What is the difference between new and ::new

Question

Many libraries like boost use ::new and ::delete .

Example from boost::make_shared

template< class T, class... Args > typename boost::detail::sp_if_not_array< T >::type make_shared( Args && ... args ) {     boost::shared_ptr< T > pt( static_cast< T* >( 0 ), BOOST_SP_MSD( T ) );      boost::detail::sp_ms_deleter< T > * pd = static_cast<boost::detail::sp_ms_deleter< T > *>( pt._internal_get_untyped_deleter() );      void * pv = pd->address();      ::new( pv ) T( boost::detail::sp_forward<Args>( args )... );     pd->set_initialized();      T * pt2 = static_cast< T* >( pv );      boost::detail::sp_enable_shared_from_this( &pt, pt2, pt2 );     return boost::shared_ptr< T >( pt, pt2 ); } 

What does this mean? and why would one use ::new over just new?

Answer 1

A class C could define its own operator new (this enables, for example, to have your own allocation policy for that class, and/or to provide some Allocator for it. Many standard containers templates accept an optional allocator argument, for example the second argument to std::vector; see also std::allocator and this example).

If you code new C, that operator would be used (if it exists).

If you code ::new C, the global new is used

Your example is using the global placement new

Answer 2

new, new[], delete, and delete[] (including the placement variants) are overridable both at class and at global scope, although doing the latter is ill-advised.

When you see ::new, you are using the global new operator.