I'm new to the concept so don't be hard on me.
why doesn't this code produce a destructor call ?
The names of the classes are self-explanatory.
The SString will print a message in ~SString().
It only prints one destructor message.
int main(int argc, TCHAR* argv[])
{
smart_ptr<SString> smt(new SString("not lost"));
new smart_ptr<SString>(new SString("but lost"));
return 0;
}
Is this a memory leak?
The impl. for smart_ptr is from here
edited:
//copy ctor
smart_ptr(const smart_ptr<T>& ptrCopy)
{
m_AutoPtr = new T(ptrCopy.get());
}
//overloading = operator
smart_ptr<T>& operator=(smart_ptr<T>& ptrCopy)
{
if(m_AutoPtr)
delete m_AutoPtr;
m_AutoPtr = new T(*ptrCopy.get());
return *this;
}
By new smart_ptr<SString>(new SString("but lost")); you are creating a new, dynamically allocated smart pointer. You don't store the result of the allocation (a pointer to a shared_ptr to a SString) anywhere, it's dangling... since you don't store the result, you also can not call delete for it - therefore it's destructor won't be called, and in turn also the SString destructor of the contained object won't be called!
If you try
smart_ptr<SString> *p = new smart_ptr<SString>(new SString("but lost"));
delete p;
instead, you will see the destructor called also for this case.
However, that's no sensible use of a smart_ptr. smart_ptr were created so that you don't need to call delete manually; therefore, don't use them that way; use them as in your first statement!
The point of a smart pointer is that you're supposed to have only automatic smart pointer objects:
{
smart_ptr<Foo> p(new Foo);
}
// bye bye Foo
Your second line, however, creates a dynamic smart pointer, whose life never ends! Thus it never gets a chance to destroy the object it's in charge of.
You would have to delete the smart pointer itself manually, so that it can in turn clean up the object:
auto sp = new smart_ptr<Foo>(new Foo);
// ^^^^^^^
// ^^^^^^^^^^^^^^ +------< dynamic Foo, handled by the SP
// |
// +---------------< dynamic smart pointer, handled by YOU
delete sp;
Yes, the smart point itself is leaked. (And anything it holds a reference to).
I cannot think of a good reason to new a smart pointer...
Yes, it is a memory leak, you're leaking the second smart pointer and its contents.
The reason is that the first smart pointer is created on the stack, so its life-time is scoped to the block it is declared in, after that it will be automatically destroyed.
The second one is created on the heap, which means it will live until you destroy it with delete at which point its destructor will be called (and with that the destructor of the SString it is holding).
Related
I learned C# first, and now I'm starting with C++. As I understand, operator new in C++ is not similar to the one in C#.
Can you explain the reason of the memory leak in this sample code?
class A { ... };
struct B { ... };
A *object1 = new A();
B object2 = *(new B());
What is happening
When you write T t; you're creating an object of type T with automatic storage duration. It will get cleaned up automatically when it goes out of scope.
When you write new T() you're creating an object of type T with dynamic storage duration. It won't get cleaned up automatically.
You need to pass a pointer to it to delete in order to clean it up:
However, your second example is worse: you're dereferencing the pointer, and making a copy of the object. This way you lose the pointer to the object created with new, so you can never delete it even if you wanted!
What you should do
You should prefer automatic storage duration. Need a new object, just write:
A a; // a new object of type A
B b; // a new object of type B
If you do need dynamic storage duration, store the pointer to the allocated object in an automatic storage duration object that deletes it automatically.
template <typename T>
class automatic_pointer {
public:
automatic_pointer(T* pointer) : pointer(pointer) {}
// destructor: gets called upon cleanup
// in this case, we want to use delete
~automatic_pointer() { delete pointer; }
// emulate pointers!
// with this we can write *p
T& operator*() const { return *pointer; }
// and with this we can write p->f()
T* operator->() const { return pointer; }
private:
T* pointer;
// for this example, I'll just forbid copies
// a smarter class could deal with this some other way
automatic_pointer(automatic_pointer const&);
automatic_pointer& operator=(automatic_pointer const&);
};
automatic_pointer<A> a(new A()); // acts like a pointer, but deletes automatically
automatic_pointer<B> b(new B()); // acts like a pointer, but deletes automatically
This is a common idiom that goes by the not-very-descriptive name RAII (Resource Acquisition Is Initialization). When you acquire a resource that needs cleanup, you stick it in an object of automatic storage duration so you don't need to worry about cleaning it up. This applies to any resource, be it memory, open files, network connections, or whatever you fancy.
