COMP6771

Advanced C++ Programming

Week 1.2

Intro & Types

First program

#include <iostream>

int main() {
  // put "Hello world\n" to the character output
  std::cout << "Hello, world!\n";
}

First program

#include <iostream>

auto main() -> int {
  // put "Hello world\n" to the character output
  std::cout << "Hello, world!\n";
}

// `int` for integers.
auto meaning_of_life = 42;
    
// `double` for rational numbers.
auto six_feet_in_metres = 1.8288;
    
// report if this expression is false
CHECK(six_feet_in_metres < meaning_of_life);

Basic types

// `string` for text.
auto course_code = std::string("COMP6771");
    
// `char` for single characters.
auto letter = 'C';
    
CHECK(course_code.front() == letter);

Basic types

// `bool` for truth
auto is_cxx = true;
auto is_danish = false;
CHECK(is_cxx != is_danish);

Basic types

Const

The const keyword specifies that a value cannot be modified
Everything should be const unless you know it will be modified
The course will focus on const-correctness as a major topic

Const

// `int` for integers.
auto const meaning_of_life = 42;
    
// `double` for rational numbers.
auto const six_feet_in_metres = 1.8288;
    
// report if this expression is false
CHECK(six_feet_in_metres < meaning_of_life);

Why Const

Clearer code (you can know a function won't try and modify something just by reading the signature)
Immutable objects are easier to reason about
The compiler may be able to make certain optimisations
Immutable objects are much easier to use in multithreading situations

auto const x = 10;
auto const y = 173;

Integral expressions

auto const sum = 183;
CHECK(x + y == sum);

auto const difference = 163;
CHECK(y - x == difference);
CHECK(x - y == -difference);

auto const product = 1730;
CHECK(x * y == product);

auto const quotient = 17;
CHECK(y / x == quotient);

auto const remainder = 3;
CHECK(y % x == remainder);

auto const x = 15.63;
auto const y = 1.23;

Floating-point expressions

auto const sum = 16.86;
CHECK(x + y == sum);

auto const difference = 14.4;
CHECK(x - y == difference);
CHECK(y - x == -difference);

auto const product = 19.2249;
CHECK(x * y == product);

auto const expected = 12.7073170732;
auto const actual = x / y;
auto const acceptable_delta = 0.0000001;
CHECK(std::abs(expected - actual) < acceptable_delta);

auto const expr = std::string("Hello, expressions!");
auto const cxx = std::string("Hello, C++!");

String expressions

CHECK(expr != cxx);
CHECK(expr.front() == cxx[0]);

auto const concat = absl::StrCat(expr, " ", cxx);
CHECK(concat == "Hello, expressions! Hello, C++!");

auto expr2 = expr;

// Abort TEST_CASE if expression is false
REQUIRE(expr == expr2);

C++ has value semantics

auto const hello = std::string("Hello!")
auto hello2 = hello;

// Abort TEST_CASE if expression is false
REQUIRE(hello == hello2);

hello2.append("2");
REQUIRE(hello != hello2);

CHECK(hello.back() == '!');
CHECK(hello2.back() == '2');

Boolean expressions

auto const is_comp6771 = true;
auto const is_about_cxx = true;
auto const is_about_german = false;

CHECK((is_comp6771 and is_about_cxx));

CHECK((is_about_german or is_about_cxx));

CHECK(not is_about_german);

Type Conversion

In C++ we are able to convert types implicitly or explicitly. We will cover this later in the course in more detail.

Implicit promoting conversions

auto const i = 0;

{
  auto d = 0.0;
  REQUIRE(d == 0.0);

  d = i; // Silent conversion from int to double
  CHECK(d == 42.0);
  CHECK(d != 41);
}

Explicit promoting conversions

auto const i = 0;

{
  // Preferred over implicit, since your intention is clear
  auto const d = static_cast<double>(i);
  CHECK(d == 42.0);
  CHECK(d != 41);
}

Explicit narrowing (lossy) conversions

auto const i = 42;

{
  // information lost, but we're saying we know
  auto const b = gsl_lite::narrow_cast<bool>(i);
  CHECK(b == true);
  CHECK(b == gsl_lite::narrow_cast<bool>(42)); // okay
}

