what does this “new” syntax mean?

Question

Recently I read a piece of code like this:

template <unsigned long size>
class FooBase
{
  bool m_bValid;
  char m_data[size];
};

template <class T>
class Foo : public FooBase<sizeof(T)>
{
  // it's constructor
  Foo(){};
  Foo(T const & t) {construct(t); m_bValid = (true);}

  T const * const GetT() const { return reinterpret_cast<T const * const>(m_data); }
  T * const GetT() { return reinterpret_cast<T * const>(m_data);}

  // could anyone help me understand this line??
  void construct(T const & t) {new (GetT()) T(t);}
};

I have sliced the code to make sure it's not that complicated, the main question is about the construct(T const & t) function.

What does new (GetT()) T(t); exactly means?

btw, which version of GetT() is called?

Answer 1

What does new (GetT()) T(t); exactly means?

It is Placement new , it allows you to place an object at a particular location in memory, which is returned by Get() .

which version of GetT() is called?

The Second one.
Whenever the compiler has option of choosing between a const and a non-const function, it chooses the non-const version.
Specifically, in this case as @James points out in comments:
Non-const version is given preference because the member function which calls it is non-const.

Answer 2

That's called " Placement new ".

It means that you create a new object on a given memory buffer.

new (buffer) T(); //means it will instantiate an object T in the buffer.

This allows you to have memory buffers and custom allocators, without having to request and allocate new memory from the operating system.

Read this: What uses are there for "placement new"?

Answer 3

This looks like a placement new call. Here no memory is allocated. new simply returns the address in parenthesis and calls the constructor on the object being virtually constructed.