This automatic_pointer thing already exists in various forms, I've just provided it to give an example. A very similar class exists in the standard library called std::unique_ptr.
There's also an old one (pre-C++11) named auto_ptr but it's now deprecated because it has a strange copying behaviour.
And then there are some even smarter examples, like std::shared_ptr, that allows multiple pointers to the same object and only cleans it up when the last pointer is destroyed.
A step by step explanation:
// creates a new object on the heap:
new B()
// dereferences the object
*(new B())
// calls the copy constructor of B on the object
B object2 = *(new B());
So by the end of this, you have an object on the heap with no pointer to it, so it's impossible to delete.
The other sample:
A *object1 = new A();
is a memory leak only if you forget to delete the allocated memory:
delete object1;
In C++ there are objects with automatic storage, those created on the stack, which are automatically disposed of, and objects with dynamic storage, on the heap, which you allocate with new and are required to free yourself with delete. (this is all roughly put)
Think that you should have a delete for every object allocated with new.
EDIT
Come to think of it, object2 doesn't have to be a memory leak.
The following code is just to make a point, it's a bad idea, don't ever like code like this:
class B
{
public:
B() {}; //default constructor
B(const B& other) //copy constructor, this will be called
//on the line B object2 = *(new B())
{
delete &other;
}
}
In this case, since other is passed by reference, it will be the exact object pointed to by new B(). Therefore, getting its address by &other and deleting the pointer would free the memory.
But I can't stress this enough, don't do this. It's just here to make a point.
Given two "objects":
obj a;
obj b;
They won't occupy the same location in memory. In other words, &a != &b
Assigning the value of one to the other won't change their location, but it will change their contents:
obj a;
obj b = a;
//a == b, but &a != &b
Intuitively, pointer "objects" work the same way:
obj *a;
obj *b = a;
//a == b, but &a != &b
Now, let's look at your example:
A *object1 = new A();
This is assigning the value of new A() to object1. The value is a pointer, meaning object1 == new A(), but &object1 != &(new A()). (Note that this example is not valid code, it is only for explanation)
Because the value of the pointer is preserved, we can free the memory it points to: delete object1; Due to our rule, this behaves the same as delete (new A()); which has no leak.
For you second example, you are copying the pointed-to object. The value is the contents of that object, not the actual pointer. As in every other case, &object2 != &*(new A()).
B object2 = *(new B());
We have lost the pointer to the allocated memory, and thus we cannot free it. delete &object2; may seem like it would work, but because &object2 != &*(new A()), it is not equivalent to delete (new A()) and so invalid.
In C# and Java, you use new to create an instance of any class and then you do not need to worry about destroying it later.
C++ also has a keyword "new" which creates an object but unlike in Java or C#, it is not the only way to create an object.
C++ has two mechanisms to create an object:
automatic
dynamic
With automatic creation you create the object in a scoped environment:
- in a function or
- as a member of a class (or struct).
In a function you would create it this way:
int func()
{
A a;
B b( 1, 2 );
}
Within a class you would normally create it this way:
class A
{
B b;
public:
A();
};
A::A() :
b( 1, 2 )
{
}
In the first case, the objects are destroyed automatically when the scope block is exited. This could be a function or a scope-block within a function.
In the latter case the object b is destroyed together with the instance of A in which it is a member.
Objects are allocated with new when you need to control the lifetime of the object and then it requires delete to destroy it. With the technique known as RAII, you take care of the deletion of the object at the point you create it by putting it within an automatic object, and wait for that automatic object's destructor to take effect.
One such object is a shared_ptr which will invoke a "deleter" logic but only when all the instances of the shared_ptr that are sharing the object are destroyed.
In general, whilst your code may have many calls to new, you should have limited calls to delete and should always make sure these are called from destructors or "deleter" objects that are put into smart-pointers.
Your destructors should also never throw exceptions.
If you do this, you will have few memory leaks.
