#include guards Practical session #2 Software Engineering

Modular programming

Header file = interface  Declarations of functions and classes.  Definitions of classes, constants and types.  Inline functions. CPP file = implementation  Definitions of functions (class members or regular functions ).  Initialization of static class members.  In order to use the identifiers from header file – it must be included.

#include  Pre-compiler command that inserts the included file in place of #include.  #include - standard library  #include “filename.h” - your own header file

#include - rule 1 typedef std::vector int_vector; //Needs #include but has no //such include File1.h #include ….. File2.h #include “File2.h” #include “File1.h” Usage1.cpp OK #include “File1.h” #include “File2.h” Usage2.cpp Compilation error! Every header file must have all #includes it requires !!! Otherwise the order of includes will matter and the files are not independent!

#include - rule 1 typedef my_int* int_vector; //Needs my_int but has no //such include File1.h typedef int my_int; ….. File2.h #include “File2.h” #include “File1.h” Usage1.cpp OK Compilation error! #include “File1.h” #include “File2.h” Usage2.cpp Every header file must have all #includes it requires !!! Otherwise the order of includes will matter and the files are not independent!

#include - a problem #include “File1.h” #include “File3.h” //Needs code C and B Usage1.cpp Compilation error! Redefinition of class A “File2.h” is included twice ! => class A is defined twice ! class A { }; //More code File2.h #include “File2.h” //Code C File1.h #include “File2.h” //Code B File3.h

#include - a solution #include “File1.h” #include “File3.h” //Needs code C and B Usage1.cpp OK #ifndef FILE2_H #define FILE2_H class A { }; //More code #endif File2.h #include “File2.h” //Code C File1.h #include “File2.h” //Code B File3.h Include guards Every header file should have include guards ! To avoid double or cyclic inclusion! These files should have include guards too !

C++ variables and types

Agenda  Variables and types  Reference 10

Variable  Variable is a place (a piece of memory) for data storage, in C++: variable=object.  Variable has a name = identifier.  Variable has a type, that determines:  The size in memory.  The values that can be stored in it.  The operations that can be applied to it.  Variables should be initialized (good practice ). Initialization syntax:  type identifier (initial_value) ; //example: int k(5);  type identifier = initial_value; //example: int k = 5; 11 The same Example: double num ( 0.0 );

Types in C Primitive types.  Examples:  bool, int, double, char… 2. Compound (or composite ) types:  Any type that is not primitive.  Any type that made up of primitive types or other compound types.  Examples:  Arrays.,  Pointers, references,  Classes,  And there are more… C++ is a statically typed language each variable has to have a type before usage and this type cannot be changed!

Primitive built-in types Minimum sizeMeaningType NABooleanbool 8 bitsCharacterchar 16 bitsWide characterwchar_t 16 bitsShort integershort 16 bitsIntegerint 32 bitsLong integerlong 32 bits (6 p)Single-precision floating pointfloat 64 bits (10 p)Double-precision floating pointdouble 128 bits (10 p)Extended-precision floating pointlong double 13 We can find out the actual size of a variable by using sizeof operator

Scope and local variable  The scope (or simply the “{ }” braces) of an identifier, is the part of the program over which the identifier can be seen and used.  Local variable is a variable which defined inside a function. Local variable can be accessed only from the scope where it was defined and cleared automatically at the end of the scope.  Example (with compiler errors) : 1. void average() 2. { { 3. for ( int i =0 ; i < 5 ; i++ ) { 4. int x, sum = 0 ; 5. std::cout << “Enter "<< i+1 <<"'th integer:“ << std::endl; 6. std::cin >> x ; 7. sum += x ; } 8. } 9. std::cout<<" The average: “<< sum / i <<std::endl; 10. } What is the error and how can we fix it? 14

 Global variable defined in the main scope – outside the scope of all functions/classes.  Examples : 1. #include 2. //Global scope 3. int x = 20; 4. void incX(){//Local scope 5. // int x = 10 ; 6. x ++; 7. } 8. int main(){ 9. std::cout<<" Global x = "<< x <<std::endl; 10. incX(); 11. std::cout<<" Global x + 1 = "<< x <<std::endl; 12. system("pause"); 13. return 0 ; 14. } Global variable What will be the result of adding this code line (5)? 15

Constant ( const) variable Constant ( const ) variable must be initialized at its definition point and cannot be changed after its definition. Example : 1. void main(){ 2. const int cInt = 5 ; 3. const double cDoub ; //ERROR 4. cInt ++ ; //ERROR 5. cDoub +=.3 ; // ERROR 6. std::cout << cInt << std::endl ; 7. system("pause"); 8. } 16

Agenda  Variables and types  Reference 17

Reference variable  Reference type can be thought as alternative name for a variable.  Reference must be initialized at definition time.  Example : 1. long index = 10 ; 2. long &refIndex = index ; 3. std::cout<< ++refIndex << endl; 4. std::cout<< index << endl; 5. long &refIndex2 ; //ERROR 6. long &refIndex3 = 10 ; //ERROR  Reference can’t be rebind after definition. refIndex is a reference to index forever. Whenever we use refIndex (after we created it) we access the object referenced. 18

const reference Reference can be const: you cannot change the object via the const reference. long longNum ; const long& longRef = longNum; //correct longRef = 5; //ERROR  The opposite is not correct: const object can have only const references or pointers to it const long longNum (5); long& longRef = longNum; // ERROR. //Because we could change const object //via non const reference! 19