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A Quick Introduction to C++
Tom Anderson
“If programming in Pascal is like being put in a straightjacket, then programming in C is like playing with knives, and programming in C++ is like juggling chainsaws.” Anonymous.
1 Introduction
This note introduces some simple C++ concepts and outlines a subset of C++ that is easier to learn and use than the full language. Although we originally wrote this note for explaining the C++ used in the Nachos project, I believe it is useful to anyone learning C++. I assume that you are already somewhat familiar with C concepts like procedures, for loops, and pointers; these are pretty easy to pick up from reading Kernighan and Ritchie’s “The C Programming Language.” I should admit up front that I am quite opinionated about C++, if that isn’t obvious already. I know several C++ purists (an oxymoron perhaps?) who violently disagree with some of the prescriptions contained here; most of the objections are of the form, “How could you have possibly left out feature X?” However, I’ve found from teaching C++ to nearly 1000 undergrads over the past several years that the subset of C++ described here is pretty easy to learn, taking only a day or so for most students to get started. The basic premise of this note is that while object-oriented programming is a useful way to simplify programs, C++ is a wildly over-complicated language, with a host of features that only very, very rarely find a legitimate use. It’s not too far off the mark to say that C++ includes every programming language feature ever imagined, and more. The natural tendency when faced with a new language feature is to try to use it, but in C++ this approach leads to disaster. Thus, we need to carefully distinguish between (i) those concepts that are fundamental (e.g., classes, member functions, constructors) – ones that everyone should know and use, (ii) those that are sometimes but rarely useful (e.g., single inheritance, templates) – ones that beginner programmers should be able to recognize (in case they run across them) but avoid using in their own programs, at least for a while, and (iii) those that are just a bad idea and should be avoided like the plague (e.g., multiple inheritance, exceptions, overloading, references, etc). Of course, all the items in this last category have their proponents, and I will admit that, like the hated goto, it is possible to construct cases when the program would be simpler using a goto or multiple inheritance. However, it is my belief that most programmers will never encounter such cases, and even if you do, you will be much more likely to misuse the feature than properly apply it. For example, I seriously doubt an undergraduate would need any of the features listed under (iii) for any course project (at least at Berkeley this is true). And if you find yourself wanting to use a feature like multiple inheritance, then, my advice is to fully implement your program both with and without the feature, and choose whichever is simpler. Sure, this takes more effort, but pretty soon you’ll know from experience when a feature is useful and when it isn’t, and you’ll be able to skip the dual implementation. A really good way to learn a language is to read clear programs in that language. I have tried to make the Nachos code as readable as possible; it is written in the subset of C++ described in this note. It is a good idea to look over the first assignment as you read this introduction. Of course, your TA’s will answer any questions you may have. You should not need a book on C++ to do the Nachos assignments, but if you are curious, there is a large selection of C++ books at Cody’s and other technical bookstores. (My wife quips that C++ was invented
This article is based on an earlier version written by Wayne Christopher.
to make researchers at Bell Labs rich from writing “How to Program in C++” books.) Most new software development these days is being done in C++, so it is a pretty good bet you’ll run across it in the future. I use Stroustrup’s ”The C++ Programming Language” as a reference manual, although other books may be more readable. I would also recommend Scott Meyer’s “Effective C++” for people just beginning to learn the language, and Coplien’s “Advanced C++” once you’ve been programming in C++ for a couple years and are familiar with the language basics. Also, C++ is continually evolving, so be careful to buy books that describe the latest version (currently 3.0, I think!).
2 C in C++
To a large extent, C++ is a superset of C, and most carefully written ANSI C will compile as C++. There are a few major caveats though:
- All functions must be declared before they are used, rather than defaulting to type int.
- All function declarations and definition headers must use new-style declarations, e.g.,
extern int foo(int a, char* b);
The form extern int foo(); means that foo takes no arguments, rather than arguments of an unspecified type and number. In fact, some advise using a C++ compiler even on normal C code, because it will catch errors like misused functions that a normal C compiler will let slide.
- If you need to link C object files together with C++, when you declare the C functions for the C++ files, they must be done like this:
extern "C" int foo(int a, char* b);
Otherwise the C++ compiler will alter the name in a strange manner.
