c++ - Why does unique_ptr instantiation compile to larger binary than raw pointer?

Question

I was always under the impression that a std::unique_ptr had no overhead compared to using a raw pointer. However, compiling the following code

#include <memory>

void raw_pointer() {
  int* p = new int[100];
  delete[] p;
}

void smart_pointer() {
  auto p = std::make_unique<int[]>(100);
}

with g++ -std=c++14 -O3 produces the following assembly:

raw_pointer():
        sub     rsp, 8
        mov     edi, 400
        call    operator new[](unsigned long)
        add     rsp, 8
        mov     rdi, rax
        jmp     operator delete[](void*)
smart_pointer():
        sub     rsp, 8
        mov     edi, 400
        call    operator new[](unsigned long)
        lea     rdi, [rax+8]
        mov     rcx, rax
        mov     QWORD PTR [rax], 0
        mov     QWORD PTR [rax+392], 0
        mov     rdx, rax
        xor     eax, eax
        and     rdi, -8
        sub     rcx, rdi
        add     ecx, 400
        shr     ecx, 3
        rep stosq
        mov     rdi, rdx
        add     rsp, 8
        jmp     operator delete[](void*)

Why is the output for smart_pointer() almost three times as large as raw_pointer()?

question from:https://stackoverflow.com/questions/40635107/why-does-unique-ptr-instantiation-compile-to-larger-binary-than-raw-pointer

Answer 1

Because std::make_unique<int[]>(100) performs value initialization while new int[100] performs default initialization - In the first case, elements are 0-initialized (for int), while in the second case elements are left uninitialized. Try:

int *p = new int[100]();

And you'll get the same output as with the std::unique_ptr.

See this for instance, which states that std::make_unique<int[]>(100) is equivalent to:

std::unique_ptr<T>(new int[100]())

If you want a non-initialized array with std::unique_ptr, you could use¹:

std::unique_ptr<int[]>(new int[100]);

_{¹ As mentioned by @Ruslan in the comments, be aware of the difference between std::make_unique() and std::unique_ptr() - See Differences between std::make_unique and std::unique_ptr.}