Member initializer list, pointer initialization without argument - c++

Question

In a large framework which used to use many smart pointers and now uses raw pointers, I come across situations like this quite often:
class A {
public:
int* m;
A() : m() {}
};
The reason is because int* m used to be a smart pointer and so the initializer list called a default constructor. Now that int* m is a raw pointer I am not certain if this is equivalent to:
class A {
public:
int* m;
A() : m(nullptr) {}
};
Without the explicit nullptr is A::m still initialized to zero? A look at no optimization objdump -d makes it appear to be yes but I am not certain. The reason I feel that the answer is yes is due to this line in the objdump -d (I posted more of the objdump -d below):
400644: 48 c7 00 00 00 00 00 movq $0x0,(%rax)
Little program that tries to find undefined behavior:
class A {
public:
int* m;
A() : m(nullptr) {}
};
int main() {
A buf[1000000];
unsigned int count = 0;
for (unsigned int i = 0; i < 1000000; ++i) {
count += buf[i].m ? 1 : 0;
}
return count;
}
Compilation, execution, and return value:
g++ -std=c++14 -O0 foo.cpp
./a.out; echo $?
0
Relevant assembly sections from objdump -d:
00000000004005b8 <main>:
4005b8: 55 push %rbp
4005b9: 48 89 e5 mov %rsp,%rbp
4005bc: 41 54 push %r12
4005be: 53 push %rbx
4005bf: 48 81 ec 10 12 7a 00 sub $0x7a1210,%rsp
4005c6: 48 8d 85 e0 ed 85 ff lea -0x7a1220(%rbp),%rax
4005cd: bb 3f 42 0f 00 mov $0xf423f,%ebx
4005d2: 49 89 c4 mov %rax,%r12
4005d5: eb 10 jmp 4005e7 <main+0x2f>
4005d7: 4c 89 e7 mov %r12,%rdi
4005da: e8 59 00 00 00 callq 400638 <_ZN1AC1Ev>
4005df: 49 83 c4 08 add $0x8,%r12
4005e3: 48 83 eb 01 sub $0x1,%rbx
4005e7: 48 83 fb ff cmp $0xffffffffffffffff,%rbx
4005eb: 75 ea jne 4005d7 <main+0x1f>
4005ed: c7 45 ec 00 00 00 00 movl $0x0,-0x14(%rbp)
4005f4: c7 45 e8 00 00 00 00 movl $0x0,-0x18(%rbp)
4005fb: eb 23 jmp 400620 <main+0x68>
4005fd: 8b 45 e8 mov -0x18(%rbp),%eax
400600: 48 8b 84 c5 e0 ed 85 mov -0x7a1220(%rbp,%rax,8),%rax
400607: ff
400608: 48 85 c0 test %rax,%rax
40060b: 74 07 je 400614 <main+0x5c>
40060d: b8 01 00 00 00 mov $0x1,%eax
400612: eb 05 jmp 400619 <main+0x61>
400614: b8 00 00 00 00 mov $0x0,%eax
400619: 01 45 ec add %eax,-0x14(%rbp)
40061c: 83 45 e8 01 addl $0x1,-0x18(%rbp)
400620: 81 7d e8 3f 42 0f 00 cmpl $0xf423f,-0x18(%rbp)
400627: 76 d4 jbe 4005fd <main+0x45>
400629: 8b 45 ec mov -0x14(%rbp),%eax
40062c: 48 81 c4 10 12 7a 00 add $0x7a1210,%rsp
400633: 5b pop %rbx
400634: 41 5c pop %r12
400636: 5d pop %rbp
400637: c3 retq
0000000000400638 <_ZN1AC1Ev>:
400638: 55 push %rbp
400639: 48 89 e5 mov %rsp,%rbp
40063c: 48 89 7d f8 mov %rdi,-0x8(%rbp)
400640: 48 8b 45 f8 mov -0x8(%rbp),%rax
400644: 48 c7 00 00 00 00 00 movq $0x0,(%rax)
40064b: 5d pop %rbp
40064c: c3 retq
40064d: 0f 1f 00 nopl (%rax)

Answer 1

Empty () initializer stands for default-initialization in C++98 and for value-initialization in C++03 and later. For scalar types (including pointers) value-initialization/default-initialization leads to zero-initialization.
Which means that in your case m() and m(nullptr) will have exactly the same effect: in both cases m is initialized as a null pointer. In C++ it was like that since the beginning of standardized times.