Separate Compilation

A key concept in programming  Two kinds of languages, compilation (C, Pascal, …) and interpretation (Lisp, …, Matlab, Phython, Javascript…)  It was a ‘milestone’ for procedural programming, some old ones like Pascal cannot, …  It is in fact a kind of ‘abstract data type’, to separate the function “implementations” from its “definitions”  It is also impossible for large projet without it

Translating a Library 3 Program Source File Library Header File Library Implementation File C++ Compiler Linker Program Object File Library Object File Program Executable File g++ -c.o e.g., g++ foo.cpp bar.o fb.o

Motivation #include using namespace std; bool even(int); bool odd(int x) { bool res; if(x == 0) res=false; else even(x-1); return res; } bool even(int x) { bool res; if (x == 0) res=true; else odd(x-1); return res; } int main() { int x; cin >> x; cout >> odd(x) >> endl; cout >> even(x) >> endl; return 0; }

#include using namespace std; bool even(int); bool odd(int); int main() { int x; cin >> x; cout >> odd(x) >> endl; cout >> even(x) >> endl; return 0; } bool odd(int x) { bool res; if(x == 0) res=false; else even(x-1); return res; } bool even(int x) { bool res; if (x == 0) res=true; else odd(x-1); return res; } It’s about ‘Organization’, organization of files!

One file  many files … 3 functions: odd() even() main() “ main.cpp ” odd() “ odd.cpp ” even() “ even.cpp ” main() “ main.cpp ” Reasons: 1. Reuse code for other programs 2. Divide work among programmers 3. Better maintenance, localize changes

#include using namespace std; bool odd(int); bool even(int); int main() { int x; while (cin >> x) { num_calls = 0; cout << boolalpha << odd(x) << endl; } return 0; } main() “ main.cpp ” #include using namespace std; bool odd(int); bool even(int x) { … } #include using namespace std; bool even(int); bool odd(int x) { … } even() “ even.cpp ” odd() “ odd.cpp ”

 g++ -c main.cpp  main.o  g++ -c even.cpp  even.o  g++ -c odd.cpp  odd.o  g++ -o odd-even main.o even.o odd.o  odd-even  -c generates an object file  -o generates an executable by linking all object files  g++ file.cpp  a.out

Separate compilation by ‘makefile’ If only “ main.cpp ” is modified, just need to re-compile “ main.cpp ” and re-link by: g++ -c main.cpp g++ -o odd-even main.o even.o odd.o No need to re-compile the object files, i.e. even.o and odd.o ‘makefile’ will automate all these things!!! ‘makefile’ is also a program in ‘script’ language that organizes your ‘command files’!

Using header files  We do not want to repeat writing the same declarations in multiple files.  Should a declaration require updating, one has to go through all files that have the declaration and make the change.  More importantly, maintaining duplicate information in multiple files is error-prone. Many declarations are repeated in “odd.cpp” and “even.cpp”, but

#include preprocessor directive  include header files  Instructs C++ preprocessor to replace directive with a copy of the contents of the specified file  Quotes for user-defined header files  Preprocessor first looks in current directory  If the file is not found, looks in C++ Standard Library directory  Angle brackets for C++ Standard Library  Preprocessor looks only in C++ Standard Library directory  #include 11

#include using namespace std; bool even(int); bool odd(int x) #include “myInclude.h” bool odd(int x) { bool res; if(x == 0) res=false; else even(x-1); return res; } #include “myInclude.h” int main() { int x; cin >> x; cout >> odd(x) >> endl; cout >> even(x) >> endl; return 0; } #include “myInclude.h” bool even(int x) { bool res; if (x == 0) res=true; else odd(x-1); return res; } myInclude.hmain.cppodd.cppeven.cpp Example of headfile

Avoiding recursive inclusions …  It is ok to have multiple declarations of a function prototype, but not for its definition  In the.h file, put the prototypes there .h files are likely to be multiply-included  In creating the.o file, there may be nested #include statement  The nested #include statement may be recursive  In main.cpp, #include “foo.h”  In foo.h, #include “bar.h”  In bar.h, #include “foo.h”  To break the infinite “recursive” inclusion, use #ifndefine #define to define a “variable” in the compilation process of.o file  If a variable has been defined, the compiler will skip the code segment between #ifndefine and #endif. 13

#define preprocessor directive  if XYZ is undefined, we process the file. Otherwise we do not process the file (jumping to the #endif).  XYZ should not appear in any other files. 14 #ifndef XYZ #define XYZ f1(); f2(); … #endif

File scope: external/internal Each file must know the existence of every variable, constant and function that it uses! Global constants: repeat their definitions (global constants are ‘ local ’ to the file, internal!) External (global) variables: add keyword “ extern ” External functions: add function prototypes (with or without “ extern ” ) “ extern ”  it ’ s (just) a declaration, but not a definition! equivalently, it means the variable / function is global and is defined in another file.