- There are a number of new keywords, which you may not use as identifiers — some common ones are new, delete, const, and class.
3 Basic Concepts
Before giving examples of C++ features, I will first go over some of the basic concepts of object-oriented languages. If this discussion at first seems a bit obscure, it will become clearer when we get to some examples.
- Classes and objects. A class is similar to a C structure , except that the definition of the data structure, and all of the functions that operate on the data structure are grouped together in one place. An object is an instance of a class (an instance of the data structure); objects share the same functions with other objects of the same class, but each object (each instance) has its own copy of the data structure. A class thus defines two aspects of the objects: the data they contain, and the behavior they have.
- Member functions. These are functions which are considered part of the object and are declared in the class definition. They are often referred to as methods of the class. In addition to member functions, a class’s behavior is also defined by:
(a) What to do when you create a new object (the constructor for that object) – in other words, initialize the object’s data.
Inside a member function, one may refer to the members of the class by their names alone. In other words, the class definition creates a scope that includes the member (function and data) definitions. Note that if you are inside a member function, you can get a pointer to the object you were called on by using the variable this. If you want to call another member function on the same object, you do not need to use the this pointer, however. Let’s extend the Stack example to illustrate this by adding a Full() function.
class Stack { public: void Push(int value); // Push an integer, checking for overflow. bool Full(); // Returns TRUE if the stack is full, FALSE otherwise. int top; // Index of the lowest unused position. int stack[10]; // A pointer to an array that holds the contents. };
bool Stack::Full() { return (top == 10); }
Now we can rewrite Push this way:
void Stack::Push(int value) { ASSERT(!Full()); stack[top++] = value; }
We could have also written the ASSERT:
ASSERT(!(this->Full());
but in a member function, the this-> is implicit. The purpose of member functions is to encapsulate the functionality of a type of object along with the data that the object contains. A member function does not take up space in an object of the class.
- Private members. One can declare some members of a class to be private , which are hidden to all but the member functions of that class, and some to be public , which are visible and accessible to everybody. Both data and function members can be either public or private. In our stack example, note that once we have the Full() function, we really don’t need to look at the top or stack members outside of the class – in fact, we’d rather that users of the Stack abstraction not know about its internal implementation, in case we change it. Thus we can rewrite the class as follows:
class Stack { public: void Push(int value); // Push an integer, checking for overflow. bool Full(); // Returns TRUE if the stack is full, FALSE otherwise.
private: int top; // Index of the top of the stack. int stack[10]; // The elements of the stack. };
Before, given a pointer to a Stack object, say s, any part of the program could access s->top, in potentially bad ways. Now, since the top member is private, only a member function, such as Full(), can access it. If any other part of the program attempts to use s->top the compiler will report an error. You can have alternating public: and private: sections in a class. Before you specify either of these, class members are private, thus the above example could have been written:
class Stack { int top; // Index of the top of the stack. int stack[10]; // The elements of the stack. public: void Push(int value); // Push an integer, checking for overflow. bool Full(); // Returns TRUE if the stack is full, FALSE otherwise. };
Which form you prefer is a matter of style, but it’s usually best to be explicit, so that it is obvious what is intended. In Nachos, we make everything explicit. What is not a matter of style: all data members of a class should be private. All operations on data should be via that class’ member functions. Keeping data private adds to the modularity of the system, since you can redefine how the data members are stored without changing how you access them.
- Constructors and the operator new. In C, in order to create a new object of type Stack, one might write:
struct Stack *s = (struct Stack *) malloc(sizeof (struct Stack)); InitStack(s, 17);
The InitStack() function might take the second argument as the size of the stack to create, and use malloc() again to get an array of 17 integers. The way this is done in C++ is as follows:
Stack *s = new Stack(17);
The new function takes the place of malloc(). To specify how the object should be initialized, one declares a constructor function as a member of the class, with the name of the function being the same as the class name:
class Stack { public: Stack(int sz); // Constructor: initialize variables, allocate space. void Push(int value); // Push an integer, checking for overflow. bool Full(); // Returns TRUE if the stack is full, FALSE otherwise. private:
delete s2;
This will deallocate the object, but first it will call the destructor for the Stack class, if there is one. This destructor is a member function of Stack called ˜Stack():
class Stack { public: Stack(int sz); // Constructor: initialize variables, allocate space. ˜Stack(); // Destructor: deallocate space allocated above. void Push(int value); // Push an integer, checking for overflow. bool Full(); // Returns TRUE if the stack is full, FALSE otherwise. private: int size; // The maximum capacity of the stack. int top; // Index of the lowest unused position. int* stack; // A pointer to an array that holds the contents. };
Stack::˜Stack() { delete [] stack; // delete an array of integers }
The destructor has the job of deallocating the data the constructor allocated. Many classes won’t need destructors, and some will use them to close files and otherwise clean up after themselves.
