Distributed Real- Time Control Systems 13/14

Size: px

Start display at page:

Download "Distributed Real- Time Control Systems 13/14"

Marjorie Malone
5 years ago
Views:

1 Distributed Real- Time Control Systems 13/14 Lecture 10 C++ Programming Operators and more A. Bernardino, C. Silvestre, IST- ACSDC 1

2 References From C programming we know that int i =3; int *p; p = &i; Symbol Type Mem Addr i int Value p int * p i=3 means that p points to the memory space representng i and *p is the value of i. A. Bernardino, C. Silvestre, IST- ACSDC 2

3 References to Objects The reference to an object is a C++ feature. (Not available in C). int i =3; int *p = &i; int &j = i; Symbol Type Mem Addr Value i, j int p int * q Int * We say that j is a reference to i. This means to give variable i a second name: j (alias). The reference mechanism is important as an efficient method to pass object as functon arguments and return values. A. Bernardino, C. Silvestre, IST- ACSDC 3

4 Reference vs Value Arguments #include <stdio.h> void symm(int & j) { //REF ARG j = - 1 * j; int main() { int i = 3; symm( i ); prinf( result: %d\n, i ); #include <stdio.h> void symm(int j) { //VALUE ARG j = - 1 * j; int main() { int i = 3; symm( i ); prinf( result: %d\n, i ); result: - 3 result: 3 A. Bernardino, C. Silvestre, IST- ACSDC 4

5 Efficiency Issues Passage by reference is much more efficient when dealing with large objects. Passage by value demands a copy of the argument (clone) to the called functon. obj obj clone main func main func A. Bernardino, C. Silvestre, IST- ACSDC 5

6 The const Qualifier The const keyword should be used to prevent inadvertedly changing variables and objects in functons that are not supposed to do so. A. Bernardino, C. Silvestre, IST- ACSDC 6

7 Simple uses of const //Two constant integers int const c1 = 10; const int c2 = 11; Advantages over the usual #define method. The compiler will issue an explicit error if the user tries to change the values of c1 or c2. Use c++ syntax and not pre- processor syntax. A. Bernardino, C. Silvestre, IST- ACSDC 7

8 const in Pointers int const *c3; const int *c4; int * const c5; int const * const c6; //A variable pointer to a constant integer. //A constant pointer to a variable integer. //A constant pointer to a constant integer. The const keyword applies to the element immediately to the leh. If there is nothing to the leh, it applies to the element immediately to the right, but this can not be a pointer sign (*). A. Bernardino, C. Silvestre, IST- ACSDC 8

9 const in FuncTons SomeTmes we want to return objects that should not be changed by the caller. Solid const * get_heaviest( Solid * solidarray) SomeTmes we want to prevent member functons from changing the state of the object. class Class2 { void Method1() const; int MemberVariable1; A. Bernardino, C. Silvestre, IST- ACSDC 9

10 Things can get nasty L const int*const mymethod(const int*const&)const Will return a pointer which is constant and points to a constant integer. The method doesn't alter either the variable pointed to by the parameter or the pointer itself and it doesn't alter any of the member variables of the object the method is called from!!! A. Bernardino, C. Silvestre, IST- ACSDC 10

11 const: A client- server contract Using the keyword const is important for optmizing client- server code. For instance, in our Solid s problem, functon CalcMass() only returns values: it does not modify object propertes. We should let the compiler know about this because it may lead to optmizatons. This is done using: virtual float CalcMass() const = 0;! Defining the functon as const will avoid the modificaton of the object and facilitates the maintenance of object propertes in a distributed system. This is called a client- server contract Note: float CalcMass() const and float CalcMass() are two different functons! A. Bernardino, C. Silvestre, IST- ACSDC 11

12 The vola/le qualifier The vola/le keywork is the opposite of const. It tells the compiler that it should not optmize access to a certain variable, and always read it from memory when instructed. It is mostly important when event external to the code can change the variable value, e.g. hardware, memory mapped I/O. bool flag = false; while(!flag) { //flag not used here // The compiler may optmize this to : while(true) { //infinite cycle A. Bernardino, C. Silvestre, IST- ACSDC 12

Inline functons FuncTon calls have overhead: Copy arguments to the stack Save the return address Save return values For small functons, the overhead can be significant.

13 Inline functons FuncTon calls have overhead: Copy arguments to the stack Save the return address Save return values For small functons, the overhead can be significant. inline suggests the compiler to copy the functon code to where it is called, instead of doing a functon call. inline int func(a,b) { return (a+1)*b; int main() { int x = 3, y = 4, w, z; z = func(x,y); w = func(x,z); int main() { int x = 3, y = 4, w, z; z = (x+1)*y; w = (x+1)*z; A. Bernardino, C. Silvestre, IST- ACSDC 13

14 Inline functons Like macros but behave like functons: Type checking Clear rules for argument passing and return. Check the problem with the following code? How to solve it? #define MYMACRO(x) ((x+1)*(x+2)) int main() { int a = 2; MYMACRO(++a); CAVEATS Because the functon code is replicated everywhere it is called, this may result in large increases in program size. Inlines should be reserved for small functons. The compiler may decide not to inline a functon, if it is too complex. A. Bernardino, C. Silvestre, IST- ACSDC 14

15 AutomaTc inlines Class member functons are automatc inlines if implemented at declaraton Tme (in- situ). class A { public: int id; //automatic inlines A(int i) { id = i ; int incr() { return id++; Class member functon inlines, if not defined in- situ, must be defined in the same file as the class declaraton. class A { public: int id; A(int); int incr(); //not automatic inlines inline A::A(int i) { id = i; inline int A::incr() { return id++; A. Bernardino, C. Silvestre, IST- ACSDC 15

