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Basic Syntax

CS253 Basic Syntax

The main function

Here’s a complete C++ program:

    int main() {
        return 0;
    }

That’s it!

main() is a function, not a method. Methods are functions inside of classes. Unlike Java, C++ doesn’t have to have a class, so main() is a function.

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 a snippet of code:

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

How do you do?

• You still need #include <iostream>.
• You still need using namespace std;.
• You still need int main() {.
• The return 0; is optional, but I always include it.
• 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 varying sorts
  • I try to return different values for different failures, e.g., 1 for “not enough arguments”, 2 for “can’t read the file”, 3 for “bad data in file”, etc.
• <0: That’s just weird. Many environments (e.g., bash) will only use the lower eight bits of the return value. E.g., -1 ⇒ 255, which will be confused with a signal return value, 128+signum.

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.
• The type of argv is always an array of C strings, even if you type arguments that look like numbers or something else.

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)
• char (not byte)
• short / int / long / long long
• float / double / long double
• size_t

Sizes

• The exact sizes of these types are determined by the implementation.
• On my home machine, sizeof(long)==4, but sizeof(long)==8 on many computers.
• The C++ standard defines minimum sizes for the various 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 such as const, constexpr, and static modify existing types.

    const long id = get_id_number();
    constexpr double PI = 3.1415926;
    static const char *program_name;

• const: not allowed to change this
• constexpr: compile-time constant
• 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 for 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:

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

7.45

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 = 3.14159;
if (pi*pi < 10) {
    cout << "Good--mathematics has not changed\n";
}
else
    cout << "Can’t even trust math any more‽\n";

Good--mathematics has not changed

switch

srand(time(nullptr));
int sides = rand() % 5;
cout << sides << ": ";
switch (sides) {
  case 3: cout << "Triangle\n";   break;
  case 4: cout << "Square\n";     break;
  case 5: cout << "Pentagon\n";   break;
  default: cout << "Can’t deal with that!\n";
}

2: Can’t deal with that!

• You can switch on integral types (char, short, int, long, …).
• You can’t switch on floating-point types or strings.
• The break prevents flow into the next case.

while

int i=4;
while (i<10)
    cout << i++ << ' ';

4 5 6 7 8 9 

do … while

int i=4;
do {
    i += 2;
    cout << i << ' ';
} while (i<15);

6 8 10 12 14 16 

for

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

01234

range-based for

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

11, 22, 33, 

int b[] = {101, 202, 303, 404};
for (int &n : b)
    n *= 3;
for (const int n : b)
    cout << n << ", ";

303, 606, 909, 1212, 

Variable declaration

The condition of an if, switch, or while can be a declaration.

const char show[] = "Star Trek";
if (const char *p = strchr(show, 'T'))
    cout << "Found it at location " << p-show << '\n';

Found it at location 5

false, 0, and a null pointer are false, everything else is true.