B object2 = *(new B());
This line is the cause of the leak. Let's pick this apart a bit..
object2 is a variable of type B, stored at say address 1 (Yes, I'm picking arbitrary numbers here). On the right side, you've asked for a new B, or a pointer to an object of type B. The program gladly gives this to you and assigns your new B to address 2 and also creates a pointer in address 3. Now, the only way to access the data in address 2 is via the pointer in address 3. Next, you dereferenced the pointer using * to get the data that the pointer is pointing to (the data in address 2). This effectively creates a copy of that data and assigns it to object2, assigned in address 1. Remember, it's a COPY, not the original.
Now, here's the problem:
You never actually stored that pointer anywhere you can use it! Once this assignment is finished, the pointer (memory in address3, which you used to access address2) is out of scope and beyond your reach! You can no longer call delete on it and therefore cannot clean up the memory in address2. What you are left with is a copy of the data from address2 in address1. Two of the same things sitting in memory. One you can access, the other you can't (because you lost the path to it). That's why this is a memory leak.
I would suggest coming from your C# background that you read up a lot on how pointers in C++ work. They are an advanced topic and can take some time to grasp, but their use will be invaluable to you.
Well, you create a memory leak if you don't at some point free the memory you've allocated using the new operator by passing a pointer to that memory to the delete operator.
In your two cases above:
A *object1 = new A();
Here you aren't using delete to free the memory, so if and when your object1 pointer goes out of scope, you'll have a memory leak, because you'll have lost the pointer and so can't use the delete operator on it.
And here
B object2 = *(new B());
you are discarding the pointer returned by new B(), and so can never pass that pointer to delete for the memory to be freed. Hence another memory leak.
If it makes it easier, think of computer memory as being like a hotel and programs are customers who hire rooms when they need them.
The way this hotel works is that you book a room and tell the porter when you are leaving.
If you program books a room and leaves without telling the porter the porter will think that the room is still is use and will not let anyone else use it. In this case there is a room leak.
If your program allocates memory and does not delete it (it merely stops using it) then the computer thinks that the memory is still in use and will not allow anyone else to use it. This is a memory leak.
This is not an exact analogy but it might help.
When creating object2 you're creating a copy of the object you created with new, but you're also losing the (never assigned) pointer (so there's no way to delete it later on). To avoid this, you'd have to make object2 a reference.
It's this line that is immediately leaking:
B object2 = *(new B());
Here you are creating a new B object on the heap, then creating a copy on the stack. The one that has been allocated on the heap can no longer be accessed and hence the leak.
This line is not immediately leaky:
A *object1 = new A();
There would be a leak if you never deleted object1 though.
How do I delete a pointer and the object it's pointing to?
Will the code below delete the object?
Object *apple;
apple = new Object();
delete apple;
And what happens if the pointer is not deleted, and gets out of scope?
Object *apple;
apple = new Object();
This might be a very basic question, but I'm coming from Java.
Your first code snippet does indeed delete the object. The pointer itself is a local variable allocated on the stack. It will be deallocated as soon as it goes out of scope.
That brings up the second point--if the pointer goes out of scope before you deallocate the object you allocated on the heap, you will never be able to deallocate it, and will have a memory leak.
Hope this helps.
Hello and welcome to C++ land! You will love how much you hate it (or something like that). C++, while appearing to be similar to java in untrained eyes might look similar, is actually quite different semantically. Lets see how these semantics play out in c++ in regards to your question. First lets take a class:
class Foo {
public:
Foo() { std::cout << "In constructor\n"; }
~Foo() { std::cout << "In destructor\n"; }
};
Foo here is just a representative of any class you might want to use. Lets see what happens when we create and play with a normal Foo object:
{
Foo bar;
do_stuff(bar);
}
If we were to run code that looked like this, we would see:
In constructor
In destructor
This is because, when an object is created, it is constructed using the constructor. When it goes out of scope, the destructor is called (~Foo in our code) which deconstructs (or destroys) the object. This is actually a fairly common and powerful feature in C++ (known as RAII, as opposed to other forms of returning memory to the system, such as Garbage Collection). Armed with this new knowledge, lets see what happens when we play with a pointer to Foo:
{
Foo *bar = new Foo();
some_more_stuff(bar);
}
What happens here is we would see:
In constructor
This is because of how pointers are allocated versus how variables are allocated. The way pointers are allocated, they don't actually go out of scope normally, but their contents do. This is known as a dangling pointer. For a better example, take a look at this:
#include <iostream>
int* get_int() {
int qux = 42;
int *foo = &qux;
return foo;
}
int main() {
int *qazal = get_int();
std::cout << *qazal;
}
Thanks to modern operating systems, this memory will still be returned when the program finishes, but not during the running of the program. If we were to delete the pointer (in the same scope it was created) via delete, then that memory will actually be returned to the operating system at that time.
