Calling constructors in c without new

Question

I ve often seen that people create objects in C   using  Thing myThing  asdf      Instead of this   Thing myThing   Thing  asdf      This seems to work  using gcc   at least as long as there are no templates involved  My question now  is the first line correct and if so should I use it

User · Answer

I played a bit with it and the syntax seems to get quite strange when a constructor takes no arguments. Let me give an example:

#include <iostream> 

using namespace std;

class Thing
{
public:
    Thing();
};

Thing::Thing()
{
    cout << "Hi" << endl;
}

int main()
{
    //Thing myThing(); // Does not work
    Thing myThing; // Works

}

so just writing Thing myThing w/o brackets actually calls the constructor, while Thing myThing() makes the compiler thing you want to create a function pointer or something ??!!

User · Answer

I assume with the second line you actually mean   Thing  thing   new Thing  uiae      which would be the standard way of creating new dynamic objects  necessary for dynamic binding and polymorphism  and storing their address to a pointer  Your code does what JaredPar described  namely creating two objects  one passed a const char   the other passed a const Thing amp    and then calling the destructor   Thing    on the first object  the const char  one    By contrast  this   Thing thing  uiae      creates a static object which is destroyed automatically upon exiting the current scope

User · Answer

Quite simply  both lines create the object on the stack  rather than on the heap as  new  does  The second line actually involves a second call to a copy constructor  so it should be avoided  it also needs to be corrected as indicated in the comments   You should use the stack for small objects as much as possible since it is faster  however if your objects are going to survive for longer than the stack frame  then it s clearly the wrong choice

User · Answer

The compiler may well optimize the second form into the first form  but it doesn t have to    include  lt iostream gt   class A       public          A     std  cerr  lt  lt   Empty constructor   lt  lt  std  endl            A const A amp     std  cerr  lt  lt   Copy constructor   lt  lt  std  endl            A const char  str    std  cerr  lt  lt   char constructor     lt  lt  str  lt  lt  std  endl             A     std  cerr  lt  lt   destructor   lt  lt  std  endl        void direct         std  cerr  lt  lt  std  endl  lt  lt   TEST     lt  lt    FUNCTION    lt  lt  std  endl      A a   FUNCTION         static cast lt void gt  a      avoid warnings about unused variables    void assignment         std  cerr  lt  lt  std  endl  lt  lt   TEST     lt  lt    FUNCTION    lt  lt  std  endl      A a   A   FUNCTION         static cast lt void gt  a      avoid warnings about unused variables    void prove copy constructor is called         std  cerr  lt  lt  std  endl  lt  lt   TEST     lt  lt    FUNCTION    lt  lt  std  endl      A a   FUNCTION         A b   a      static cast lt void gt  b      avoid warnings about unused variables    int main         direct        assignment        prove copy constructor is called        return 0      Output from gcc 4 4   TEST  direct char constructor  direct destructor  TEST  assignment char constructor  assignment destructor  TEST  prove copy constructor is called char constructor  prove copy constructor is called Copy constructor destructor destructor

User · Answer

Both lines are in fact correct but do subtly different things     The first line creates a new object on the stack by calling a constructor of the format Thing const char       The second one is a bit more complex   It essentially does the following   Create an object of type Thing using the constructor Thing const char   Create an object of type Thing using the constructor Thing const Thing amp   Call  Thing   on the object created in step  1

User · Answer

In append to JaredPar answer 1-usual ctor  2nd-function-like-ctor with temporary object  Compile this source somewhere here http   melpon org wandbox  with different compilers    turn off rvo for clang  gcc with  -fno-elide-constructors    include  lt stdio h gt  class Thing   public      Thing const char   puts   FUNCTION           Thing const Thing amp   puts   FUNCTION               Thing   puts   FUNCTION         int main int   argc    const char     argv          Thing myThing   Thing  quot asdf quot       And you will see the result  From ISO IEC 14882 2003-10-15  8 5   part 12  Your 1st 2nd construction are called direct-initialization  12 1  part 13 A functional notation type conversion  5 2 3  can be used to create new objects of its type   Note  The syntax looks like an explicit call of the constructor        An object created in this way is unnamed   Note  12 2 describes the lifetime of temporary objects      Note  explicit constructor calls do not yield lvalues  see 3 10      Where to read about RVO   12 Special member functions   12 8 Copying class objects  Part 15 When certain criteria are met  an implementation is allowed to omit the copy construction of a class object  even if the copy constructor and or destructor for the object have side effects   Turn off it with compiler flag from comment to view such copy-behavior

User · Answer

Ideally  a compiler would optimize the second  but it s not required  The first is the best way  However  it s pretty critical to understand the distinction between stack and heap in C    sine you must manage your own heap memory

[c++] Calling constructors in c++ without new

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