Show Lecture.BasicSyntax as a slide show.

CS253 Basic Syntax

The main() function

int main() {
    return 0;
}

• This is a complete C program.
• main() is a function, not a method.
• Methods are functions inside of classes.
  • methods ⊂ functions
  • All methods are functions.
  • Not all functions are methods.
• Unlike Java, C++ doesn’t have to have a class, so main() is a function.
• We sometimes call this a free function.
  • It’s not part of a class. It’s free!

Slightly more

Here’s a complete C++ program that creates some output:

#include <iostream>

using namespace std;

int main() {
    cout << "Hello, world!\n";
    return 0;
}

Hello, world!

• You need #include <iostream> to access I/O facilities.
• You need using namespace std; because you just have to, for the moment.

How Examples Work

Many examples in these slides are just snippets of code:

cout << "How do you do?" << '\n';

How do you do?

• They’re turned into complete programs via webserver magic.
• You still need #include <iostream>.
• You still need using namespace std;.
• You still need int main() {.
• The return 0; is optional in main(), which is stoooopid, so I always include it (unless I run out of slide space).
• You still need the } to close main().

The main() function

Here’s the other valid definition of main():

int main(int argc, char *argv[]) {
    // Display all arguments, including program name.
    for (int i=0; i<argc; i++)
        cout << "argv[" << i << "]: \"" << argv[i] << "\"\n";
    return 0;
}

argv[0]: "./a.out"

That is all

Don’t even ask about void main().

It does not exist.

⚠ ☢ ☣ ☠️

Return value

main() returns an int, a success/failure code to the invoker.

• 0: success
• >0: failures of various sorts. Return different values for different failures, e.g.:
  • 1: “memory too expensive”
  • 2: “fish encountered in file”
  • 3: “user is ugly”
• <0: That’s just weird. Only the least-significant eight bits of the return value count, so −1 ⇒ 255.

Arguments

    int main(int argc, char *argv[])

• argc: number of arguments, including the program name as argv[0].
  • argc stands for “argument count”.
• argv: array of C-style strings, corresponding to program arguments.
  • argv stands for “argument vector”.
• argc is always ≥ 1, except in strange embedded environments.
• argv is always an array of C strings, even if you type arguments that look like numbers or something else.

Arguments example

% cat ~cs253/Example/show-args.cc
#include <iostream>
using namespace std;

int main(int argc, char *argv[]) {
    for (int i=0; i<argc; i++)
	cout << "argv[" << i << "]: " << argv[i] << '\n';
}
% g++ -Wall ~cs253/Example/show-args.cc
% ./a.out CS253: "best class ever!"
argv[0]: ./a.out
argv[1]: CS253:
argv[2]: best class ever!
% mv a.out pinkiepie
% ./pinkiepie Cupcakes
argv[0]: ./pinkiepie
argv[1]: Cupcakes

argv[0] is the real program name, as executed—nice for error messages.

Compatibility

• Why is argv[] an array of char *? Surely they should be C++ strings?
  • C++ strings didn’t exist in C.
• Why is argv[] an array of char *? Surely they should be const char *?
  • C didn’t have const when this was all invented.
  • It’s your memory, you should be able to change it.

Basic types

C++ has a number of built-in types:

• void
• bool (not boolean)
• byte (a collection of bits, not a small integer as in Java)
• char / short / int / long / long long
• unsigned versions of those
• float / double / long double
• size_t (an alias for an unsigned integer type)

Modifiers

long, short, and unsigned are modifiers, not types. However, it’s common to use them alone:

In a bagel shop, you don’t say “I’d like a whole wheat bagel.” You just ask for a “whole wheat”, and the “bagel” is implied. Similarly, int is implied.

Sizes

• The sizes of types are determined by the implementation.
• On my tiny laptop, sizeof(long)==4, but sizeof(long)==8 on many computers.
• The C++ standard defines minimum sizes for types:
  • short, int: 16 bits
  • long: 32 bits
  • long long: 64 bits
  • sizeof(char) == 1
  • sizeof(char) ≤ sizeof(short) ≤ sizeof(int) ≤
    sizeof(long) ≤ sizeof(long long)
• Pointer types often vary.
  • sizeof(int *)==4 on a 32-bit machine
  • sizeof(int *)==8 on a 64-bit machine

Qualifiers

Qualifiers (const, constexpr, static) modify existing types.

const auto now = time(nullptr);
constexpr double PI = 3.14159265;
static long csuid = 800000000;
cout << now << ' ' << PI << ' ' << ++csuid << '\n';

1714033882 3.14159 800000001

• const: not allowed to change this
• constexpr: compile-time constant, never changes
• static: longer-than-function lifetime, private if a global

Derived types

Via pointers & arrays, a vast number of derived types exist:

    int a[10];		// 10 ints
    int *b;		// A pointer to any number of ints
    int *c[10];		// An array of 10 pointers-to-ints
    int (*d)[10];	// A pointer to an array of 10 ints

If you find complex types confusing, build intermediate types with typedef:

    typedef float cash; // cash is now a synonym for float (note the order)
    typedef int *intp;	// New type intp, a pointer to ints
    intp e[10];		// An array of 10 such pointers

Aliases via typedef

With typedef, you can create an alias for an existing type. It does not create a new type.

typedef int counter;    // typedef old new;
counter c = 42;
cout << c << '\n';

42

To use typedef, think of it in two steps:

1. Declare a variable: int counter;
2. Slap typedef in front of it: typedef int counter;

Aliases via using

We’re familiar with using from using namespace std; but it can also be used to make aliases for types:

using counter = int;    // using new = old;
counter c = 43;
cout << c << '\n';

43

Lazy Programmer’s Declaration

Or, you can just declare variables with auto (which is not a type!):

auto i=4;           // an int
auto r=3.45;        // a double
cout << i+r << '\n';

7.45

But you must initialize the variable, so that the compiler knows its type:

auto foo;
foo = 'x';

c.cc:1: error: declaration of 'auto foo' has no initializer

Why auto is useful

Consider this common code:

time_t now = time(nullptr);
cout << "Seconds since the start of 1970: " << now << '\n';

Seconds since the start of 1970: 1714033882

time_t is an alias (via typedef) for short, int, long, or long long. It can hold whatever time() produces. Many functions have a related type for their return value, and they clutter up your brain.

auto now = time(nullptr);
cout << "Seconds since the start of 1970: " << now << '\n';

Seconds since the start of 1970: 1714033882

What type is now? I don’t care. It’s the type that time() returned.

