Is struct member pointers automatically initialized to zero?

Question

If I have the following struct:

struct MyStruct {
    int *a;
    int *b;
};

And initializes it like this:

int some_var;
MyStruct s{
    .a = &some_var
};

can I be sure s.b will be initialized to nullptr?

EDIT:

Full compiling code, tested with g++ 7.3.0:

// test.cpp
struct MyStruct {
    int *a;
    int *b;
};

int main()
{
    int some_var;
    MyStruct s{
        .a = &some_var
    };
}

If I print the variable values at this sample, b is indeed 0. But I want to know if this behavior is guaranteed by the standard.

Answer 1

Note, this is C99 syntax, which is not supported by C++11 standard, but supported by GCC. GCC documentation states so that that omitted field members are implicitly initialized the same as objects that have static storage duration.

Alternate syntax would be

MyStruct s{
        a: &some_var
    };

If the same field is initialized multiple times, it would be initialized with the value from the last initialization. If such initialization causes side-effect, it is unspecified whether the side effect happens or not. GCC discards them, only last initialization happens.

Here is C++11 compatible initialization:

MyStruct s { &some_var };  // second field would be initialized as one with static duration

GCC would issue a warning about missing initializers in both cases.