Functions

C++ has functions just like other languages. We will explore some together

Nullary function (no parameters)

auto is_about_cxx() -> bool {
  return true;
}

CHECK(is_about_cxx());

Unary function (one parameter)

auto square(int const x) -> int {
  return x * x;
}

CHECK(square(2) == 4);

Binary function (two parameters)

auto area(int const width, int const length) -> int {
  return width * length;
}

CHECK(area(2, 4) == 8);

Default Arguments

Functions can use default arguments, which is used if an actual argument is not specified when a function is called
Default values are used for the trailing parameters of a function call - this means that ordering is important
Formal parameters: Those that appear in function definition
Actual parameters (arguments): Those that appear when calling the function

std::string rgb(short r = 0, short g = 0, short b = 0);
rgb();// rgb(0, 0, 0);
rgb(100);// Rgb(100, 0, 0);
rgb(100, 200);     // Rgb(100, 200, 0)
rgb(100, , 200);   // error

Function overloading

auto square(int const x) -> int {
  return x * x;
}

auto square(double const x) -> double {
  return x * x;
}

CHECK(square(2) == 4);
CHECK(square(2.0) == 4.0);
CHECK(square(2.0) != 4);

Function overloading refers to a family of functions in the same scope that have the same name but different formal parameters.
This can make code easier to write and understand

Overload Resolution

This is the process of "function matching"
Step 1: Find candidate functions: Same name
Step 2: Select viable ones: Same number arguments + each argument convertible
Step 3: Find a best-match: Type much better in at least one argument

When writing code, try and only create overloads that are trivial
- If non-trivial to understand, name your functions differently

auto g() -> void;
auto f(int) -> void;
auto f(int, int) -> void;
auto f(double, double = 3.14) -> void;
f(5.6); // calls f(double, double)

Errors in function matching are found during compile time

Return types are ignored. Read more about this here.

Conditional expressions

auto is_even(int const x) -> bool {
  return x % 2 == 0;
}

auto collatz_point_conditional(int const x) -> int {
  return is_even(x) ? x / 2
                    : 3 * x + 1;
}

CHECK(collatz_point_conditional(6) == 3);
CHECK(collatz_point_conditional(5) == 16);

https://en.wikipedia.org/wiki/Collatz_conjecture

if-statement

auto collatz_point_if_statement(int const x) -> int {
  if (is_even(x)) {
    return x / 2;
  }

  return 3 * x + 1;
}

CHECK(collatz_point_if_statement(6) == 3);
CHECK(collatz_point_if_statement(5) == 16);

switch-statement

auto is_digit(char const c) -> bool {
	switch (c) {
	case '0': [[fallthrough]];
	case '1': [[fallthrough]];
	case '2': [[fallthrough]];
	case '3': [[fallthrough]];
	case '4': [[fallthrough]];
	case '5': [[fallthrough]];
	case '6': [[fallthrough]];
	case '7': [[fallthrough]];
	case '8': [[fallthrough]];
	case '9': return true;
	default: return false;
	}
}

CHECK(is_digit('6'));
CHECK(not is_digit('A'));

Sequenced collections

auto const single_digits = std::vector<int>{
  0, 1, 2, 3, 4, 5, 6, 7, 8, 9
};

auto more_single_digits = single_digits;
REQUIRE(single_digits == more_single_digits);

more_single_digits[2] = 0;
CHECK(single_digits != more_single_digits);

more_single_digits.push_back(0);
CHECK(more_single_digits.size() == 11);

Sequenced collections

auto const single_digits = std::vector<int>{
  0, 1, 2, 3, 4, 5, 6, 7, 8, 9
};

more_single_digits.push_back(0);
CHECK(ranges::count(more_single_digits, 0) == 2);

more_single_digits.pop_back();
CHECK(ranges::count(more_single_digits, 0) == 1);

CHECK(std::erase(more_single_digits, 0) == 1);
CHECK(ranges::count(more_single_digits, 0) == 0);
CHECK(ranges::distance(more_single_digits) == 8);

Sequenced collections

#include <vector>

Values and references

We can use pointers in C++ just like C, but generally we don't want to
A reference is an alias for another object: You can use it as you would the original object
Similar to a pointer, but:
- Don't need to use -> to access elements
- Can't be null
- You can't change what they refer to once set

auto i = 1;
auto& j = i;
j = 3;

CHECK(i == 3)

