In the C++20 standard, it is said that array types are implicit lifetime type.
Does it mean that an array to a non implicit lifetime type can be implicitly created? The implicit creation of such an array would not cause creation of the array's elements?
Consider this case:
//implicit creation of an array of std::string
//but not the std::string elements:
void * ptr = operator new(sizeof (std::string) * 10);
//use launder to get a "pointer to object" (which object?)
std::string * sptr = std::launder(static_cast<std::string*>(ptr));
//pointer arithmetic on not created array elements well defined?
new (sptr+1) std::string("second element");
Is this code not UB any more since C++20?
Maybe this way is better?
//implicit creation of an array of std::string
//but not the std::string elements:
void * ptr = operator new(sizeof (std::string) * 10);
//use launder to get a "pointer to object" (actually not necessary)
std::string (* sptr)[10] = std::launder(static_cast<std::string(*)[10]>(ptr));
//pointer arithmetic on an array is well defined
new (*sptr+1) std::string("second element");
TC Answer + Comments conclusion:
launder
in the first example cause UB, and is
not necessary in the second example.The right code is:
//implicit creation of an array of std::string
//but not the std::string elements:
void * ptr = operator new(sizeof (std::string) * 10);
//the pointer already points to the implicitly created object
//so casting is enough
std::string (* sptr)[10] = static_cast<std::string(*)[10]>(ptr);
//pointer arithmetic on an array is well defined
new (*sptr+1) std::string("second element");
Does it means that an array to a non implicit lifetime type can be implicitly created?
Yes.
The implicit creation of such an array would not cause creation of the array's elements?
Yes.
This is what makes std::vector
implementable in ordinary C++.
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