Do the parentheses after the type name make a difference with new

Question

If  Test  is an ordinary class  is there any difference between   Test  test   new Test    and  Test  test   new Test

User · Answer

No  they are the same  But there is a difference between   Test t          create a Test called t   and  Test t         declare a function called t which returns a Test   This is because of the basic C    and C  rule  If something can possibly be a declaration  then it is a declaration   Edit  Re the initialisation issues regarding POD and non-POD data  while I agree with everything that has been said  I would just like to point out that these issues only apply if the thing being new d or otherwise constructed does not have a user-defined constructor  If there is such a constructor it will be used  For 99 99  of sensibly designed classes there will be such a constructor  and so the issues can be ignored

User · Answer

new Thing    is explicit that you want a constructor called whereas new Thing  is taken to imply you don t mind if the constructor isn t called   If used on a struct class with a user-defined constructor  there is no difference  If called on a trivial struct class  e g  struct Thing   int i      then new Thing  is like malloc sizeof Thing    whereas new Thing    is like calloc sizeof Thing    - it gets zero initialized   The gotcha lies in-between   struct Thingy      Thingy       No-longer a trivial class   virtual WaxOn      int i       The behavior of new Thingy  vs new Thingy    in this case changed between C  98 and C  2003  See Michael Burr s explanation for how and why

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In general we have default-initialization in first case and value-initialization in second case   For example  in case with int  POD type     int  test   new int - we have any initialization and value of  test can be any  int  test   new int   -  test will have 0 value    next behaviour depended from your type Test  We have defferent cases  Test have defult constructor  Test have generated default constructor  Test contain POD member  non POD member

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The rules for new are analogous to what happens when you initialize an object with automatic storage duration  although  because of vexing parse  the syntax can be slightly different   If I say  int my int     default-initialize   indeterminate  non-class type   Then my int has an indeterminate value  since it is a non-class type  Alternatively  I can value-initialize my int  which  for non-class types  zero-initializes  like this  int my int       value-initialize   zero-initialize  non-class type    Of course  I can t use    because that would be a function declaration  but int   works the same as int   to construct a temporary   Whereas  for class types  Thing my thing     default-initialize   default ctor  class type  Thing my thing       value-initialize   default-initialize   default ctor  class type   The default constructor is called to create a Thing  no exceptions  So  the rules are more or less   Is it a class type   YES  The default constructor is called  regardless of whether it is value-initialized  with     or default-initialized  without       There is some additional prior zeroing behavior with value-initialization  but the default constructor is always given the final say   NO  Were    used   YES  The object is value-initialized  which  for non-class types  more or less just zero-initializes  NO  The object is default-initialized  which  for non-class types  leaves it with an indeterminate value  it effectively isn t initialized        These rules translate precisely to new syntax  with the added rule that    can be substituted for    because new is never parsed as a function declaration  So  int  my new int   new int     default-initialize   indeterminate  non-class type  Thing  my new thing   new Thing     default-initialize   default ctor  class type  int  my new zeroed int   new int       value-initialize   zero-initialize  non-class type       my new zeroed int   new int       ditto        my new thing   new Thing       value-initialize   default-initialize   default ctor  class type    This answer incorporates conceptual changes in C  11 that the top answer currently does not  notably  a new scalar or POD instance that would end up an with indeterminate value is now technically now default-initialized  which  for POD types  technically calls a trivial default constructor   While this does not cause much practical change in behavior  it does simplify the rules somewhat

User · Answer

Assuming that Test is a class with a defined constructor  there s no difference  The latter form makes it a little clearer that Test s constructor is running  but that s about it

User · Answer

Let s get pedantic  because there are differences that can actually affect your code s behavior  Much of the following is taken from comments made to an  Old New Thing  article   Sometimes the memory returned by the new operator will be initialized  and sometimes it won t depending on whether the type you re newing up is a POD  plain old data   or if it s a class that contains POD members and is using a compiler-generated default constructor    In C  1998 there are 2 types of initialization  zero and default In C  2003 a 3rd type of initialization  value initialization was added    Assume   struct A   int m        POD struct B    B    int m        non-POD  compiler generated default ctor struct C   C     m        C    int m        non-POD  default-initialising m   In a C  98 compiler  the following should occur    new A   - indeterminate value new A   - zero-initialize new B   - default construct  B  m is uninitialized  new B   - default construct  B  m is uninitialized  new C   - default construct  C  m is zero-initialized  new C   - default construct  C  m is zero-initialized    In a C  03 conformant compiler  things should work like so    new A    - indeterminate value new A    - value-initialize A  which is zero-initialization since it s a POD  new B    - default-initializes  leaves B  m uninitialized  new B    - value-initializes B which zero-initializes all fields since its default ctor is compiler generated as opposed to user-defined  new C    - default-initializes C  which calls the default ctor  new C    - value-initializes C  which calls the default ctor    So in all versions of C   there s a difference between new A and new A   because A is a POD   And there s a difference in behavior between C  98 and C  03 for the case new B     This is one of the dusty corners of C   that can drive you crazy  When constructing an object  sometimes you want need the parens  sometimes you absolutely cannot have them  and sometimes it doesn t matter

[c++] Do the parentheses after the type name make a difference with new?

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