References and const

A reference to const means you can't modify the object using the reference
The object is still able to be modified, just not through this reference

auto i = 1;
auto const& ref = i;
std::cout << ref << '\n';
i++; // This is fine
std::cout << ref << '\n';
ref++; // This is not

auto const j = 1;
auto const& jref = j; // this is allowed
auto& ref = j; // not allowed

Functions: Pass by value

The actual argument is copied into the memory being used to hold the formal parameters value during the function call/execution

#include <iostream>

auto swap(int x, int y) -> void {
  auto const tmp = x;
  x = y;
  y = tmp;
}

auto main() -> int {
  auto i = 1;
  auto j = 2;
  std::cout << i << ' ' << j << '\n'; // prints 1 2
  swap(i, j);
  std::cout << i << ' ' << j << '\n'; // prints 1 2... not swapped?
}

Functions: pass by reference

The formal parameter merely acts as an alias for the actual parameter
Anytime the method/function uses the formal parameter (for reading or writing), it is actually using the actual parameter
Pass by reference is useful when:
- The argument has no copy operation
- The argument is large

#include <iostream>

auto swap(int& x, int& y) -> void {
  auto const tmp = x;
  x = y;
  y = tmp;
}

auto main() -> int {
  auto i = 1;
  auto j = 2;
  std::cout << i << ' ' << j << '\n'; // 1 2
  swap(i, j);
  std::cout << i << ' ' << j << '\n'; // 2 1
}

// C equivalent
#include <stdio.h>

void swap(int* x, int* y) {
  auto const tmp = *x;
  *x = *y;
  *y = tmp;
}

int main() {
  int i = 1;
  int j = 2;
  printf("%d %d\n", i, j);
  swap(&i, &j);
  printf("%d %d\n", i, j)
}

Values and references

auto by_value(std::string const sentence) -> char;

// takes ~153.67 ns
by_value(two_kb_string);

auto by_reference(std::string const& sentence) -> char;

// takes ~8.33 ns
by_reference(two_kb_string);

auto by_value(std::vector<std::string> const long_strings) -> char;

// takes ~2'920 ns
by_value(sixteen_two_kb_strings);

// takes ~13 ns
by_reference(sixteen_two_kb_strings);

auto by_reference(std::vector<std::string> const& long_strings) -> char;

range-for-statements

auto all_computer_scientists(std::vector<std::string> const& names) -> bool {
	auto const famous_mathematician = std::string("Gauss");
	auto const famous_physicist = std::string("Newton");

	for (auto const& name : names) {
		if (name == famous_mathematician or name == famous_physicist) {
			return false;
		}
	}

	return true;
}

for-statements

auto square_vs_cube() -> bool {
	// 0 and 1 are special cases, since they're actually equal.
	if (square(0) != cube(0) or square(1) != cube(1)) {
		return false;
	}

	for (auto i = 2; i < 100; ++i) {
		if (square(i) == cube(i)) {
			return false;
		}
	}

	return true;
}

User-defined types: enumerations

enum class computing_courses {
	intro,
	data_structures,
	engineering_design,
	compilers,
	cplusplus,
};

auto const computing101 = computing_courses::intro;
auto const computing102 = computing_courses::data_structures;
CHECK(computing101 != computing102);

User-defined types: structures

struct scientist {
	std::string family_name;
	std::string given_name;
	field_of_study primary_field;
	std::vector<field_of_study> secondary_fields;
};

Defining two objects

auto const famous_physicist = scientist{
    .family_name = "Newton",
    .given_name = "Isaac",
    .primary_field = field_of_study::physics,
    .secondary_fields = {field_of_study::mathematics,
                         field_of_study::astronomy,
                         field_of_study::theology},
};

auto const famous_mathematician = scientist{
    .family_name = "Gauss",
    .given_name = "Carl Friedrich",
    .primary_field = field_of_study::mathematics,
    .secondary_fields = {field_of_study::physics},
};

Member access

CHECK(famous_physicist.given_name
   != famous_mathematician.given_name);

CHECK(famous_physicist.family_name
   != famous_mathematician.family_name);

CHECK(famous_physicist.primary_field
   != famous_mathematician.primary_field);

CHECK(famous_physicist.secondary_fields
   != famous_mathematician.secondary_fields);

Wouldn't it be nicer if we could say this?