When you call delete on a pointer it frees the memory of the thing pointed to. In other words you don't need to free the memory that makes up the pointer variable, just the thing that is pointed to. So your code:
Object *apple;
apple = new Object();
delete apple;
correctly deletes the object and there will be no memory leak.
If you don't call delete and the variable goes out of scope then you'll have a memory leak. These can be difficult to track down, so it's advisable to use a smart pointer class.
Operator delete deletes an object pointed to by the pointer that previously was allocated with operator new.
The pointer itself is not changed and even will have the same value as before the calling the operator. However its value becomes invalid after deleteing the object.
If the pointer itself is a local variable it will be destroyed after the control leaves the declaration region of the pointer.
If the pointer has the static storage duration then it will be destroyed when the program finishes its execution.
Take into account that you can use smart pointers instead of raw pointers.
For example
std::unique_ptr<Object> apple( new Object() );
in this case you need not to call delete It is the smart pointer that will do all the work itself.
Will the code below delete the object?
Yes, it will. But the pointer isn't deleted and accidentally using the pointer after deletion will lead to error.
And what happens if the pointer is not deleted, and gets out of scope?
Memory leak will happen.
I have the following code and I get stack overflow error can anyone please explain me What's wrong here. from my understanding this pointer points to current object so why I cant delete it in a destructor;
class Object
{
private:
static int objCount;
public:
int getCount()
{
int i =10;
return i++;
}
Object()
{
cout<< "Obj Created = "<<++objCount<<endl;
cout <<endl<<this->getCount()<<endl;
}
~Object()
{
cout<<"Destructor Called\n"<<"Deleted Obj="<<objCount--<<endl;
delete this;
}
};
int Object::objCount = 0;
int _tmain(int argc, _TCHAR* argv[])
{
{
Object obj1;
}
{
Object *obj2 = new Object();
}
getchar();
return 0;
}
You're doing delete this; in your class's destructor.
Well, delete calls the class's destructor.
You're doing delete this; in your class's destructor.
...
<!<!<!Stack Overflow!>!>!>
(Sorry guys I feel obliged to include this... this might probably spoil it sorrrryyyy!
Moral of the boring story, don't do delete this; on your destructor (or don't do it at all!)
Do [1]
Object *obj = new Object();
delete obj;
or much better, just
Object obj;
[1]#kfsone's answer more accurately points out that the stack overflow was actually triggered by obj1's destructor.
'delete this' never makes sense. Either you're causing an infinite recursion, as here, or you're deleting an object while it is still in use by somebody else. Just remove it. The object is already being deleted: that's why you're in the destructor.
The crash you are having is because of the following statement:
{
Object obj1;
}
This allocates an instance of "Object" on the stack. The scope you created it in ends, so the object goes out of scope, so the destructor (Object::~Object) is invoked.
{
Object obj1;
// automatic
obj1.~Object();
}
This means that the next instruction the application will encounter is
delete this;
There are two problems right here:
delete calls the object's destructor, so your destructor indirectly calls your destructor which indirectly calls your destructor which ...
After the destructor call, delete attempts to return the object's memory to the place where new obtains it from.
By contrast
{
Object *obj2 = new Object();
}
This creates a stack variable, obj2 which is a pointer. It allocates memory on the heap to store an instance of Object, calls it's default constructor, and stores the address of the new instance in obj2.
Then obj2 goes out of scope and nothing happens. The Object is not released, nor is it's destructor called: C++ does not have automatic garbage collection and does not do anything special when a pointer goes out of scope - it certainly doesn't release the memory.
This is a memory leak.
Rule of thumb: delete calls should be matched with new calls, delete [] with new []. In particular, try to keep new and delete in matching zones of authority. The following is an example of mismatched ownership/authority/responsibility:
auto* x = xFactory();
delete x;
Likewise
auto* y = new Object;
y->makeItStop();
Instead you should prefer
// If you require a function call to allocate it, match a function to release it.
auto* x = xFactory();
xTerminate(x); // ok, I just chose the name for humor value, Dr Who fan.