16 Operator Overloading Operators are like functons. They can also be overloaded. We can redefine what the following operators do when applied to objects of our classes: + - * / = < > += - = *= /= << >> <<= >>= ==!= <= >= % & ^! ~ &= ^= = && %= [] () - >* - > new delete new[] delete[], A. Bernardino, C. Silvestre, IST- ACSDC 16

17 Operators are functons When we write It is the same as writng: MyClass a, b;... a+b; MyClass a, b;... a.operator+(b); operator+() is a functon. A. Bernardino, C. Silvestre, IST- ACSDC 17

18 Example Let us redefine the + operator in the class Solid to represent the additon of the mass of two Solids. class Solid { public: float operator+(solid &s); ; float Solid::operator+ (Solid &s) { return CalcMass() + s.calcmass(); A. Bernardino, C. Silvestre, IST- ACSDC 18

19 The this pointer The keyword this represents a pointer to the object whose member functon is being executed - it is a pointer to the object itself. One of its uses can be to check if a parameter passed to a member functon is the object itself class CDummy { public: int isitme (CDummy& param); ; int CDummy::isitme (CDummy& param) { if (&param == this) return true; else return false; int main () { CDummy a; CDummy *b = &a; if ( b- >isitme(a) ) prinf("yes, &a is b ); return 0; A. Bernardino, C. Silvestre, IST- ACSDC 19

20 Assignment Operator When we write It is the same as writng: MyClass a, b;... b = a; MyClass a, b;... b.operator=(a); operator=() is called the assignment operator. A. Bernardino, C. Silvestre, IST- ACSDC 20

21 Assignment Operator Let us define an assignment operator to our Cube class. class Cube : public Solid { public: // Assignment operator Cube& operator= (const Cube & ); ; Cube& Cube::operator= (const Cube& c) { if ( this == &c) return (*this) ; _s = c._s; return (*this); The assignment operator returns a reference to the current object. Consider the assignment c2=c1:! The purpose is to assign the propertes of c1 to c2. We are invoking a member functon of c2 with argument c1. Variable side refers to c2 and c.side refers to c1. The invocaton of base the class (Solid) assignment operator is not automatc. A. Bernardino, C. Silvestre, IST- ACSDC 21

22 Cascading Operators Why returning pointer this? A er assigning the width, our objectve is complete. However we should also support operatons like: c3 = c2 = c1 ó c3 = (c2 = c1) ó c3.operator=( c2.operator=(c1) ) This means: assign c1 to c2, then assign c2 to c3. What happens in this case: The assignment operator of c2 is called with a parameter of type reference to c1. The member variable side of c1 is assigned to c2 and a reference to c2 is returned. The assignment operator of c3 is called with the reference to c2. Then the member variable side of c2 is assigned to c3. A. Bernardino, C. Silvestre, IST- ACSDC 22

23 The copy constructor Code 1 Code 2 Cube c1(1.0); Cube c2(2.0); c1 = c2; Cube c1(1.0) Cube c2=c1; What are the differences? The result is quite similar but what happens is substantally different. In the le, the compiler just invokes the method operator= in object c2. In the right, a new object is constructed, being initalized with c1. This requires a special constructor: the Copy Constructor. A. Bernardino, C. Silvestre, IST- ACSDC 23

24 The copy constructor for Cubes class Cube : public Solid { public: // declaraton Cube (const Cube & c);... ; // implementaton Cube::Cube (const Cube& c){ _s = c._s; The copy constructor does not return a value. The argument is const because it is not allowed to change it. On the contrary to other operators, the compiler always create a default copy constructor. However, it is safer to define explicitly the copy constructor. A. Bernardino, C. Silvestre, IST- ACSDC 24

25 AutomaTc InvokaTon of the Copy Constructor The copy constructor is automatcally invoked in: object initalizaton. passing and returning value arguments in functons. class MyClass {... ; MyClass func( MyClass a ) { //object a is created in the stack and //initalized to obj1 from main() Myclass b = a; // object b is created in the stack // and initalized to a. return b; int main() { MyClass obj1; MyClass obj2 = func(obj1); // object obj2 is created // and initalized to the return value A. Bernardino, C. Silvestre, IST- ACSDC 25

26 PrevenTng Object Cloning SomeTmes objects should not be cloned. E.g. when holding unique resources in the system. To prevent object cloning, the copy constructor and the assignment operator should be made private. class A { private: A (const A &); A& operator= (const A & ); public: A(); ~A();... ; // TO EXEMPLIFY PRIVATE COPY CONSTRUCTOR AND ASSIGNMENT OPERATOR. A. Bernardino, C. Silvestre, IST- ACSDC 26

27 Friend Classes and FuncTons The friend qualifier allows access to protected or private fields to other classes or functons. Only functons or classes declared friend can use private class a ributes We have declared a class View3D inside our Cube and Cylinder classes. Thus any method of View3D can access the protected or private members of Cube and Cylinder objects We also declared a functon SameVol as friend, so it can access the protected or private members. Note that this functon is not a member of any class. class Cube { friend class View3D; friend bool SameVol(Cube &, Cylinder&); ; class View3D { Show(Cube&); //3D display a cube Show(Cylinder&); //3D dsplay a cylinder bool SameVol(Cube& a, Cylinder& b) { return a._s*a_s*a._s == PI*b._r*b._r*b._h; A. Bernardino, C. Silvestre, IST- ACSDC 27

Distributed Real-Time Control Systems. Lecture 14 Intro to C++ Part III

Distributed Real-Time Control Systems Lecture 14 Intro to C++ Part III 1 Class Hierarchies The human brain is very efficient in finding common properties to different entities and classify them according