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";
}
// `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 = static_cast<bool>(i);
  CHECK(b == true);
  CHECK(b == 42); // bad
  CHECK(b == static_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

What purpose do these two lines serve?

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-based for loops

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 = std::unordered_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 <unordered_set>

Hash maps

auto country_codes = std::unordered_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(
  std::unordered_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 <unordered_map>

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

COMP6771 21T2 - 1.2 - Intro & Types

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