// If you have to allocate it yourself, you should be responsible for releasing it.
auto* y = new Object;
delete y;
C++ has container classes that will manage object lifetime of pointers for you, see std::shared_ptr, std::unique_ptr.
There are two issues here:
You are using delete, which is generally a code smell
You are using delete this, which has several issues
Guideline: You should not use new and delete.
Rationale:
using delete explicitly instead of relying on smart pointers (and automatic cleanup in general) is brittle, not only is the ownership of a raw pointer unclear (are you sure you should be deleting it ?) but it is also unclear whether you actually call delete on every single codepath that needs it, especially in the presence of exceptions => do your sanity (and that of your fellows) a favor, don't use it.
using new is also error-prone. First of all, are you sure you need to allocate memory on the heap ? C++ allows to allocate on the stack and the C++ Standard Library has containers (vector, map, ...) so the actual instances where dynamic allocation is necessary are few and far between. Furthermore, as mentioned, if you ever reach for dynamic allocation you should be using smart pointers; in order to avoid subtle order of execution issues it is recommend you use factory functions: make_shared and make_unique (1) to build said smart pointers.
(1) make_unique is not available in C++11, only in C++14, it can trivially be implemented though (using new, of course :p)
Guideline: You shall not use delete this.
Rationale:
Using delete this means, quite literally, sawing off the branch you are sitting on.
The argument to delete should always be a dynamically allocated pointer; therefore should you inadvertently allocate an instance of the object on the stack you are most likely to crash the program.
The execution of the method continues past this statement, for example destructors of local objects will be executed. This is like walking on the ghost of the object, don't look down!
Should a method containing this statement throw an exception or report an error, it is difficult to appraise whether the object was successfully destroyed or not; and trying again is not an option.
I have seen several example of usage, but none that could not have used a traditional alternative instead.
I learned C# first, and now I'm starting with C++. As I understand, operator new in C++ is not similar to the one in C#.
Can you explain the reason of the memory leak in this sample code?
class A { ... };
struct B { ... };
A *object1 = new A();
B object2 = *(new B());
What is happening
When you write T t; you're creating an object of type T with automatic storage duration. It will get cleaned up automatically when it goes out of scope.
When you write new T() you're creating an object of type T with dynamic storage duration. It won't get cleaned up automatically.
You need to pass a pointer to it to delete in order to clean it up:
However, your second example is worse: you're dereferencing the pointer, and making a copy of the object. This way you lose the pointer to the object created with new, so you can never delete it even if you wanted!
What you should do
You should prefer automatic storage duration. Need a new object, just write:
A a; // a new object of type A
B b; // a new object of type B
If you do need dynamic storage duration, store the pointer to the allocated object in an automatic storage duration object that deletes it automatically.
template <typename T>
class automatic_pointer {
public:
automatic_pointer(T* pointer) : pointer(pointer) {}
// destructor: gets called upon cleanup
// in this case, we want to use delete
~automatic_pointer() { delete pointer; }
// emulate pointers!
// with this we can write *p
T& operator*() const { return *pointer; }
// and with this we can write p->f()
T* operator->() const { return pointer; }
private:
T* pointer;
// for this example, I'll just forbid copies
// a smarter class could deal with this some other way
automatic_pointer(automatic_pointer const&);
automatic_pointer& operator=(automatic_pointer const&);
};
automatic_pointer<A> a(new A()); // acts like a pointer, but deletes automatically
automatic_pointer<B> b(new B()); // acts like a pointer, but deletes automatically
This is a common idiom that goes by the not-very-descriptive name RAII (Resource Acquisition Is Initialization). When you acquire a resource that needs cleanup, you stick it in an object of automatic storage duration so you don't need to worry about cleaning it up. This applies to any resource, be it memory, open files, network connections, or whatever you fancy.
This automatic_pointer thing already exists in various forms, I've just provided it to give an example. A very similar class exists in the standard library called std::unique_ptr.
There's also an old one (pre-C++11) named auto_ptr but it's now deprecated because it has a strange copying behaviour.
And then there are some even smarter examples, like std::shared_ptr, that allows multiple pointers to the same object and only cleans it up when the last pointer is destroyed.
