COMP6771

Advanced C++ Programming

Week 3.1

Class Types

  • Scope
  • Class Types
  • Assignment 1 / Other:
    • Creating a new directory (+CMakeLists)
    • Creating and running a new single file
    • Committing those changes with git
    • Questions about ass1 (might do at end)

Today we are covering

  • The scope of a variable is the part of the program where it is accessible
    • Scope starts at variable definition
    • Scope (usually) ends at next "}"
    • You're probably familiar with this even if you've never seen the term
  • Define variables as close to first usage as possible
  • This is the opposite of what you were taught in first year undergrad
    • Defining all variables at the top is especially bad in C++

Scope

#include <iostream>

int i = 1;
int main() {
	std::cout << i << "\n";
	if (i > 0) {
		int i = 2;
		std::cout << i << "\n";
		{
			int i = 3;
			std::cout << i << "\n";
		}
		std::cout << i << "\n";
	}
	std::cout << i << "\n";
}

lecture-3/demo301-scope.cpp

Object Lifetimes

  • An object is a piece of memory of a specific type that holds some data
    • All variables are objects
    • Unlike many other languages, this does not add overhead
  • Object lifetime starts when it comes in scope
    • "Constructs" the object
    • Each type has 1 or more constructor that says how to construct it
  • Object lifetime ends when it goes out of scope
    • "Destructs" the object
    • Each type has a different "destructor" which tells the compiler how to destroy it

 

This is the behavior that primitive types follow, but you probably knew that intuitively. With classes, we tend to think a bit more explicitly about it.

Construction

  • Eg. https://en.cppreference.com/w/cpp/container/vector/vector
  • Generally use () to call functions, and {} to construct objects
    • () can only be used for functions, and {} can be used for either
    • There are some rare occasions these are different
      • Sometimes it is ambiguous between a constructor and an initialize list
auto main() -> int {
  // Always use auto on the left for this course, but you may see this elsewhere.
  std::vector<int> v11; // Calls 0-argument constructor. Creates empty vector.
  
  // There's no difference between these:
  // T variable = T{arg1, arg2, ...}
  // T variable{arg1, arg2, ...}
  auto v12 = std::vector<int>{}; // No different to first
  auto v13 = std::vector<int>(); // No different to the first
  
  {
  	auto v2 = std::vector<int>{v11.begin(), v11.end()}; // A copy of v11.
  	auto v3 = std::vector<int>{v2}; // A copy of v2.
  } // v2 and v3 destructors are called here

  auto v41 = std::vector<int>{5, 2}; // Initialiser-list constructor {5, 2}
  auto v42 = std::vector<int>(5, 2); // Count + value constructor (5 * 2 => {2, 2, 2, 2, 2})
} // v11, v12, v13, v41, v42 destructors called here

lecture-3/demo302-construction.cpp

Construction

  • Also works for your basic types
    • But the default constructor has to be manually called
      • This potential bug can be hard to detect due to how function stacks work (variable may happen to be 0)
      • Can be especially problematic with pointers
#include <iostream>

double f() {
	return 1.1;
}

int main() {
	// One of the reasons we do auto is to avoid ununitialized values.
	// int n; // Not initialized (memory contains previous value)

	int n21{}; // Default constructor (memory contains 0)
	auto n22 = int{}; // Default constructor (memory contains 0)
	auto n3{5};

	// Not obvious you know that f() is not an int, but the compiler lets it through.
	// int n43 = f();

	// Not obvious you know that f() is not an int, and the compiler won't let you (narrowing
	// conversion)
	// auto n41 = int{f()};

	// Good code. Clear you understand what you're doing.
	auto n42 = static_cast<int>(f());

	// std::cout << n << "\n";
	std::cout << n21 << "\n";
	std::cout << n22 << "\n";
	std::cout << n3 << "\n";
	std::cout << n42 << "\n";
}

lecture-3/demo303-construction2.cpp

Why are object lifetimes useful?