The destructor for an object is called when the object is deallocated. If the object was created with new, then you must call delete on the object, or else the object will continue to occupy space until the program is over – this is called “a memory leak.” Memory leaks are bad things – although virtual memory is supposed to be unlimited, you can in fact run out of it – and so you should be careful to always delete what you allocate. Of course, it is even worse to call delete too early – delete calls the destructor and puts the space back on the heap for later re-use. If you are still using the object, you will get random and non-repeatable results that will be very difficult to debug. In my experience, using data that has already been deleted is major source of hard-to-locate bugs in student (and professional) programs, so hey, be careful out there!
If the object is an automatic, allocated on the execution stack of a function, the destructor will be called and the space deallocated when the function returns; in the test() example above, s1 will be deallocated when test() returns, without you having to do anything.
In Nachos, we always explicitly allocate and deallocate objects with new and delete, to make it clear when the constructor and destructor is being called. For example, if an object contains another object as a member variable, we use new to explicitly allocated and initialize the member variable, instead of implicitly allocating it as part of the containing object. C++ has strange, non-intuitive rules for the order in which the constructors and destructors are called when you implicitly allocate and deallocate objects. In practice, although simpler, explicit allocation is slightly slower and it makes it more likely that you will forget to deallocate an object (a bad thing!), and so some would disagree with this approach.
When you deallocate an array, you have to tell the compiler that you are deallocating an array, as opposed to a single element in the array. Hence to delete the array of integers in Stack::˜Stack:
delete [] stack;
3.2 Other Basic C++ Features
Here are a few other C++ features that are useful to know.
- When you define a class Stack, the name Stack becomes usable as a type name as if created with typedef. The same is true for enums.
- You can define functions inside of a class definition, whereupon they become inline functions , which are expanded in the body of the function where they are used. The rule of thumb to follow is to only consider inlining one-line functions, and even then do so rarely. As an example, we could make the Full routine an inline.
class Stack { ... bool Full() { return (top == size); }; ... };
There are two motivations for inlines: convenience and performance. If overused, inlines can make your code more confusing, because the implementation for an object is no longer in one place, but spread between the .h and .c files. Inlines can sometimes speed up your code (by avoiding the overhead of a procedure call), but that shouldn’t be your principal concern as a student (rather, at least to begin with, you should be most concerned with writing code that is simple and bug free). Not to mention that inlining sometimes slows down a program, since the object code for the function is duplicated wherever the function is called, potentially hurting cache performance.
- Inside a function body, you can declare some variables, execute some statements, and then declare more variables. This can make code a lot more readable. In fact, you can even write things like:
for (int i = 0; i < 10; i++) ;
Depending on your compiler, however, the variable i may still visible after the end of the for loop, however, which is not what one might expect or desire.
- Comments can begin with the characters // and extend to the end of the line. These are usually more handy than the /* */ style of comments.
- C++ provides some new opportunities to use the const keyword from ANSI C. The basic idea of const is to provide extra information to the compiler about how a variable or function is used, to allow it to flag an error if it is being used improperly. You should always look for ways to get the compiler to catch bugs for you. After all, which takes less time? Fixing a compiler-flagged error, or chasing down the same bug using gdb? For example, you can declare that a member function only reads the member data, and never modifies the object:
class Stack { ... bool Full() const; // Full() never modifies member data ... };
Nachos contains a few examples of the correct use of inheritance and templates, but realize that Nachos does not use them everywhere. In fact, if you get confused by this section, don’t worry, you don’t need to use any of these features in order to do the Nachos assignments. I omit a whole bunch of details; if you find yourself making widespread use of inheritance or templates, you should consult a C++ reference manual for the real scoop. This is meant to be just enough to get you started, and to help you identify when it would be appropriate to use these features and thus learn more about them!
