Analog Devices ee-112 Application Notes
a Engineer To Engineer Note EE-112
Technical Notes on using Analog Devices’ DSP components and development tools
Phone: (800) ANALOG-D, FAX: (781) 461-3010, EMAIL: dsp.support@analog.com, FTP: ftp.analog.com, WEB: www.analog.com/dsp
Class Implementation in Analog C++
Contributed by Graham Andrews,
C/C++ Compiler Developer,
DSP Development Tools Product Line
Overview
A "class" is a type in the C++ programming
language. The most simple form of this type is
likened directly to a struct type in C. In fact, the
keyword "class" can be replaced with the
class A{
public: // public access
int x;
double y;
};
All the examples described in this note show
equivalent layouts in C++ and C terms. The
examples demonstrate how an object
representation in C++ translates to C, so there is
a direct correspondence to the generated code.
keyword "struct" with no effect on the layout of
the internal representation of the object. The
difference is that C++ can place restrictions on
the accessibility of its class members. A struct
that is valid in C is valid in C++, and the same
rules of layout and parameter passing exist in
both C and C++. Member functions declared
within a class have no effect on the layout of a
given class object unless the member function is
declared virtual. When inheritance, virtual base
classes, and virtual functions are present, the
layout of C++ classes is more complicated.
A C struct is a valid C++ construct with the
same object layout:
struct A{
int x;
double y;
};
If the keyword "struct" is replaced with "class"
in C++, then the members x and y are implicitly
defined as private . Access specifiers define the
Inheritance
A class can be derived from one or more
classes. The classes from which it is derived are
called base classes. An object of a derived class
inherits all the members of the base classes. A
derived class can have base classes, which
themselves are derived, and each class may have
more than one base class. The layout of class
objects follows a simple pattern. This pattern is
illustrated by the following examples, which
show the underlying class layout in terms of C.
Example 1. Base & Derived Classes
class A{
int a1;
double a2;
};
class B:A{
int b1;
int b2;
};
C layout equivalents:
rules for using these members with the '->' and
'.' operators. The keyword "struct" makes these
members implicitly public in C++.
In C++, the previous example could be equally
written as:
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struct A{
int a1;
double a2;
};
struct B{
struct A base_A;
int b1;
int b2;
};
Example 2. Inheritance Using Classes A & B
from Example 1
class C{
char c1[10];
};
class D : B{
float d1;
short d2;
};
class E: C ,D {
unsigned int e1:2;
unsigned int e2:6;
unsigned e3:17;
};
C layout equivalents:
struct A {
int a1;
double a2;
};
struct B {
struct A __b_A;
int b1;
int b2;
};
struct C {
char c1[10];
};
class object layout to point to the instance of the
base.
In Example 2, class B has class A as a base class,
and class C has class A as a base class. If class D
has both B and C as base classes, then there
would be two class A parts of class D. However,
if class A is declared to be a virtual base of class
B and of class C, then only one A sub-object is
present in a D object.
Example 3. Virtual Base Classes
class Z{
int z1;
int z2;
};
class A: Z{
int a1;
};
class B : virtual A{
int b1;
};
class C : virtual A{
int c1;
};
class D: B, C{
int d1;
};
struct D {
struct B __b_B;
float d1;
short d2;
};
struct E {
struct C base_C;
struct D base_D;
unsigned int e1: 2;
unsigned int e2: 6;
unsigned int e3: 17;
};
Virtual Base Classes
Every class in a hierarchy that specifies a base
class to be virtual, shares a single object of that
base class within a derived object. In terms of
the layout, the compiler adds pointers to the
EN-112 Page 2
Technical Notes on using Analog Devices’ DSP components and development tools
Phone: (800) ANALOG-D, FAX: (781) 461-3010, EMAIL: dsp.support@analog.com, FTP: ftp.analog.com,
WEB: www.analog.com/dsp
C layout equivalents:
struct Z {
int z1;
int z2;
};
struct A {
struct Z __b_Z;
int a1;
};
struct B {
int b1;
struct A *__p_A;
/* pointer to A part of a B
object */
struct A __v_A;
/* instance of A part in B
object */
};
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