The goal of this lab is to explore how C++ adds minimal extensions to C in order to provide several features that have become common in object-oriented languages. The subsequent lab will have you use these features to work with some basic C++ code.

1 How this lab works

1.1 Passing off

Each subsequent section of this lab writeup describes a C++ feature and ends with a set of questions about that feature. You’ll pass off the lab by sharing your answers to a TA-selected subset of these questions with a TA. The TA may also ask a variation on one or more question to ensure understanding of the underlying concepts.

We recommend you write down your answers as you determine them.

1.2 Compiling

Many parts of this writeup will ask you to inspect various aspects of compiled C++ code. You can run a C++ compiler by using clang++ (or g++); to access the resulting assembly use the -S flag (e.g., clang++ -S ex.cpp generates assembly in ex.s); to inspect the data set by the code use lldb (or gdb).

You’ll likely find that -O1 will generally create much shorter code without removing useless code that you might have wanted to test something.

You should always give your C++ files the extension .cpp. Some

2 Function name overloading

In C, if you name a function baz then it is compiled with label baz. That means you cannot re-use the name baz for more than one function.

In C++, function names are only part of the compiled label: argument types and where the name appeared are also part of it. This allows code like

int abs_val(int x) { return x < 0 ? -x : x; }
double abs_val(double x) { return x < 0 ? -x : x; }
int main() {
    printf("%d %f", abs_val(-23), abs_val(-20.3f));
}

to be compiled; the compiler picks which version of abs_val to invoked in each instance using information derived during type checking.

This renaming of functions to arrive at labels is called name mangling.

3 Namespaces

In C, every function and global variable’s name must be unique among all the files you link with. When you write all your own code, that’s not too hard to do; but when you are using third-party libraries there is no reason to expect them not to conflict in names. Because of this, major C libraries generally pick some kind of prefix to all function names, but it can be annoying to read and write code when virtually every function starts with the same prefix.

Later languages have added in a full module or package system, with full scoping and so on, but C++ uses a simpler mechanism called namespaces. These are included in the name mangling to further decrease the chance of name collisions.

Name mangling for namespaces uses a length-before-name technique, so that namespace welcome would become 7welcome in the mangled name. This is important to be able to have numbers in the namespace hierarchy: 7welcome2it represents the two nested namespaces welcome::it while 10welcome2it is the single namespace welcome2it

4 Operator overloading

C++ allows you to redefine what almost every operator does. You can define functions whose name is operator+, operator<<=, etc, and they will be used in place of built-in behavior for +, <<=, etc, when those operators are applied to the appropriate types.

#include <stdio.h>

typedef struct nonint { int x; } nonint;

nonint operator+ (nonint a, nonint b) {
    nonint ans = { a.x - b.x };
    return ans;
}

int main() {
    nonint a = { 3 };
    nonint b = { 5 };
    nonint c = a + b;
    printf("%d + %d = %d\n", a.x, b.x, c.x);
}

5 Other odds and ends

C++ compilation also makes use of a few other syntactic sugar pieces to help make code easier to write. One example is the “pass by reference” syntax, a short-hand way of using pointers. For example, what in C might be

char *strsep_c(char **txt, char delim) {
    if (!txt || !*txt) return 0;
    char *ans = *txt;
    while(**txt && **txt != delim) *txt += 1;
    if (**txt == delim) {
        *txt = 0;
        *txt += 1;
    }
    return ans;
}

can be written in C++ as

char *strsep_c(char *& txt, char delim) {
    if (!txt) return 0;
    char *ans = txt;
    while(*txt && *txt != delim) txt += 1;
    if (*txt == delim) {
        txt = 0;
        txt += 1;
    }
    return ans;
}

These two compile to virtually the assembly except that the compiler takes care of ensuring that the second, which uses pass-by-reference syntax, is always given a valid reference so the two-part check in the first is just a 1-part check in the second

Recent versions of C++ have added many other similar tricks for efficient coding with more compile-time checking, such as more careful casts, self-deallocating pointers, etc.

6 Classes

C++ is often characterized as “C supporting objected oriented coding”. This section explores several of those concepts:

C++ adds a class keyword that is identical to struct in almost every way, except a class defaults to private: members and a struct to public:. C++ also removes the need to typedef to get a simple name for a struct or class.

6.1 Member functions

C++ also extends struct and class to allow them to declare member functions as well as member attributes:

class gleam {
    int a;
    int b;
public:
    int inOrder();
};

int gleam::inOrder() {
    return this->a <= this->b;
}

6.2 constructors, destructors, new and delete

Constructors are an important part of how OO objects are initialized. C++ has added an operator new that acts like “malloc¹ and then invoke a constructor” and an operator delete that acts like “free and then invoke a destructor”,

Thus, we could test the code above with

gleam *a = new gleam();
int b = a->inOrder();

To add a constructor, make a member function with the same name as the class; a destructor’s name is preceded by a tilde ~. Neither should have a return type specified. Destructors are usually used to delete anything the constructor newed, and are typically omitted if there is no need to do that.

class gleam2 {     
    int a;        
    int b;        
public:           
    int inOrder();
    gleam2(int x, int y) { this->a = x; this->b = y; }
    ~gleam2() { puts("Gone"); }
};

6.3 Inheritance and overloading

We’ll talk about how C++ implements inheritance and overloading in lecture.

6.4 Templates

C++ also has a “template language”, which looks superficially like Java’s generic syntax but is actually a full programming language in its own right. Most casual C++ programmers I know only use the basic type-generic aspect of templates, not any of their advanced features.