Reassign unique_ptr object with make_unique statements - Memory leak? - c++

Question

I don't get what following statement would do (specially second line)?
auto buff = std::make_unique<int[]>(128);
buff = std::make_unique<int[]>(512);
Will the second call to make_unique followed by assignment operator will de-allocate memory allocated by first call, or will there be memory leak? Must I have to use buff.reset(new int[512]); ?
I've debugged it, but didn't find any operator= being called, nor any destructor be invoked (by unique_ptr).

Answer 1

Move assignment operator is called, which does
if (this != &_Right)
{ // different, do the swap
reset(_Right.release());
this->get_deleter() = _STD move(_Right.get_deleter());
}
No leak here, as it does reset, which will deallocate.

Answer 2

There is no memory leak here, the assignment will deallocate the resources associated with the first allocation. Not seeing it in the debugger most likely means the relevant call was just optimised out. Try compiling with -O0 if you want to see it.

Answer 3

gcc 5.3:
#include <memory>
extern void emit(int*);
int main()
{
// declare and initialise buf
auto buff = std::make_unique<int[]>(128);
// make_unique on the RHS returns a temporary
// - therefore an r-value reference
// therefore this becomes and operator=(unique_ptr&&)
// (move-assignment)
buff = std::make_unique<int[]>(512);
// something to get the compiler to emit code
emit(buff.get());
}
yields assembly:
main:
pushq %r12
movl $512, %edi
pushq %rbp
pushq %rbx
call operator new[](unsigned long) ; <-- new (1)
movl $64, %ecx
movq %rax, %rbp
xorl %eax, %eax
movq %rbp, %rdi
rep stosq
movl $2048, %edi
call operator new[](unsigned long) ; <<-- new (2)
movl $2048, %edx
xorl %esi, %esi
movq %rax, %rdi
movq %rax, %rbx
call memset
movq %rbp, %rdi
call operator delete[](void*) ;<-- delete (1)
movq %rbx, %rdi
call emit(int*)
movq %rbx, %rdi
call operator delete[](void*) ;<-- delete (2)
popq %rbx
xorl %eax, %eax
popq %rbp
popq %r12
ret
movq %rax, %r12
movq %rbp, %rbx
.L3:
movq %rbx, %rdi
vzeroupper
call operator delete[](void*) ;<-- handles a failed assignment
movq %r12, %rdi
call _Unwind_Resume
movq %rax, %r12
jmp .L3