C++ Program Structure (and tools)

1. C++ Program Structure (and tools) Today we’ll talk generally about C++ development (plus a few platform specifics) • We’ll develop, submit, and grade code in Windows • It’s also helpful to become familiar with Linux • E.g., on shell.cec.wustl.edu • For example, running code through two different compilers can catch a lot more “easy to make” errors

2. Writing a C++ Program Visual Studio Eclipse emacs Makefile (ASCII text) editor 1 source file = 1 compilation unit C++ source files (ASCII text) .cpp Also: .C .cxx .cc Programmer (you) Also: .H .hxx .hpp C++ header files (ASCII text) .h readme (ASCII text)

3. What Goes Into a C++ Program? • Declarations: data types, function signatures, classes • Allows the compiler to check for type safety, correct syntax • Usually kept in “header” (.h) files • Included as needed by other files (to keep compiler happy) class Simple { typedef unsigned int UINT32; public: Simple (int i); int usage (char * program_name); void print_i (); private: struct Point2D { int i_; double x_; }; double y_; }; • Definitions: static variable initialization, function implementation • The part that turns into an executable program • Usually kept in “source” (.cpp) files void Simple::print_i () { cout << “i_ is ” << i_ << endl; } • Directives: tell compiler (or precompiler) to do something • More on this later

4. A Very Simple C++ Program #include <iostream> // precompiler directive using namespace std; // compiler directive // definition of function named “main” int main (int, char *[]) { cout << “hello, world!” << endl; return 0; }

5. What is #include <iostream> ? • #include tells the precompiler to include a file • Usually, we include header files • Contain declarations of structs, classes, functions • Sometimes we include template definitions • Varies from compiler to compiler • Advanced topic we’ll cover later in the semester • <iostream> is the C++ label for a standard header file for input and output streams

6. What is using namespace std; ? • The using directive tells the compiler to include code from libraries that have separate namespaces • Similar idea to “packages” in other languages • C++ provides a namespace for its standard library • Called the “standard namespace” (written as std) • cout, cin, and cerr standard iostreams, and much more • Namespaces reduce collisions between symbols • Rely on the :: scoping operator to match symbols to them • If another library with namespace mylib defined cout we could say std::cout vs. mylib::cout • Can also apply using more selectively: • E.g., just using std::cout

7. What is int main (int, char*[]) { ... } ? • Defines the main function of any C++ program • Who calls main? • The runtime environment, specifically a function often called something like crt0 or crtexe • What about the stuff in parentheses? • A list of types of the input arguments to function main • With the function name, makes up its signature • Since this version of main ignores any inputs, we leave off names of the input variables, and only give their types • What about the stuff in braces? • It’s the body of function main, its definition

8. What’s cout << “hello, world!” << endl; ? • Uses the standard output iostream, named cout • For standard input, use cin • For standard error, use cerr • << is an operator for inserting into the stream • A member operator of the ostream class • Returns a reference to stream on which its called • Can be applied repeatedly to references left-to-right • “hello, world!”is a C-style string • A 14-postion character array terminated by ‘\0’ • endl is an iostream manipulator • Ends the line, by inserting end-of-line character(s) • Also flushes the stream

9. What about return 0; ? • The main function should return an integer • By convention it should return 0 for success • And a non-zero value to indicate failure • The program should not exit any other way • Letting an exception propagate uncaught • Dividing by zero • Dereferencing a null pointer • Accessing memory not owned by the program • Indexing an array “out of range” can do this • Dereferencing a “stray” pointer can do this

10. A Slightly Bigger C++ Program #include <iostream> using namespace std; int main (int argc, char * argv[]) { for (int i = 0; i < argc; ++i) { cout << argv[i] << endl; } return 0; }

11. int argc, char * argv[] • A way to affect the program’s behavior • Carry parameters with which program was called • Passed as parameters to main from crt0 • Passed by value (we’ll discuss what that means) • argc • An integer with the number of parameters (>=1) • argv • An array of pointers to C-style character strings • Its array-length is the value stored in argc • The name of the program is kept in argv[0]

12. for (int i = 0; i < argc; ++i) • Standard (basic) C++ for loop syntax • Initialization statement done once at start of loop • Test expression done before running each time • Expression to increment after running each time • int i = 0 • Declares integer i (scope is the loop itself) • Initializes i to hold value 0 (not an assignment!) • i < argc • Tests whether or not we’re still inside the array! • Reading/writing memory we don’t own can crash the program (if we’re really lucky!) • ++i • increments the array position (why prefix?)

13. {cout << argv[i] << endl;} • Body of the for loop • I strongly prefer to use braces with for, if, while, etc., even w/ single-statement body • Avoids maintenance errors when adding/modifying code • Ensures semantics/indentation say same thing • argv[i] • An example of array indexing • Specifies ith position from start of argv

14. Lifecycle of a C++ Program xterm An “IDE” window console/terminal/window Makefile WebCAT make turnin/checkin E-mail “make” utility Runtime/utility libraries (binary) .lib .a .dll .so Eclipse compile link Visual Studio C++ source code Programmer (you) debugger precompiler gcc, etc. link compiler linker executable program compiler object code (binary, one per compilation unit) .o

15. Development Environment Studio • We’ll follow a similar format most days in the course • Around 30 minutes of lecture and discussion • Then about 60 minutes of studio time • Except for open studio/lab days, reviews before the midterm and final, and the day of the midterm itself • In the studios, please work in groups of 2 or 3 • Exercises are posted on the course web page • Record your answers to the exercises, and e-mail your answers to the course account when you’re done • We’ll migrate throughout the studio to answer questions • Use studio time to develop skills and understanding • A good chance to explore ideas you can use for the labs • Exams will test understanding of the studio material • You’re encouraged to try variations beyond the exercises