Re: Article on possible improvements to C++

"Alf P. Steinbach" <>
Sun, 22 Nov 2009 06:53:02 +0100
* dragan:

Alf P. Steinbach wrote:

"Placement new"

is manually calling a constructor. It is "CONSTRUCTION" (a bad choice of
terminology, but so it is in C++).

It sounds as if you degree with my description. In that case, sorry but whatever
you mean is incorrect. Placement new does exactly the same as non-placement new:
calling an allocation function with specified allocation function arguments, and
if that succeeds, calling a constructor with specified constructor arguments.

For example,

   #include <stdio.h>

   struct S
       void* operator new( size_t const size, char const* const blah )
           printf( "allocation function called, arg = \"%s\"\n", blah );
           return ::operator new( size );

       S( int const whatever )
           printf( "constructor called, arg = %d\n", whatever );

   int main()
       S* const p = new( "Get it?" ) S( 42 );

 was probably originally referring to the ability to
specify some existing storage, a placement of the object, but in the
standard's terminology "placement" refers to any allocation function
with extra arguments, or new expression supplying such arguments.

Technically a placement new expression does the same as a
non-placement new expression.

Also, there's no big difference for a global new operator.

Cheers & hth.,

- Alf

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