new和:new的区别是什么

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

许多库如boost use::new和:delete。

Example from boost::make_shared

boost::make_shared的例子

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?

这是什么意思?为什么要用:new而不是just new?

5 个解决方案

#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).

类C可以定义自己的操作符new(例如，它允许为该类拥有自己的分配策略，并/或为该类提供一些分配器。许多标准容器模板接受可选的分配器参数，例如std::vector的第二个参数;参见std::分配器和这个例子)。

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

如果您编写新的C，将使用该操作符(如果存在的话)。

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

如果您编码::new C，则使用全局new

Your example is using the global placement new

您的示例使用了全局布局new。

#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.

新的、新的[]、删除和删除[](包括位置变体)在类和全局作用域中都是可重写的，尽管这样做是不明智的。

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

当您看到::new时，您正在使用全局新操作符。

#3

In general whenever scope resolution ( ::)operator is used without any specifier at LHS, it refers to the global scope. Here also it is same.

通常，当在LHS中使用范围解析(::)操作符而不使用任何说明符时，它指的是全局范围。这里也是一样的。

Coming to the operator new, it can be overloaded at local and outer scope. Hence to access the global variant scope resolution operator is used.

到操作符new，它可以在本地和外部的范围重载。因此，使用全局变量范围解析操作符。

#4

new can be overriden and replaced. So simply stating new won't always get the new you want.

新的可能会被推翻，取而代之。所以简单地说新的并不总是得到你想要的新的。

In this specific case, we are looking at placement new:

在这个具体的案例中，我们正在寻找新的定位:

void * pv = pd->address();

::new( pv ) T( boost::detail::sp_forward<Args>( args )... );

this is where we are trying to construct a T in the location pv.

这是我们试图在位置pv中构造T的地方。

To avoid the possibility that overrids of new are invoked instead of the "real" placement new, you have to use ::new( void pointer here ) type( arguments... );. That specific new can neither be replaced nor overridden.

为了避免调用new的覆盖而不是“真正的”位置新，您必须使用:new(void指针)类型(参数…);这个新特性既不能被替换，也不能被重写。

#5

-1

The normal new operator is implemented at the class scale and can be overriden while ::new is implemented in a global scale and can't be overriden.

正常的新操作符是在类级别实现的，可以是overriden，而:new是在全局级别实现的，不能是overriden。

#1