AAA

• I’m part of the AAA (almost always auto) school of thought.
  • Don’t declare your variables until you have a value for them.
  • That value will determine the type of the variable.
• Sure, sometimes you can’t. Those are the exceptions.
• It can be tricky:

string a      = "Alvin";     // a std::string
const char *s = "Simon";     // a const char *
char t[]      = "Theodore";  // an array of char
auto d        = "Dave";      // which one?
cout << sizeof(a) << '\n'
     << sizeof(s) << '\n'
     << sizeof(t) << '\n'
     << sizeof(d) << '\n';

32
8
9
8

Literals

Names

• Variable/function/method/class/type names are quite similar to Java.
• Start with a letter, continue with letters, digits, or underscore.
• Constants, by convention, are all upper case (PI).
• User-defined classes are often capitalized (class Foo);
• Avoid starting anything with an underscore (_), or containing a double underscore.
• Modern C++ allows amazing Unicode variable names.
  • Wish we had one o’ them modern compilers on this here website!

cout << __VERSION__;

8.3.1 20191121 (Red Hat 8.3.1-5)

double π = 355/113.0; cout << π;

c.cc:1: error: stray '\317' in program

Control Flow

• if
• switch
• while
• do … while
• for
• range-based for

if

int x = 42;
if (x < 100)
    cout << "This is true\n";

This is true

double pi = 3.14159265;
if (pi*pi > 10) {
    cout << "Can’t even trust math any more‽\n";
}
else
    cout << "Good--math hasn’t changed.\n";

Good--math hasn’t changed.

• The braces are optional, for just one statement.
• Omitting them is a good way to start an argument with a programmer.

Truth & falsity

C++ is quite liberal about what if/while/for consider to be trueish values:

if

• An if statement can contain a variable declaration. The condition succeeds if the value is trueish.

if (const char *p = getenv("PATH"))
    cout << "PATH is " << p << '\n';

PATH is /bin

• This is handy if the code computes a value that is only needed inside the if, which is quite common.
• The scope of the new variable is restricted to the if statement, even without braces. Smaller scope = better code!
• This works well if the desired value is trueish.

if

• A C++17 addition: an if statement can contain a variable declaration, much like a for loop.
• This is helpful for the same reasons as the previous example.
• Sometimes, the useful value doesn’t correspond to a trueish value, e.g., the error value is -1, not 0.

if (auto now = time(nullptr); now != -1)
    cout << "Time is " << now << '\n';

Time is 1714033882

switch

auto now = time(nullptr);
switch (localtime(&now)->tm_hour) {
  case 2: case 3: case 5: case 7:
  case 11: case 13: case 17: case 19:
    cout << "Prime time!\n"; break;
  default:
    cout << "Composite time!\n"; break;
  case 1:
    cout << "Bed time!\n";
}

Prime time!

• switch on integer types (char, short, int, long, …).
• You can’t switch on floating-point types or strings.
• The break prevents flow into the next case.
• In C++17, a declaration is allowed before the switch value, as in if.

while

char c = 'f';
while (c < 'x')
    cout << c++ << ' ';

f g h i j k l m n o p q r s t u v w 

• No, C++17 doesn’t allow a variable declaration. We already have a for loop for that.

do … while

double age = 0.0;
do {
    age += 1.0/13.0;
    cout << age << '\n';
} while (age < 1);

0.0769231
0.153846
0.230769
0.307692
0.384615
0.461538
0.538462
0.615385
0.692308
0.769231
0.846154
0.923077
1
1.07692

Isn’t that one too many iterations?

do … while

double age = 0.0;
cout << fixed << setprecision(16);
do {
    age += 1.0/13.0;
    cout << age << '\n';
} while (age < 1);

0.0769230769230769
0.1538461538461539
0.2307692307692308
0.3076923076923077
0.3846153846153846
0.4615384615384616
0.5384615384615385
0.6153846153846154
0.6923076923076923
0.7692307692307692
0.8461538461538460
0.9230769230769229
0.9999999999999998
1.0769230769230766

Printing more digits shows that floating-point numbers are inherently imprecise.

for

for (int i=0; i<5; i++)
    cout << i;

01234

Declare your loop variable inside the for loop, so the scope of the loop variable is just the for loop. There are exceptions, sure. Focus on the 99% case.

int j;
for (j=0; j<5; j++);
    cout << j;

5

The error (extra semicolon) was not detected.

for (int k=0; k<5; k++);
    cout << k;

c.cc:2: error: 'k' was not declared in this scope

The error was detected.

Range-based for

// C array:
int a[] = {11,22,33};
for (int v : a)
    cout << v << ", ";

11, 22, 33, 

// C++ container:
vector<short> b = {101, 202, 303, 404};
for (short &n : b)
    n *= 3;
for (const short n : b)
    cout << n << ", ";

303, 606, 909, 1212, 

// Immediate list:
for (auto q : {12, 34, 56, 78})
    cout << q << " and ";

12 and 34 and 56 and 78 and