Initialize unnamed struct pointer/array

Question

struct A
{
    int member1;
    int member2;
    struct B
    {
        int member3;
        int member4;
    } *b;
};

How can I initialize A and at the same time create an array of B's to fill the b field? I want a static variable, so preferably function calling.

I tried this but it does not work (I didn't think it would):

static A a = {
    1,
    2,
    & { {3, 4}, {5, 6} },
}

Do you want this in C++ or C? The [c++] and [c] tags are not synonymous; the answers will vary depending on the language. — In silico
– In silico, Commented Jan 14, 2011 at 1:00
And I agree with @Insilico. Pick a language, both in the question tags and in the language you use. They are different languages. — Lightness Races in Orbit
– Lightness Races in Orbit, Commented Jan 14, 2011 at 1:04
I'm using a C++ compiler, but now I'm curious what the difference is between C and C++ when it comes to this. — Oliver Zheng
– Oliver Zheng, Commented Jan 14, 2011 at 1:21
"The b field" is not an array and cannot be "filled". Pointers and arrays are not in any sense the same. — Karl Knechtel
– Karl Knechtel, Commented Jan 14, 2011 at 2:32

templatetypedef · Accepted Answer · 2011-01-14 01:20:32Z

3

I think the problem with this code is this line:

& { {3, 4}, {5, 6} }

The problem is that {{3, 4}, {5, 6}} is an rvalue (a value, not an object) and you can't take the address of an rvalue. After all, where is the memory for this object going to come from? However, you might be able to get away with rewriting this to

A::B elems[2] = {{3, 4}, {5, 6}};
static A a = {
    1,
    2,
    elems
};

Since now you have an lvalue you can point at.

edited Jan 14, 2011 at 1:20

answered Jan 14, 2011 at 1:04

templatetypedef

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1 Comment

Oliver Zheng Over a year ago

That's what I thought could work. I expected that to work because this does: const char *ptr = "blah".

caf · Accepted Answer · 2011-01-14 05:24:35Z

1

You have tagged this with both C and C++; the following answer is for C.

In C89, you can do:

static struct B a_b_init[] = { {3, 4}, {5, 6} };
static struct A a = {
    1,
    2,
    a_b_init
};

In C99, you can use a compound literal, which has very similar syntax to your attempt:

static struct A a = {
    1,
    2,
    (struct B []){ {3, 4}, {5, 6} }
};

answered Jan 14, 2011 at 5:24

caf

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3 Comments

Oliver Zheng Over a year ago

That is exactly what I was looking for. Thank you! And what would be the equivalent in C++? I still want a static-scoped struct and not an STL container.

Oliver Zheng Over a year ago

I noticed that this syntax compiles with GCC for C, but not for C++. The error is braces around scalar initializer for 'B*'.

caf Over a year ago

@MTsoul: The first option should work for C++ too, and is equivalent in effect to the second (it's just that the static struct array gets a name, rather than being anonymous).

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