4.1 Inheritance
Inheritance captures the idea that certain classes of objects are related to each other in useful ways. For example, lists and sorted lists have quite similar behavior – they both allow the user to insert, delete, and find elements that are on the list. There are two benefits to using inheritance:
- You can write generic code that doesn’t care exactly which kind of object it is manipulating. For example, inheritance is widely used in windowing systems. Everything on the screen (windows, scroll bars, titles, icons) is its own object, but they all share a set of member functions in common, such as a routine Repaint to redraw the object onto the screen. This way, the code to repaint the entire screen can simply call the Repaint function on every object on the screen. The code that calls Repaint doesn’t need to know which kinds of objects are on the screen, as long as each implements Repaint.
- You can share pieces of an implementation between two objects. For example, if you were to imple- ment both lists and sorted lists in C, you’d probably find yourself repeating code in both places – in fact, you might be really tempted to only implement sorted lists, so that you only had to debug one version. Inheritance provides a way to re-use code between nearly similar classes. For example, given an implementation of a list class, in C++ you can implement sorted lists by replacing the insert member function – the other functions, delete, isFull, print, all remain the same.
4.1.1 Shared Behavior
Let me use our Stack example to illustrate the first of these. Our Stack implementation above could have been implemented with linked lists, instead of an array. Any code using a Stack shouldn’t care which implemen- tation is being used, except that the linked list implementation can’t overflow. (In fact, we could also change the array implementation to handle overflow by automatically resizing the array as items are pushed on the stack.) To allow the two implementations to coexist, we first define an abstract Stack, containing just the public member functions, but no data.
class Stack { public: Stack(); virtual ˜Stack(); // deallocate the stack virtual void Push(int value) = 0; // Push an integer, checking for overflow. virtual bool Full() = 0; // Is the stack is full? };
// For g++, need these even though no data to initialize. Stack::Stack {} Stack::˜Stack() {}
The Stack definition is called a base class or sometimes a superclass. We can then define two different derived classes , sometimes called subclasses which inherit behavior from the base class. (Of course, inheri- tance is recursive – a derived class can in turn be a base class for yet another derived class, and so on.) Note that I have prepended the functions in the base class is prepended with the keyword virtual, to signify that they can be redefined by each of the two derived classes. The virtual functions are initialized to zero, to tell the compiler that those functions must be defined by the derived classes. Here’s how we could declare the array-based and list-based implementations of Stack. The syntax : public Stack signifies that both ArrayStack and ListStack are kinds of Stacks, and share the same behavior as the base class.
class ArrayStack : public Stack { // the same as in Section 2 public: ArrayStack(int sz); // Constructor: initialize variables, allocate space. ˜ArrayStack(); // Destructor: deallocate space allocated above. void Push(int value); // Push an integer, checking for overflow. bool Full(); // Returns TRUE if the stack is full, FALSE otherwise. private: int size; // The maximum capacity of the stack. int top; // Index of the lowest unused position. int *stack; // A pointer to an array that holds the contents. };
class ListStack : public Stack { public: ListStack(); ˜ListStack(); void Push(int value); bool Full(); private: List *list; // list of items pushed on the stack };
ListStack::ListStack() { list = new List; }
ListStack::˜ListStack() { delete list; }
void ListStack::Push(int value) { list->Prepend(value); }
bool ListStack::Full() { return FALSE; // this stack never overflows! }
The neat concept here is that I can assign pointers to instances of ListStack or ArrayStack to a variable of type Stack, and then use them as if they were of the base type.