A step by step explanation:
// creates a new object on the heap:
new B()
// dereferences the object
*(new B())
// calls the copy constructor of B on the object
B object2 = *(new B());
So by the end of this, you have an object on the heap with no pointer to it, so it's impossible to delete.
The other sample:
A *object1 = new A();
is a memory leak only if you forget to delete the allocated memory:
delete object1;
In C++ there are objects with automatic storage, those created on the stack, which are automatically disposed of, and objects with dynamic storage, on the heap, which you allocate with new and are required to free yourself with delete. (this is all roughly put)
Think that you should have a delete for every object allocated with new.
EDIT
Come to think of it, object2 doesn't have to be a memory leak.
The following code is just to make a point, it's a bad idea, don't ever like code like this:
class B
{
public:
B() {}; //default constructor
B(const B& other) //copy constructor, this will be called
//on the line B object2 = *(new B())
{
delete &other;
}
}
In this case, since other is passed by reference, it will be the exact object pointed to by new B(). Therefore, getting its address by &other and deleting the pointer would free the memory.
But I can't stress this enough, don't do this. It's just here to make a point.
Given two "objects":
obj a;
obj b;
They won't occupy the same location in memory. In other words, &a != &b
Assigning the value of one to the other won't change their location, but it will change their contents:
obj a;
obj b = a;
//a == b, but &a != &b
Intuitively, pointer "objects" work the same way:
obj *a;
obj *b = a;
//a == b, but &a != &b
Now, let's look at your example:
A *object1 = new A();
This is assigning the value of new A() to object1. The value is a pointer, meaning object1 == new A(), but &object1 != &(new A()). (Note that this example is not valid code, it is only for explanation)
Because the value of the pointer is preserved, we can free the memory it points to: delete object1; Due to our rule, this behaves the same as delete (new A()); which has no leak.
For you second example, you are copying the pointed-to object. The value is the contents of that object, not the actual pointer. As in every other case, &object2 != &*(new A()).
B object2 = *(new B());
We have lost the pointer to the allocated memory, and thus we cannot free it. delete &object2; may seem like it would work, but because &object2 != &*(new A()), it is not equivalent to delete (new A()) and so invalid.
In C# and Java, you use new to create an instance of any class and then you do not need to worry about destroying it later.
C++ also has a keyword "new" which creates an object but unlike in Java or C#, it is not the only way to create an object.
C++ has two mechanisms to create an object:
automatic
dynamic
With automatic creation you create the object in a scoped environment:
- in a function or
- as a member of a class (or struct).
In a function you would create it this way:
int func()
{
A a;
B b( 1, 2 );
}
Within a class you would normally create it this way:
class A
{
B b;
public:
A();
};
A::A() :
b( 1, 2 )
{
}
In the first case, the objects are destroyed automatically when the scope block is exited. This could be a function or a scope-block within a function.
In the latter case the object b is destroyed together with the instance of A in which it is a member.
Objects are allocated with new when you need to control the lifetime of the object and then it requires delete to destroy it. With the technique known as RAII, you take care of the deletion of the object at the point you create it by putting it within an automatic object, and wait for that automatic object's destructor to take effect.
One such object is a shared_ptr which will invoke a "deleter" logic but only when all the instances of the shared_ptr that are sharing the object are destroyed.
In general, whilst your code may have many calls to new, you should have limited calls to delete and should always make sure these are called from destructors or "deleter" objects that are put into smart-pointers.
Your destructors should also never throw exceptions.
If you do this, you will have few memory leaks.
B object2 = *(new B());
This line is the cause of the leak. Let's pick this apart a bit..
object2 is a variable of type B, stored at say address 1 (Yes, I'm picking arbitrary numbers here). On the right side, you've asked for a new B, or a pointer to an object of type B. The program gladly gives this to you and assigns your new B to address 2 and also creates a pointer in address 3. Now, the only way to access the data in address 2 is via the pointer in address 3. Next, you dereferenced the pointer using * to get the data that the pointer is pointing to (the data in address 2). This effectively creates a copy of that data and assigns it to object2, assigned in address 1. Remember, it's a COPY, not the original.
Now, here's the problem:
You never actually stored that pointer anywhere you can use it! Once this assignment is finished, the pointer (memory in address3, which you used to access address2) is out of scope and beyond your reach! You can no longer call delete on it and therefore cannot clean up the memory in address2. What you are left with is a copy of the data from address2 in address1. Two of the same things sitting in memory. One you can access, the other you can't (because you lost the path to it). That's why this is a memory leak.
