3. Using the C++ Compiler Options
4.1.1 Compatibility with Microsoft Windows
4.3 Overriding With Less Restrictive Virtual Functions
4.4 Making Forward Declarations of enum Types and Variables
4.5 Using Incomplete enum Types
4.6 Using an enum Name as a Scope Qualifier
4.7 Using Anonymous struct Declarations
4.8 Passing the Address of an Anonymous Class Instance
4.9 Declaring a Static Namespace-Scope Function as a Class Friend
4.10 Using the Predefined __func__ Symbol for Function Name
6. Creating and Using Templates
9. Improving Program Performance
10. Building Multithreaded Programs
12. Using The C++ Standard Library
13. Using the Classic iostream Library
The following attributes, invoked by __attribute__ ((keyword)), or alternatively by [[keyword]], are implemented by the compiler for compatibility:
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This attribute, attached to struct or union type definition, specifies that each member (other than zero-width bitfields) of the structure or union is placed to minimize the memory required. When attached to an enum definition, it indicates that the smallest integral type should be used.
Specifying this attribute for struct and union types is equivalent to specifying the packed attribute on each of the structure or union members.
In the following example struct my_packed_struct's members are packed closely together, but the internal layout of its s member is not packed. To do that, struct my_unpacked_struct would also need to be packed.
struct my_unpacked_struct { char c; int i; ; struct __attribute__ ((__packed__)) my_packed_struct { char c; int i; struct my_unpacked_struct s; };
You may only specify this attribute on the definition of an enum, struct, or union, and not on a typedef that does not also define the enumerated type, structure, or union.