I have the following code that intends to create an array, but without initializing its default objects. I would like to ideally move to a new place, which seems to happen, but I found that the object destructor is called inside the emplace function.
#include <iostream>
#include <memory> // std::uninitialized_copy, std::allocator...
#include <utility> // std::move...
#include <bitset>
struct Int {
int i;
Int ( ) : i ( -1 ) { std::cout << "default constructed\n"; }
Int ( const int i_ ) : i ( i_ ) { std::cout << i << " constructed\n"; }
Int ( Int && int_ ) : i ( std::move ( int_.i ) ) { std::cout << i << " move constructed\n"; }
Int ( const Int & int_ ) : i ( int_.i ) { std::cout << i << " copy constructed\n"; }
~Int ( ) { std::cout << i << " destructed\n"; i = -1; }
};
template <typename T, size_t S = 64>
class NoInitArray {
std::bitset<S> m_used;
T *m_array = reinterpret_cast < T* > ( ::operator new ( sizeof ( T ) * S ) );
public:
T const &operator [ ] ( const size_t idx_ ) const {
return m_array [ idx_ ];
}
NoInitArray ( ) { }
~NoInitArray ( ) {
for ( size_t idx = 0; idx < S; ++idx ) {
if ( m_used [ idx ] ) {
reinterpret_cast< const T* > ( m_array + idx )->~T ( );
}
}
}
template<typename ...Args>
void emplace ( const size_t idx_, Args &&... value_ ) {
std::cout << "start emplace\n";
m_used [ idx_ ] = 1;
new ( m_array + idx_ ) T ( std::forward<T> ( value_ ) ... );
std::cout << "end emplace\n";
}
};
int main ( ) {
NoInitArray<Int> nia;
nia.emplace ( 0, 0 );
nia.emplace ( 1, 1 );
std::cout << nia [ 1 ].i << std::endl;
nia.emplace ( 2, 2 );
return 0;
}
The result of starting this program is as follows:
start emplace
0 constructed
0 move constructed
0 destructed
end emplace
start emplace
1 constructed
1 move constructed
1 destructed
end emplace
1
start emplace
2 constructed
2 move constructed
2 destructed
end emplace
0 destructed
1 destructed
2 destructed
This shows that objects are built once and destroyed twice (which is obviously UB), once inside the emplace function, and then once upon destruction of NoInitArray.
Question: "Why is the destructor of my Int object called inside the emplace function"?
Compiler, the latest Clang / LLVM on Windhoze.
EDIT1: I added move and copy constructors to the Int structure, now the counters match, i.e. 2 designs and 2 destruction.
EDIT2: new ( m_array + idx_ ) T ( std::forward<T> ( value_ ) ... ); new ( m_array + idx_ ) T ( value_ ... ); / .
EDIT3: . , NoInitArray() . delete m_array - , ( Clang/LLVM) m_array [0] (, , , UB). std:: malloc/std:: free, , , , , , .