CHECK(famous_physicist != famous_mathematician);

User-defined types: structures

struct scientist {
	std::string family_name;
	std::string given_name;
	field_of_study primary_field;
	std::vector<field_of_study> secondary_fields;


};

	auto operator==(scientist const&) const -> bool = default;

Hash sets

auto computer_scientists = absl::flat_hash_set<std::string>{
   "Lovelace",
   "Babbage",
   "Turing",
   "Hamilton",
   "Church",
   "Borg",
};

REQUIRE(ranges::distance(computer_scientists) == 6);
CHECK(computer_scientists.contains("Lovelace"));
CHECK(not computer_scientists.contains("Gauss"));

Inserting an element

computer_scientists.insert("Gauss");
CHECK(ranges::distance(computer_scientists) == 7);
CHECK(computer_scientists.contains("Gauss"));

computer_scientists.erase("Gauss");
CHECK(ranges::distance(computer_scientists) == 6);
CHECK(not computer_scientists.contains("Gauss"));

Finding an element

auto ada = computer_scientists.find("Lovelace");
REQUIRE(ada != computer_scientists.end());

CHECK(*ada == "Lovelace");

An empty set

computer_scientists.clear();
CHECK(computer_scientists.empty());

auto const no_names = absl::flat_hash_set<std::string>{};
REQUIRE(no_names.empty());

CHECK(computer_scientists == no_names);

Where can I find it?

#include "absl/containers/flat_hash_set.h"

Hash maps

auto country_codes = absl::flat_hash_map<std::string, std::string>{
   {"AU", "Australia"},
   {"NZ", "New Zealand"},
   {"CK", "Cook Islands"},
   {"ID", "Indonesia"},
   {"DK", "Denmark"},
   {"CN", "China"},
   {"JP", "Japan"},
   {"ZM", "Zambia"},
   {"YE", "Yemen"},
   {"CA", "Canada"},
   {"BR", "Brazil"},
   {"AQ", "Antarctica"},
};

CHECK(country_codes.contains("AU"));
CHECK(not country_codes.contains("DE")); // Germany not present

country_codes.emplace("DE", "Germany");
CHECK(country_codes.contains("DE"));

Hash maps

auto check_code_mapping(
  absl::flat_hash_map<std::string, std::string> const& country_codes,
  std::string const& code,
  std::string const& name) -> void {
	auto const country = country_codes.find(code);
	REQUIRE(country != country_codes.end());

	auto const [key, value] = *country;
	CHECK(code == key);
	CHECK(name == value);
}

Hash maps

#include "absl/container/flat_hash_map.h"

Declarations vs Definitions

void declared_fn(int arg);
class declared_type;

// This class is defined, but not all the methods are.
class defined_type {
  int declared_member_fn(double);
  int defined_member_fn(int arg) { return arg; }
};

// These are all defined.
int defined_fn() { return 1; }

int i;
int const j = 1;
auto vd = std::vector<double>{};

A declaration makes known the type and the name of a variable
A definition is a declaration, but also does extra things
- A variable definition allocates storage for, and constructs a variable
- A class definition allows you to create variables of the class' type
- You can call functions with only a declaration, but must provide a definition later
Everything must have precisely one definition

Program errors

There are 4 types of program errors that we will discuss

Compile-time
Link-time
Run-time
Logic

Compile-time Errors

auto main() -> int {
  a = 5; // Compile-time error: type not specified
}

Link-time Errors

#include "catch2/catch.hpp"

auto is_cs6771() -> bool;

TEST_CASE("This is all the code")
  CHECK(is_cs6771()); // Link-time error: is_cs6771 not defined.
}

[build] ld.lld: error: unable to find library -ltest_main
[build] clang: error: linker command failed with exit code 1 (use -v to see invocation)

Remember how we couldn't demo yesterday? That was due to a linker error because a whole file was missing!

Run-time errors

// attempting to open a file...
if (auto file = std::ifstream("hello.txt"); not file) {
    throw std::runtime_error("Error: file not found.\n");
}

Logic (programmer) Errors

auto const empty = std::string("");
CHECK(empty[0] == 'C'); // Logic error: bad character access

Logic (programmer) Errors

auto const s = std::string("");
assert(not s.empty());
CHECK(s[0] == 'C'); // Logic error: bad character access