Question

In “The C++ Programming Language” book, in list of operations (article 6.2), Bjarne Stoustrup wrote this:

create (place) new ( expr-list ) type

create (place and initialize) new ( expr-list ) type ( expr-list )

I’ve never heard about this kind of the new operator, and I’m interested what does it do.

Answer 1

This is called placement-new. You can create an object over already existing memory.

Here’s an explanation and an useful question in SO

You can also have nothrow, for example:

char* pzNewBuffer = new (nothrow) char [2048];

which tells, that new will not throw std::bad_alloc in case of out of memory, but it will return NULL, instead.

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Another example, that comes to my mind – the standard containers (probably) use placement new: when you call reserve, memory is allocated, but nothing is constructed/initialzed on this memory. So, when you insert (with std::vector<T>::push_back for example), if there’s allocated, but not initialized memory – placement new is used.