Can you think of a thing where you always have to remember to do something when you're done?

void ReadWords(const std::string& filename) {
  std::ifstream f{filename};
  std::vector<std::string> words;
  std::copy(std::istream_iterator<std::string>{f}, {}, std::back_inserter{words});
  f.close();
}
  • What happens if we omit f.close() here (assume similar behavior to c/java/python)?
  • How easy to spot is the mistake
  • How easy would it be for a compiler to spot this mistake for us?
    • How would it know where to put the f.close()?

Namespaces

// lexicon.hpp
namespace lexicon {
    std::vector<std::string> read_lexicon(std::string const& path);

    void write_lexicon(std::vector<std::string> const&, std::string const& path);
} // namespace lexicon

Used to express that names belong together.

Prevent similar names from clashing.

// word_ladder.hpp
namespace word_ladder {
    absl::flat_hash_set<std::string> read_lexicon(std::string const& path);
} // namespace word_ladder

Namespaces

// word_ladder.hpp
namespace word_ladder {
    absl::flat_hash_set<std::string> read_lexicon(std::string const& path);
} // namespace word_ladder
// read_lexicon.cpp
namespace word_ladder {
    absl::flat_hash_set<std::string> read_lexicon(std::string const& path) {
        // open file...
        // read file into flat_hash_set...
        // return flat_hash_set
    }
} // namespace word_ladder

Nested namespaces

namespace comp6771::word_ladder {
    std::vector<std::vector<std::string>>
    word_ladder(std::string const& from, std::string const& to);
} // namespace comp6771::word_ladder
namespace comp6771 {
    // ...
    
    namespace word_ladder {
        std::vector<std::vector<std::string>>
        word_ladder(std::string const& from, std::string const& to);
    } // namespace word_ladder
} // namespace comp6771

Prefer top-level and occasionally two-tier namespaces to multi-tier.

It's okay to own multiple namespaces per project, if they logically separate things.

Unnamed namespaces

In C you had static functions that made functions local to a file.

C++ uses "unnamed" namespaces to achieve the same effect.

Functions that you don't want in your public interface should be put into unnamed namespaces.

namespace word_ladder {
    namespace {
        bool valid_word(std::string const& word);
    } // namespace
} // namespace word_ladder

Unlike named namespaces, it's okay to nest unnamed namespaces.

Namespace aliases

namespace views = ranges::views;
namespace chrono = std::chrono;

Gives a namespace a new name. Often good for shortening nested namespaces.

Always fully qualify your function calls...

int main() {
    auto const x = 10.0;
    auto const x2 = std::pow(x, 2);
    
    auto const ladders = word_ladder::generate("at", "it");
    
    auto const x2_as_int = gsl_lite::narrow_cast<int>(x2);
}

There are certain complex rules about how overload resolution works that will surprise you, so it's a best practice to always fully-qualify your function calls.

Using a namespace alias counts as "fully-qualified" only if the alias was fully qualified.

...even if you're in the same namespace

namespace word_ladder {
    namespace {
        bool valid_word(std::string const& word);
    } // namespace
    
    std::vector<std::vector<std::string>>
    generate(std::string const& from, std::string const& to) {
        // ...
        auto const result = word_ladder::valid_word(word);
        // ...
    }
} // namespace word_ladder

There are certain complex rules about how overload resolution works that will surprise you, so it's a best practice to always fully-qualify your function calls.

Using a namespace alias counts as "fully-qualified" only if the alias was fully qualified.

...even if you're in the same nested namespace

namespace word_ladder::something::very_long {
    namespace {
        bool valid_word(std::string const& word);
    } // namespace
    
    std::vector<std::vector<std::string>>
    generate(std::string const& from, std::string const& to) {
        // ...
        auto const result = word_ladder::something::very_long::valid_word(word);
        // ...
    }
} // namespace word_ladder

There are certain complex rules about how overload resolution works that will surprise you, so it's a best practice to always fully-qualify your function calls.

