c++ - 具有零计数和无效指针的 std::memset

Question

当count 等于 0？

Answer 1

不，那会导致未定义的行为。例如:

void* p = get_address(); // may return null
size_t sz = get_size();  // zero if previous returned null

memset(p, 0, sz); // Compiler may assume that p is not null

if (p) { // this null-check can be omitted since we "know" p is not null
  foo(p);
}

事实上，如果您查看 code generated by GCC :

main:
        push    rbx
        call    get_address()
        mov     rbx, rax
        call    get_size()
        mov     rdi, rbx
        xor     esi, esi
        mov     rdx, rax
        call    memset
        mov     rdi, rbx
        call    foo(void*) ; <-- unconditional call
        xor     eax, eax
        pop     rbx
        ret

你可以看到“if”分支被省略了。