[Qt-interest] Placement new in Qt

[Brad Howes]

On Jul 1, 2009, at 12:49 PM, Eric Clark wrote:

> else if (QTypeInfo<T>::isComplex) new (n) T(t);
>
> What does Qt expect it to do? Does it expect realloc to get called,  
> or does it just expect "n" to be returned?


This should just use the address of 'n' for storage, and initializing  
the new T object as a copy of 't'. No reallocations are possible here.  
Somewhere else, Qt must have made sure that 'n' points to an object  
that is large enough to hold T. The 'isComplex' check (I assume) is to  
differentiate between POD types like int or double and non-POD types,  
since the final clause

> else *reinterpret_cast<T*>(n) = t;


is just doing a (fast) bit-copy from the object to copy 't' to the  
memory pointed to by 't' which is only safe for PODs.

Brad

-- 
Brad Howes
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