Struct inheritance causing non-aggregate type error on static initialization

Question

I currently have the following:

struct MsgHeader_t
{
    int   a;
    float b;
};

which initializes a default const as

const MsgHeader_t default = { 0, 0.0f };

which was working fine. Now I have to extend it and it MUST inherit from another struct. So now I have.

struct MsgId_t
{
    unsigned int id;
};

struct MsgHeader_t : public MsgId_t
{
    int   a;
    float b;
};

But now when I try to initialize that default const (const MsgHeader_t aMessage = { 0, 0, 0.0f };) I get

error: braces around initializer for non-aggregate type 'const MsgHeader_t'

`MsgHeader_t` is POD but no longer an aggregate type https://stackoverflow.com/questions/23808357/brace-initialization-for-inherited-pod — Cory Kramer, Apr 14 '20 at 15:45

score 1 · Accepted Answer · answered Apr 14 '20 at 15:46

1

Once you have inheritance, then your class can no longer be an aggregate type. From this Draft C++ Standard (bold italics mine):

8.5.1 Aggregates

¹ An aggregate is an array or a class (Clause 9) with no user-provided constructors (12.1), no private or protected non-static data members (Clause 11), no base classes (Clause 10), and no virtual functions (10.3).

answered Apr 14 '20 at 15:46

Adrian Mole

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I made be a little dense in my reading but does that mean I should be able to provide a ctor for it and all should be good? Also, what is a POD? – jiveturkey Apr 14 '20 at 15:50
POD = "Plain Old Data." Yes, a constructor would work, and this could use 'brace' initialization for the base class. Also, it's not for me to assess your [specific gravity](https://www.google.co.uk/search?source=hp&ei=YNyVXuPQEZGxU8j4lagI&q=specific+gravity&oq=specific+gravity&gs_lcp=CgZwc3ktYWIQAxgAMgIIADICCAAyAggAMgIIADICCAAyAggAMgIIADICCAAyAggAMgIIADoFCAAQgwFKJwgXEiMwZzgzZzk1ZzY2ZzgwZzExM2c2NWc1NWc2NGc3MGc3Nmc3MUobCBgSFzBnMWcxZzFnMWcxZzFnMWcxZzFnNWcyUPsFWO4dYIAlaABwAHgAgAFliAHaCJIBBDE1LjGYAQCgAQGqAQdnd3Mtd2l6&sclient=psy-ab). – Adrian Mole Apr 14 '20 at 15:55
Very funny. Thanks for making me look things up :) – jiveturkey Apr 14 '20 at 15:58

score 0 · Answer 2 · answered Apr 14 '20 at 16:31

0

Note that since C++17, you might do, as you expect:

const MsgHeader_t aMessage = { 0, 0, 0.0f };. Demo
or more properly const MsgHeader_t aMessage = { {0}, 0, 0.0f };. Demo

answered Apr 14 '20 at 16:31

Jarod42

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I wish. Unfortunately I'm in C++89. – jiveturkey Apr 14 '20 at 16:45
2

That the kind of info which should be in question, IMO. – Jarod42 Apr 14 '20 at 16:50

Struct inheritance causing non-aggregate type error on static initialization

2 Answers2