c++ allocator: operator new or placement new

Question

After reading this link , I've known that placement new was too hard to use properly. Then I found std::allocator , so I thought std::allocator should have used placement new because it could separate allocation and do construction in two steps.

However, it seems that How are allocator in C++ implemented? tells me that std::allocator is implemented by operator new , instead of placement new . I'm confused now. If it doesn't use placement new , how could it separate allocation and do construction in two steps?

Answer 1

You have to differ between the operator new function and new expressions .

The former (the operator new function) only allocates memory. The latter ( new expression) calls operator new to allocate memory and then invokes the constructor.

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std::allocator have two functions to implement allocation and construction: allocate and construct .

The allocate function uses the operator new function to allocate memory. The construct function uses placement-new to construct objects. Which seems to be the two-step process you want.