16 #include #include "library.h" using namespace std; int Int=99; int main(){ cout << "Hello"<<endl; cout << f(100) << endl; return 0; } #include "library.h" int f(int i){ return (Int + i); } main.cpp source.cpp Output: Hello 199 library.h extern int Int; int f(int);

Separate compilation with classes

Interface and Implementation  In C++ it is more common to separate the class interface from its implementation. Abstract data type  The interface lists the class and its members (data and functions).  The implementation provides implementations of the functions. 18 What to do? How to do?

19 Class interface  header file *.h Class implementation  source code file *.cpp Client/application/user  driver file main.cpp The implementation is hidden from the users!

Separate Files .cpp source-code files .h header files  Separate files in which class definitions are placed  Allow compiler to recognize the classes when used elsewhere  Generally have.h filename extensions  Driver files  Program used to test software (such as classes)  Contains a main function so it can be executed 20

Class Libraries 21  Class declarations placed in header file  Given. h extension  Contains data items and prototypes  Implementation file  Same prefix name as header file  Given. cpp extension  Programs which use this class library called client programs

Example  The interface, implementation and driver separated into three files  Interface contains data members and member function prototypes only  Note the #ifndef statements to prevent multiple inclusions 22 #ifndef GRADEBOOK_H #define GRADEBOOK_H class GradeBook { public: GradeBook( string ); void setCourseName( string ); string getCourseName(); void displayMessage(); private: string courseName; }; #endif Gradebook6.h (with Validity Checking)

 set functions perform validity checking to keep courseName in a consistent state  GradeBook implementation is placed in a separate file  Include the header file to access the class name GradeBook  Binary scope resolution operator :: “ties” a function to its class 23 #include "GradeBook6.h" // include definition of class GradeBook void GradeBook::setCourseName( string name ) { // if name has 25 or fewer characters if ( name.length() <= 25 ) courseName = name; // if name has more than 25 characters if ( name.length() > 25 ) { // set courseName to first 25 characters of parameter name courseName = name.substr( 0, 25 ); cout << "Name \"" << name << "\" exceeds maximum length (25).\n" << "Limiting courseName to first 25 characters."; } Gradebook6.cpp

 Include the header file to use the class GradeBook 24 #include "GradeBook6.h" // include definition of class GradeBook int main() { // initial course name of gradeBook1 is too long GradeBook gradeBook1( "COMP104 Introduction to Programming in C++" ); GradeBook gradeBook2( "COMP152 OOP and Data Structures" ); cout << "gradeBook1's initial course name is: " << gradeBook1.getCourseName() << "\ngradeBook2's initial course name is: " << gradeBook2.getCourseName() << endl; driver6.cpp (1/2)

 Call set function to perform validity checking directly  In Linux, compile them all together using g++ Gradebook6.cpp driver6.cpp  Or using object files: g++ -c Gradebook6.cpp; g++ -c driver6.cpp; g++ driver6.o driver6.o 25 // modify myGradeBook's courseName (with a valid-length string) gradeBook1.setCourseName( "COMP104 C++ Programming" ); // display each GradeBook's courseName cout << "\ngradeBook1's course name is: " << gradeBook1.getCourseName() << "\ngradeBook2's course name is: " << gradeBook2.getCourseName() << endl; return 0; } driver6.cpp (2/2)

26 Name "COMP104 Introduction to Programming in C++" exceeds maximum length (25). Limiting courseName to first 25 characters. gradeBook1's initial course name is: COMP104 Introduction to P gradeBook2's initial course name is: COMP152 OOP and Data Stru gradeBook1's course name is: COMP104 C++ Programming gradeBook2's course name is: COMP152 OOP and Data Stru driver6.cpp Sample Output