Why does the use of 'new' cause memory leaks?

Question

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

Answer 1

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.