effective C++, 3rd edition, by Scott Meyers (2005)
C++ Cookbook, by D.R. Stephens, C. Diggins, J. Turkanis, and J. Cogswell (2005)
The C++ Standard Library, by Nicolai M. Josuttis (1999)
Absolute C++, 3rd edition, by Walter Savitch (2007)

[Daved]

You can check out the stickied Book Recommendations thread. I'd recommend Accelerated C++ as a beginner book over Absolute C++.

The others on your list are good but aren't exactly "teach the language" type books.

[mmfuller]

Thanks for the recommendation.

Like many in the life sciences, I've had one semester in C++ and the rest of my education has come from reading, working with other scientists, and working in the same lab with a few software engineers.

So I'm still a novice, with just enough knowledge to get into trouble!

Mike