What does : public after struct mean?

Question

    struct hpet : public description_table_header
{
    uint8_t hardware_rev_id;
    uint8_t comparator_count:5;
    uint8_t counter_size:1;
    uint8_t reserved:1;
    uint8_t legacy_replacement:1;
    pci_vendor_t pci_vendor_id;
    address_structure address;
    uint8_t hpet_number;
    uint16_t minimum_tick;
    uint8_t page_protection;
} __attribute__((packed));

Why is there a public after the struct name and also __attribute__((packed)), why is packed inside paranthesis???
This is a table for a HPET(High Precision Event Timer).

This is sturcture inheritance in C++. `packed` for structure alignment. — sujin, Dec 29 '13 at 14:03

score 2 · Answer 1 · edited May 23 '17 at 11:49

2

In C++, a colon : followed by optional public/private/protected designator and a type name is the syntax for specifying inheritance.

In your code, the hpet class inherits the description_table_header class.

See this answer for a discussion of the differences between public, private, and protected inheritance.

edited May 23 '17 at 11:49

Community

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answered Dec 29 '13 at 14:04

Sergey Kalinichenko

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this is C++ (which is an excellent language for low level code) You struct is publically inheriting (which struct can do - the same way classes do) – RichardPlunkett Dec 29 '13 at 14:09

score 2 · Accepted Answer · edited May 23 '17 at 12:11

The code is C++ and not C as you tagged it. That's quite an important detail.

The public here is the access specifier for the inheritance (the struct inherits from description_table_header). This will be covered in all good C++ textbooks. A relevant question here on SO is: Difference between private, public, and protected inheritance. The specification of public for a struct is not actually needed since, for a struct, that is the default.

The __attribute__((packed)) is a compiler specific extension that specifies the layout of the struct. In this case the struct is packed so that there is no padding in the struct.

What does : public after struct mean?

2 Answers2