protected: Stack(); // initialize data private: int numPushed; };
Stack::Stack() { numPushed = 0; }
void Stack::Push(int value) { numPushed++; }
int Stack::NumPushed() { return numPushed; }
We can then modify both ArrayStack and ListStack to make use the new behavior of Stack. I’ll only list one of them here:
class ArrayStack : public Stack { public: ArrayStack(int sz); ˜ArrayStack(); void Push(int value); bool Full(); private: int size; // The maximum capacity of the stack. int *stack; // A pointer to an array that holds the contents. };
ArrayStack::ArrayStack(int sz) : Stack() { size = sz; stack = new int[size]; // Let’s get an array of integers. }
void ArrayStack::Push(int value) { ASSERT(!Full()); stack[NumPushed()] = value; Stack::Push(); // invoke base class to increment numPushed }
There are a few things to note:
- The constructor for ArrayStack needs to invoke the constructor for Stack, in order to initialize numPushed. It does that by adding : Stack() to the first line in the constructor:
ArrayStack::ArrayStack(int sz) : Stack()
The same thing applies to destructors. There are special rules for which get called first – the construc- tor/destructor for the base class or the constructor/destructor for the derived class. All I should say is, it’s a bad idea to rely on whatever the rule is – more generally, it is a bad idea to write code which requires the reader to consult a manual to tell whether or not the code works!
- I introduced a new keyword, protected, in the new definition of Stack. For a base class, protected signifies that those member data and functions are accessible to classes derived (recursively) from this class, but inaccessible to other classes. In other words, protected data is public to derived classes, and private to everyone else. For example, we need Stack’s constructor to be callable by ArrayStack and ListStack, but we don’t want anyone else to create instances of Stack. Hence, we make Stack’s constructor a protected function. In this case, this is not strictly necessary since the compiler will complain if anyone tries to create an instance of Stack because Stack still has an undefined virtual functions, Push. By defining Stack::Stack as protected, you are safe even if someone comes along later and defines Stack::Push. Note however that I made Stack’s data member private, not protected. Although there is some debate on this point, as a rule of thumb you should never allow one class to see directly access the data in another, even among classes related by inheritance. Otherwise, if you ever change the implementation of the base class, you will have to examine and change all the implementations of the derived classes, violating modularity.
- The interface for a derived class automatically includes all functions defined for its base class, with- out having to explicitly list them in the derived class. Although we didn’t define NumPushed() in ArrayStack, we can still call it for those objects:
ArrayStack *s = new ArrayStack(17);
ASSERT(s->NumPushed() == 0); // should be initialized to 0
- Conversely, even though we have defined a routine Stack::Push(), because it is declared as virtual, if we invoke Push() on an ArrayStack object, we will get ArrayStack’s version of Push:
Stack *s = new ArrayStack(17);
if (!s->Full()) // ArrayStack::Full s->Push(5); // ArrayStack::Push
- Stack::NumPushed() is not virtual. That means that it cannot be re-defined by Stack’s derived classes. Some people believe that you should mark all functions in a base class as virtual; that way, if you later want to implement a derived class that redefines a function, you don’t have to modify the base class to do so.
- Member functions in a derived class can explicitly invoke public or protected functions in the base class, by the full name of the function, Base::Function(), as in:
void ArrayStack::Push(int value) { ... Stack::Push(); // invoke base class to increment numPushed }
private: int size; // The maximum capacity of the stack. int top; // Index of the lowest unused position. T *stack; // A pointer to an array that holds the contents. };
To define a template, we prepend the keyword template to the class definition, and we put the param- eterized type for the template in angle brackets. If we need to parameterize the implementation with two or more types, it works just like an argument list: template <class T, class S>. We can use the type parameters elsewhere in the definition, just like they were normal types. When we provide the implementation for each of the member functions in the class, we also have to declare them as templates, and again, once we do that, we can use the type parameters just like normal types:
// template version of Stack::Stack template Stack::Stack(int sz) { size = sz; top = 0; stack = new T[size]; // Let’s get an array of type T }
// template version of Stack::Push template void Stack::Push(T value) { ASSERT(!Full()); stack[top++] = value; }
Creating an object of a template class is similar to creating a normal object:
void test() { Stack s1(17); Stack *s2 = new Stack(23);
s1.Push(5); s2->Push(’z’); delete s2; }
Everything operates as if we defined two classes, one called Stack – a stack of integers, and one called Stack – a stack of characters. s1 behaves just like an instance of the first; s2 behaves just like an instance of the second. In fact, that is exactly how templates are typically implemented – you get a complete copy of the code for the template for each different instantiated type. In the above example, we’d get one copy of the code for ints and one copy for chars. So what’s wrong with templates? You’ve all been taught to make your code modular so that it can be re-usable, so everything should be a template, right? Wrong. The principal problem with templates is that they can be very difficult to debug – templates are easy to use if they work, but finding a bug in them can be difficult. In part this is because current generation C++
debuggers don’t really understand templates very well. Nevertheless, it is easier to debug a template than two nearly identical implementations that differ only in their types. So the best advice is – don’t make a class into a template unless there really is a near term use for the template. And if you do need to implement a template, implement and debug a non-template version first. Once that is working, it won’t be hard to convert it to a template. Then all you have to worry about code explosion – e.g., your program’s object code is now megabytes because of the 15 copies of the hash table/list/... routines, one for each kind of thing you want to put in a hash table/list/... (Remember, you have an unhelpful compiler!)