Using a namespace alias counts as "fully-qualified" only if the alias was fully qualified.

What is OOP

  • A class uses data abstraction and encapsulation to define an abstract data type:
    • Interface: the operations used by the user (an API)
    • Implementation: the data members the bodies of the functions in the interface and any other functions not intended for general use
    • Abstraction: separation of interface from implementation
      • Useful as class implementation can change over time
    • Encapsulation: enforcement of this via information hiding

 

Example: Bookstore :

  • bookstore.h (interface)
  • bookstore.cpp (implementation)
  • bookstore_main.cpp (knows the interface).

C++ classes

Since you've completed COMP2511 (or equivalent), C++ classes should be pretty straightforward and at a high level follow very similar principles.

  • A class:
    • Defines a new type
    • Is created using the keywords class or struct
    • May define some members (functions, data)
    • Contains zero or more public and private sections
    • Is instantiated through a constructor
  • A member function:
    • must be declared inside the class
    • may be defined inside the class (it is then inline by default)
    • may be declared const, when it doesn’t modify the data members
  • The data members should be private, representing the state of an object.

Member access control

This is how we support encapsulation and information hiding in C++

class foo {
 public:
  // Members accessible by everyone
  foo(); // The default constructor.

 protected:
  // Members accessible by members, friends, and subclasses
  // Will discuss this when we do advanced OOP in future weeks.

 private:
  // Accessible only by members and friends
  void private_member_function();
  int private_data_member_;

 public:
  // May define multiple sections of the same name
};

A simple example

C++ classes behave how you expect

#include <iostream>
#include <string>

class person {
public:
	person(std::string const& name, int age);
	auto get_name() -> std::string const&;
	auto get_age() -> int const&;

private:
	std::string name_;
	int age_;
};

person::person(std::string const& name, int const age) {
	name_ = name;
	age_ = age;
}

auto person::get_name() -> std::string const& {
	return name_;
}

auto person::get_age() -> int const& {
	return age_;
}

auto main() -> int {
	person p("Hayden", 99);
	std::cout << p.get_name() << "\n";
}

lecture-3/demo304-classbasic.cpp

Classes and structs in C++

  • A class and a struct in C++ are almost exactly the same
  • The only difference is that:
    • All members of a struct are public by default
    • All members of a class are private by default
    • People have all sorts of funny ideas about this. This is the only difference
  • We use structs only when we want a simple type with little or no methods and direct access to the data members (as a matter of style)
    • This is a semantic difference, not a technical one
    • A std::pair or std::tuple may be what you want, though
class foo {
 int member_; // default private
};
struct foo {
 int member_; // default public
};

Class Scope

  • Anything declared inside the class needs to be accessed through the scope of the class
    • Scopes are accessed using "::" in C++
// foo.h

class Foo {
 public:
  // Equiv to typedef int Age
  using Age = int;

  Foo();
  Foo(std::istream& is);
  ~Foo();

  void member_function();
};
// foo.cpp
#include "foo.h"

Foo::Foo() {
}

Foo::Foo(std::istream& is) {
}

Foo::~Foo() {
}

void Foo::member_function() {
  Foo::Age age;
  // Also valid, since we are inside the Foo scope.
  Age age;
}

Incomplete types

  • An incomplete type may only be used to define pointers and references, and in function declarations (but not definitions)
  • Because of the restriction on incomplete types, a class cannot have data members of its own type.

 

 

 

 

 

  • But the following is legal, since a class is considered declared once its class name has been seen:
struct node {
	int data;
	// Node is incomplete - this is invalid
	// This would also make no sense. What is sizeof(Node)
	node next;
};
struct node {
	int data;
	node* next;
};

Constructors

  • Constructors define how class data members are initalised
  • A constructor has the same name as the class and no return type
  • Default initalisation is handled through the default constructor
  • Unless we define our own constructors the compile will declare a default constructor
    • This is known as the synthesized default constructor
for each data member in declaration order
  if it has an in-class initialiser
    Initialise it using the in-class initialiser
  else if it is of a built-in type (numeric, pointer, bool, char, etc.)
    do nothing (leave it as whatever was in memory before)
  else
    Initialise it using its default constructor

