Is there a memory leak in this code ?

[Diwa]

// -----------------------------------

class Column
{
public:
string name;
vector<int> values;
};

// -----------------------------------

void loadValues()
{
Column p = new Column();

p->values.push_back(55); // <--- Line 1
p->values.push_back(66); // <--- Line 2

delete p; // <--- Line 3
}

// -----------------------------------

Are the values inserted (Line 1 and 2) on
the stack or on the heap ?

Is there a memory leak for the two inserted
values inspite of the "delete p" at line 3 ?

Thanks
Diwakar

 

[Victor Bazarov]

// -----------------------------------

class Column
{
public:
string name;
vector<int> values;
};

// -----------------------------------

void loadValues()
{
Column p = new Column();

You probably meant

Column *p = new Column();

p->values.push_back(55); // <--- Line 1
p->values.push_back(66); // <--- Line 2

delete p; // <--- Line 3
}

// -----------------------------------

Are the values inserted (Line 1 and 2) on
the stack or on the heap ?

Values that appear in the code (the literals '55' and '66')
are usually neither in the stack nor in the heap (free store)
since they are literals, they are usually in the code (parts
of the instruction that places them where arguments are
transferred to the function). Values that the container
'p->values' stores are _always_ in the free store, the vector
allocates all its values there, unless you provide some kind
of custom allocator, which you didn't.

Is there a memory leak for the two inserted
values inspite of the "delete p" at line 3 ?

No.

V

 

[Diwa]

    Column *p = new Column();





Values that the container
'p->values' stores are _always_ in the free store, the vector
allocates all its values there, unless you provide some kind
of custom allocator, which you didn't.


No.

I suspect there is a memory leak. Here is my reasoning.

When "Column *p = new Column( )" is done it allocates,
lets say, 20 bytes. Just before the memory address
returned by "new", maybe it stores the num of bytes.

The "push_back()" done later at line 1 and line 2 may
results in some more "new" but still it will not
change the value (num of bytes) just before addr "p"

So at line 3, when "delete p" executes, it sees the
velue "20" just before p and then deletes only 20
bytes.

Am I missing something ?

 

[Erik Wikström]

I suspect there is a memory leak. Here is my reasoning.

When "Column *p = new Column( )" is done it allocates,
lets say, 20 bytes. Just before the memory address
returned by "new", maybe it stores the num of bytes.

The "push_back()" done later at line 1 and line 2 may
results in some more "new" but still it will not
change the value (num of bytes) just before addr "p"

So at line 3, when "delete p" executes, it sees the
velue "20" just before p and then deletes only 20
bytes.

Am I missing something ?

Yes, when you push back the numbers on line 1 and 2 those are stored in
memory managed by the vector 'values'. When you, on line 3, delete p it
will call the destructor of the Column class pointed to by p. When this
happens it will call the destructors of its members 'name' and 'value'.
When the vector's destructor is run it will free whatever memory was
used by the vector, including that used to store the elements that you
pushed back on line 1 and 2. You only have to worry about the memory you
explicitly allocated using new, nothing else.

 

[Diwa]

Yes, when you push back the numbers on line 1 and 2 those are stored in
memory managed by the vector 'values'. When you, on line 3, delete p it
will call the destructor of the Column class pointed to by p. When this
happens it will call the destructors of its members 'name' and 'value'.
When the vector's destructor is run it will free whatever memory was
used by the vector, including that used to store the elements that you
pushed back on line 1 and 2. You only have to worry about the memory you
explicitly allocated using new, nothing else.

Ah, now I feel stupid. Obviously, when 'delete p' is done it
will invoke the dtor of "Column" class in which case all
memory deletion is taken care of automatically. I don't why
but my brain was processing the behaviour of "delete p" as
behaviour of "free p". Thanks anyways, Erik and Victor.

 

[Ron Natalie]

You probably meant

Column *p = new Column();

What they probably really wanted is
Column p;
p.values.push_back...

It's the Java syndrome. You do not "new" everything
in C++.

 

[jkherciueh]

// -----------------------------------

class Column
{
public:
string name;
vector<int> values;
};

// -----------------------------------

void loadValues()
{
Column p = new Column();

p->values.push_back(55); // <--- Line 1
p->values.push_back(66); // <--- Line 2

delete p; // <--- Line 3
}

// -----------------------------------

Are the values inserted (Line 1 and 2) on
the stack or on the heap ?

They are in free storage (you probably would call that the heap) since you
used the standard allocator with the vectors.

Is there a memory leak for the two inserted
values inspite of the "delete p" at line 3 ?

The memory will leak when Line 1 or Line 2 throws an exception (e.g., due to
reallocation). Otherwise, there is no leak.


Best

Kai-Uwe Bux