5 Features To Avoid Like the Plague
Despite the length of this note, there are numerous features in C++ that I haven’t explained. I’m sure each fea- ture has its advocates, but despite programming in C and C++ for over 15 years, I haven’t found a compelling reason to use them in any code that I’ve written (outside of a programming language class!) Indeed, there is a compelling reason to avoid using these features – they are easy to misuse, resulting in programs that are harder to read and understand instead of easier to understand. In most cases, the features are also redundant – there are other ways of accomplishing the same end. Why have two ways of doing the same thing? Why not stick with the simpler one? I do not use any of the following features in Nachos. If you use them, caveat hacker.
- Multiple inheritance. It is possible in C++ to define a class as inheriting behavior from multiple classes (for instance, a dog is both an animal and a furry thing). But if programs using single inheritance can be difficult to untangle, programs with multiple inheritance can get really confusing.
- References. Reference variables are rather hard to understand in general; they play the same role as pointers, with slightly different syntax (unfortunately, I’m not joking!) Their most common use is to declare some parameters to a function as reference parameters , as in Pascal. A call-by-reference parameter can be modified by the calling function, without the callee having to pass a pointer. The effect is that parameters look (to the caller) like they are called by value (and therefore can’t change), but in fact can be transparently modified by the called function. Obviously, this can be a source of obscure bugs, not to mention that the semantics of references in C++ are in general not obvious.
- Operator overloading. C++ lets you redefine the meanings of the operators (such as + and >>) for class objects. This is dangerous at best (”exactly which implementation of ’+’ does this refer to?”), and when used in non-intuitive ways, a source of great confusion, made worse by the fact that C++ does implicit type conversion, which can affect which operator is invoked. Unfortunately, C++’s I/O facilities make heavy use of operator overloading and references, so you can’t completely escape them, but think twice before you redefine ’+’ to mean “concatenate these two strings”.
- Function overloading. You can also define different functions in a class with the same name but different argument types. This is also dangerous (since it’s easy to slip up and get the unintended version), and we never use it. We will also avoid using default arguments (for the same reason). Note that it can be a good idea to use the same name for functions in different classes, provided they use the same arguments and behave the same way – a good example of this is that most Nachos objects have a Print() method.
- Standard template library. An ANSI standard has emerged for a library of routines implementing such things as lists, hash tables, etc., called the standard template library. Using such a library should make programming much simpler if the data structure you need is already provided in the library. Alas, the standard template library pushes the envelope of legal C++, and so virtually no compilers (including
Was the intent really x == y? After all, it’s pretty easy to mistakenly leave off the extra equals sign. By never using assignment within a conditional, you can tell by code inspection whether you’ve made a mistake.
- Using #define when you could use enum. When a variable can hold one of a small number of values, the original C practice was to use #define to set up symbolic names for each of the values. enum does this in a type-safe way – it allows the compiler to verify that the variable is only assigned one of the enumerated values, and none other. Again, the advantage is to eliminate a class of errors from your program, making it quicker to debug.
