Creating an object with or without new

Question

Possible Duplicate    What is difference between instantiating an object using new vs  without       This is probably a basic question  and might have already been asked  say  here   yet I still don t understand it  So  let me ask it   Consider the following C   class   class Obj      char  str  public      Obj char  s           str   s          cout  lt  lt  str             Obj            cout  lt  lt   Done  n           delete str            See the comment of  Loki Astari  below on why this line of code is bad practice            what s the difference between the following code snippets   Obj o1   Hi n      and  Obj  o2   new Obj  Hi n      Why the former calls the destructor  but the latter doesn t  without explicit call to delete    Which one is preferred

User · Answer

The first allocates an object with automatic storage duration, which means it will be destructed automatically upon exit from the scope in which it is defined.

The second allocated an object with dynamic storage duration, which means it will not be destructed until you explicitly use delete to do so.

User · Answer

Both do different things   The first creates an object with automatic storage duration  It is created  used  and then goes out of scope when the current block           ends  It s the simplest way to create an object  and is just the same as when you write int x   0   The second creates an object with dynamic storage duration and allows two things    Fine control over the lifetime of the object  since it does not go out of scope automatically  you must destroy it explicitly using the keyword delete  Creating arrays with a size known only at runtime  since the object creation occurs at runtime   I won t go into the specifics of allocating dynamic arrays here     Neither is preferred  it depends on what you re doing as to which is most appropriate   Use the former unless you need to use the latter   Your C   book should cover this pretty well  If you don t have one  go no further until you have bought and read  several times  one of these   Good luck     Your original code is broken  as it deletes a char array that it did not new  In fact  nothing newd the C-style string  it came from a string literal  deleteing that is an error  albeit one that will not generate a compilation error  but instead unpredictable behaviour at runtime    Usually an object should not have the responsibility of deleteing anything that it didn t itself new  This behaviour should be well-documented  In this case  the rule is being completely broken

[c++] Creating an object: with or without `new`

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