The synthesized default constructor

  • Is generated for a class only if it declares no constructors
  • For each member, calls the in-class initialiser if present
    • Otherwise calls the default constructor (except for trivial types like int)
  • Cannot be generated when any data members are missing both in-class initialisers and default constructors
class C {
  int i_{0}; // in-class initialiser
  int j_; // Untouched memory
  A a_;
  // This stops default constructor
  // from being synthesized.
  B b_;
};
class A {
  int a_;
};
class B {
  B(int b): b_{b} {}
  int b_;
};

Constructor initialiser list

  • The initialisation phase occurs before the body of the constructor is executed, regardless of whether the initialiser list is supplied
  • A constructor will:
    1. Construct all data members in order of member declaration (using the same rules as those used to initialise variables)
    2. Execute the body of constructor: the code may assign values to the data members to override the initial values
#include <string>

class nodefault {
public:
	explicit nodefault(int i)
	: i_{i} {}

private:
	int i_;
};

int const b_default = 5;

class b {
	// Constructs s_ with value "Hello world"
	explicit b(int& i)
	: s_{"Hello world"} , const_{b_default} , no_default_{i} , ref_{i} {}

	// Doesn't work - constructed in order of member declaration.
	/*explicit b(int& i)
	: s_{"Hello world"} , const_{5} , ref_{i} , no_default_{ref_} {}*/

	/*explicit b(int& i) {
	   // Constructs s_ with an empty string, then reassigns it to "Hello world"
	   // Extra work done (but may be optimised out).
	   s_ = "Hello world";

	   // Fails to compile (modifying a const object).
	   const_string_ = "Goodbye world";
	   // Fails to compile (references *must* be initialized in the constructor).
	   ref_ = i;
	   // This is fine, but it can't construct it initially.
	   no_default_ = nodefault{1};
	}*/

	std::string s_;
	// All of these will break compilation if you attempt to put them in the body.
	const int const_;
	nodefault no_default_;
	int& ref_;
};

lecture-3/demo306-initlist.cpp

Delegating constructors

  • A constructor may call another constructor inside the initialiser list
    • Since the other constructor must construct all the data members, do not specify anything else in the constructor initialiser list
    • The other constructor is called completely before this one.
    • This is one of the few good uses for default values in C++
      • Default values may be used instead of overloading and delegating constructors

Delegating constructors

#include <string>

class dummy {
public:
	explicit dummy(int const& i)
	: s_{"Hello world"}
	, val_{i} {}
	explicit dummy()
	: dummy(5) {}
	std::string const& get_s() {
		return s_;
	}
	int get_val() {
		return val_;
	}

private:
	std::string s_;
	const int val_;
};

auto main() -> int {
	dummy d1(5);
	dummy d2{};
}

lecture-3/demo307-deleg.cpp

Destructors

  • Called when the object goes out of scope
    • What might this be handy for?
  • Why might destructors be handy?
    • Freeing pointers
    • Closing files
    • Unlocking mutexes (from multithreading)
    • Aborting database transactions
class MyClass {
  ~MyClass() noexcept;
};
MyClass::~MyClass() noexcept {
  // Definition here
}

Explicit type conversions

  • If a constructor for a class has 1 parameter, the compiler will create an implicit type conversion from the parameter to the class
  • This may be the behaviour you want (but usually not)
    • You have to opt-out of this implicit type conversion with the explicit keyword
class age {
public:
	age(int age)
	: age_{age} {}

private:
	int age_;
};

auto main() -> int {
	// Explicitly calling the constructor
	age a1{20};

	// Explicitly calling the constructor
	age a2 = age{20};

	// Attempts to use an integer
	// where an age is expected.
	// Implicit conversion done.
	// This seems reasonable.
	age a3 = 20;
}