I would suggest coming from your C# background that you read up a lot on how pointers in C++ work. They are an advanced topic and can take some time to grasp, but their use will be invaluable to you.
Well, you create a memory leak if you don't at some point free the memory you've allocated using the new operator by passing a pointer to that memory to the delete operator.
In your two cases above:
A *object1 = new A();
Here you aren't using delete to free the memory, so if and when your object1 pointer goes out of scope, you'll have a memory leak, because you'll have lost the pointer and so can't use the delete operator on it.
And here
B object2 = *(new B());
you are discarding the pointer returned by new B(), and so can never pass that pointer to delete for the memory to be freed. Hence another memory leak.
If it makes it easier, think of computer memory as being like a hotel and programs are customers who hire rooms when they need them.
The way this hotel works is that you book a room and tell the porter when you are leaving.
If you program books a room and leaves without telling the porter the porter will think that the room is still is use and will not let anyone else use it. In this case there is a room leak.
If your program allocates memory and does not delete it (it merely stops using it) then the computer thinks that the memory is still in use and will not allow anyone else to use it. This is a memory leak.
This is not an exact analogy but it might help.
When creating object2 you're creating a copy of the object you created with new, but you're also losing the (never assigned) pointer (so there's no way to delete it later on). To avoid this, you'd have to make object2 a reference.
It's this line that is immediately leaking:
B object2 = *(new B());
Here you are creating a new B object on the heap, then creating a copy on the stack. The one that has been allocated on the heap can no longer be accessed and hence the leak.
This line is not immediately leaky:
A *object1 = new A();
There would be a leak if you never deleted object1 though.
As referenced in this site...
http://www.parashift.com/c++-faq-lite/dtors.html#faq-11.10
But i did not find the reason, why we should explicitly call the desturctor?
You can think of it as a call to delete, but since you used placement new, you don't want to use delete, as that would attempt to free the memory. If you wanted it to be called automatically, you could use RAII:
// Could use a templated version, or find an existing impl somewhere:
void destroy_fred(Fred* f) {
f->~Fred();
}
void someCode()
{
char memory[sizeof(Fred)];
void* p = memory;
boost::shared_ptr<Fred> f(new(p) Fred(), destroy_fred);
// ...
// No need for an explicit destructor, cleaned up even during an exception
}
Normally, newed objects are destructed when delete is called. delete is never called for placement new, so you have to figure out when to destruct the object yourself, and do so explicitly.
You can't call ordinary delete because that would attempt free the memory, which was not allocated with ordinary new, so you call the destructor directly and then handle the underlying memory allocation depending on the architecture you are using.
Normally, an object is created using new, and destroyed when you call delete on the object. However, because 'delete' will also try to free the underlying memory back to the heap, you should never call delete on an object which which was instantiated using placement new.
Nevertheless, (no matter how it was allocated) every object ought to be destroyed (because its destructor might do something important).
Therefore instead of the object's being destroyed using delete, you have to destroy the object yourself, and do so explicitly, by invoking its destructor as shown in the FAQ you cited.
The example from your link:
void someCode()
{
char memory[sizeof(Fred)];
void* p = memory;
Fred* f = new(p) Fred();
...
f->~Fred(); // Explicitly call the destructor for the placed object
}
"placement new" return the memory offset that we provide. Right after new the compiler adds a call to the ctor ("placement new" is a also a trick to explicity call a ctor).
"memory" is allocated in the stack so we can not delete it.
string *ps = new string("Hello ");
The new in the above statement is called new operator which do two things
Call operator new(allocates
necessary memory like malloc) and
Call constructor
on raw memory created by operator
new to initialize
delete ps;
Again delete corresponds to
Destruction of object and
Deallocation of memory(i.e, free)
In c++ user is not allowed to call constructor directly,Placement new is used when you have raw memory allocated by some means and wanted to initialize the bits with constructor.Because the memory allocation process did not happen through operator new, delete cannot deallocate the memory.
void someCode()
{
char memory[sizeof(Fred)];//memory allocated on stack
void* p = memory;
Fred* f = new(p) Fred();//placement new
...
f->~Fred(); // Explicitly call the destructor for the placed object
}