6 Style Guidelines
Even if you follow the approach I’ve outlined above, it is still as easy to write unreadable and undebuggable code in C++ as it is in C, and perhaps easier, given the more powerful features the language provides. For the Nachos project, and in general, we suggest you adhere to the following guidelines (and tell us if you catch us breaking them):
- Words in a name are separated SmallTalk-style (i.e., capital letters at the start of each new word). All class names and member function names begin with a capital letter, except for member functions of the form getSomething() and setSomething(), where Something is a data element of the class (i.e., accessor functions). Note that you would want to provide such functions only when the data should be visible to the outside world, but you want to force all accesses to go through one function. This is often a good idea, since you might at some later time decide to compute the data instead of storing it, for example.
- All global functions should be capitalized, except for main and library functions, which are kept lower- case for historical reasons.
- Minimize the use of global variables. If you find yourself using a lot of them, try and group some together in a class in a natural way or pass them as arguments to the functions that need them if you can.
- Minimize the use of global functions (as opposed to member functions). If you write a function that operates on some object, consider making it a member function of that object.
- For every class or set of related classes, create a separate .h file and .cc file. The .h file acts as the interface to the class, and the .cc file acts as the implementation (a given .cc file should include it’s respective .h file). If using a particular .h file requires another .h file to be included (e.g., synch.h needs class definitions from thread.h) you should include the dependency in the .h file, so that the user of your class doesn’t have to track down all the dependencies himself. To protect against multiple inclusion, bracket each .h file with something like:
#ifndef STACK_H #define STACK_H
class Stack { ... };
#endif
Sometimes this will not be enough, and you will have a circular dependency. For example, you might have a .h file that uses a definition from one .h file, but also defines something needed by that .h file.
In this case, you will have to do something ad-hoc. One thing to realize is that you don’t always have to completely define a class before it is used. If you only use a pointer to class Stack and do not access any member functions or data from the class, you can write, in lieu of including stack.h:
class Stack;
This will tell the compiler all it needs to know to deal with the pointer. In a few cases this won’t work, and you will have to move stuff around or alter your definitions.
- Use ASSERT statements liberally to check that your program is behaving properly. An assertion is a condition that if FALSE signifies that there is a bug in the program; ASSERT tests an expression and aborts if the condition is false. We used ASSERT above in Stack::Push() to check that the stack wasn’t full. The idea is to catch errors as early as possible, when they are easier to locate, instead of waiting until there is a user-visible symptom of the error (such as a segmentation fault, after memory has been trashed by a rogue pointer). Assertions are particularly useful at the beginnings and ends of procedures, to check that the procedure was called with the right arguments, and that the procedure did what it is supposed to. For example, at the beginning of List::Insert, you could assert that the item being inserted isn’t already on the list, and at the end of the procedure, you could assert that the item is now on the list. If speed is a concern, ASSERTs can be defined to make the check in the debug version of your program, and to be a no-op in the production version. But many people run with ASSERTs enabled even in production.
- Write a module test for every module in your program. Many programmers have the notion that testing code means running the entire program on some sample input; if it doesn’t crash, that means it’s work- ing, right? Wrong. You have no way of knowing how much code was exercised for the test. Let me urge you to be methodical about testing. Before you put a new module into a bigger system, make sure the module works as advertised by testing it standalone. If you do this for every module, then when you put the modules together, instead of hoping that everything will work, you will know it will work. Perhaps more importantly, module tests provide an opportunity to find as many bugs as possible in a localized context. Which is easier: finding a bug in a 100 line program, or in a 10000 line program?
7 Compiling and Debugging
The Makefiles we will give you works only with the GNU version of make, called “gmake”. You may want to put “alias make gmake” in your .cshrc file. You should use gdb to debug your program rather than dbx. Dbx doesn’t know how to decipher C++ names, so you will see function names like Run__9SchedulerP6Thread. On the other hand, in GDB (but not DBX) when you do a stack backtrace when in a forked thread (in homework 1), after printing out the correct frames at the top of the stack, the debugger will sometimes go into a loop printing the lower-most frame (ThreadRoot), and you have to type control-C when it says “more?”. If you understand assembly language and can fix this, please let me know.
8 Example: A Stack of Integers
We’ve provided the complete, working code for the stack example. You should read through it and play around with it to make sure you understand the features of C++ described in this paper. To compile the simple stack test, type make all – this will compile the simple stack test (stack.cc), the inherited stack test (inheritstack.cc), and the template version of stacks (templatestack.cc).
