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c++gccplacement-new

What is the underlying implementation of placement new?

T *p = ::operator new(sizeof(T));
new (p) T;

I wonder how that syntax is working, given that placement new is declared as:

void* operator new(std::size_t, void*)

The second argument void* seems point to the storage of the object, so how does C++ connect the new (p) T to the void*?

In gcc libsupc++ the implementation is simply:

// Default placement versions of operator new.
_GLIBCXX_NODISCARD inline void* operator new(std::size_t, void* __p) _GLIBCXX_USE_NOEXCEPT
{ return __p; }

_GLIBCXX_NODISCARD inline void* operator new[](std::size_t, void* __p) _GLIBCXX_USE_NOEXCEPT
{ return __p; }

Is the compiler responsible for translating the syntax new(*T) T to the calling of a placement operator new?

Solution

  • Is it compiler's responsibility to translate the syntax new(*T) T to the calling of a placement operator new?

    Yes, exactly.

    Placement new can not be implemented as a library function, it needs special handling from the compiler anyway. The actual implementation that you saw in the libsupc++ source code can be misleading here, as most of the crucial properties of a placement new call are not reflected by that implementation.