#include <vector>

class intvec {
public:
	// This one allows the implicit conversion
	// intvec(std::vector<int>::size_type length)
	// : vec_(length, 0);

	// This one disallows it.
	explicit intvec(std::vector<int>::size_type length)
	: vec_(length, 0) {}

private:
	std::vector<int> vec_;
};

auto main() -> int {
	int const size = 20;
	// Explictly calling the constructor.
	intvec container1{size}; // Construction
	intvec container2 = intvec{size}; // Assignment

	// Implicit conversion.
	// Probably not what we want.
	// intvec container3 = size;
}

lecture-3/demo310-explicit1.cpp

lecture-3/demo310-explicit2.cpp

Const objects

  • Member functions are by default only be possible on non-const objects
    • You can declare a const member function which is valid on const objects
    • A const member function may only modify mutable members
      • A mutable member should mean that the state of the member can change without the state of the object changing
      • Good uses of mutable members are rare
      • Mutable is not something you should set lightly
      • One example where it might be useful is a cache

Const member functions

#include <iostream>
#include <string>

class person {
 public:
	person(std::string const& name) : name_{name} {}
    auto set_name(std::string const& name) -> void {
    	name_ = name;
    }
    auto get_name() -> std::string const& {
    	return name_;
    }

 private:
	std::string name_;
};

auto main() -> int {
	person p1{"Hayden"};
	p1.set_name("Chris");
	std::cout << p1.get_name() << "\n";
    
	person const p1{"Hayden"};
	p1.set_name("Chris"); // WILL NOT WORK... WHY NOT?
	std::cout << p1.get_name() << "\n"; // WILL NOT WORK... WHY NOT?
}

lecture-3/demo308-const.cpp

This pointer

  • A member function has an extra implicit parameter, named this
    • ​This is a pointer to the object on behalf of which the function is called
    • A member function does not explicitly define it, but may explicitly use it
    • The compiler treats an unqualified reference to a class member as being made through the this pointer.
  • For the next few slides, we'll be taking a look at the BookSale example in the course repo
class foo {
 public:
 	foo(int const miles) {
    	this->kilometres_ = miles / 1.159;
    }
 private:
	int kilometres_; // default private
};

class foo {
 public:
 	foo(int const miles) {
    	kilometres_ = miles / 1.159;
    }
 private:
	int kilometres_; // default private
};

Static members

  • Static functions and members belong to the class (i.e. every object), as opposed to a particular object.
  • These are essentially globals defined inside the scope of the class
    • Use static members when something is associated with a class, but not a particular instance
    • Static data has global lifetime (program start to program end)
// For use with a database
class user {
  public:
	user(std::string const& name) : name_{name} {}
	static auto valid_name(std::string const& name) -> bool {
		return name.length() < 20;
	}
  private:
	std::string name_;
}

auto main() -> int {
	auto n = std::string{"Santa Clause"};
	if (user::valid_name(n)) {
		user user1{n};
	}
}

lecture-3/demo309-static.cpp

OOP design

  • There are several special functions that we must consider when designing classes
  • For each of these functions, ask yourself:
    • Is it sane to be able to do this? Does it have a defined, obvious implementation?
  • If the answer to either of these is no, write "<function declaration> = delete;"
  • Then ask yourself "is this the behaviour of the compiler-synthesized one"
    • If so, write "<function declaration> = default;"
    • If not, write your own definition
  • Revise "The synthesized default constructor"
#include <vector>

class intvec {
public:
	// This one allows the implicit conversion
	explicit intvec(std::vector<int>::size_type length)
	: vec_(length, 0) {}
	// intvec(intvec const& v) = default;
	// intvec(intvec const& v) = delete;

private:
	std::vector<int> vec_;
};

auto main() -> int {
	intvec a{4};
	// intvec b{a}; // Will this work?
}

lecture-3/demo311-delete.cpp

COMP6771 20T2 - 3.1 - Class Types

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COMP6771 20T2 - 3.1 - Class Types

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