| language | filename | contributors | ||||||||||||||||||
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C++ |
learncpp.cpp |
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C++ is a systems programming language that, according to its inventor Bjarne Stroustrup, was designed to
- be a "better C"
- support data abstraction
- support object-oriented programming
- support generic programming
Though its syntax can be more difficult or complex than newer languages, it is widely used because it compiles to native instructions that can be directly run by the processor and offers tight control over hardware (like C) while offering high-level features such as generics, exceptions, and classes. This combination of speed and functionality makes C++ one of the most widely-used programming languages.
//////////////////
// Comparison to C
//////////////////
// C++ is almost a superset of C and shares its basic syntax for
// variable declarations, primitive types, and functions.
// Just like in C, your program's entry point is a function called
// main with an integer return type.
// This value serves as the program's exit status.
// See https://en.wikipedia.org/wiki/Exit_status for more information.
int main(int argc, char** argv)
{
// Command line arguments are passed in by argc and argv in the same way
// they are in C.
// argc indicates the number of arguments,
// and argv is an array of C-style strings (char*)
// representing the arguments.
// The first argument is the name by which the program was called.
// argc and argv can be omitted if you do not care about arguments,
// giving the function signature of int main()
// An exit status of 0 indicates success.
return 0;
}
// However, C++ varies in some of the following ways:
// In C++, character literals are chars, therefore the size is 1
sizeof('c') == sizeof(char)
// In C, character literals are ints, therefore the size is 4
sizeof('c') == sizeof(int)
// C++ has strict prototyping
void func(); // function which accepts no arguments
void func(void); // same as earlier
// In C
void func(); // function which may accept any number of arguments with unknown type
void func(void); // function which accepts no arguments
// Use nullptr instead of NULL in C++
int* ip = nullptr;
// Most C standard headers are available in C++.
// C headers generally end with .h, while
// C++ headers are prefixed with "c" and have no ".h" suffix.
// The C++ standard version:
#include <cstdio>
// The C standard version:
#include <stdio.h>
int main()
{
printf("Hello, world!\n");
return 0;
}
///////////////////////
// Function overloading
///////////////////////
// C++ supports function overloading
// provided each function takes different parameters.
void print(char const* myString)
{
printf("String %s\n", myString);
}
void print(int myInt)
{
printf("My int is %d\n", myInt);
}
int main()
{
print("Hello"); // Resolves to void print(const char*)
print(15); // Resolves to void print(int)
}
/////////////////////////////
// Default function arguments
/////////////////////////////
// You can provide default arguments for a function
// if they are not provided by the caller.
void doSomethingWithInts(int a = 1, int b = 4)
{
// Do something with the ints here
}
int main()
{
doSomethingWithInts(); // a = 1, b = 4
doSomethingWithInts(20); // a = 20, b = 4
doSomethingWithInts(20, 5); // a = 20, b = 5
}
// Default arguments must be at the end of the arguments list.
void invalidDeclaration(int a = 1, int b) // Error!
{
}
/////////////
// Namespaces
/////////////
// Namespaces provide separate scopes for variable, function,
// and other declarations.
// Namespaces can be nested.
namespace First {
namespace Nested {
void foo()
{
printf("This is First::Nested::foo\n");
}
} // end namespace Nested
} // end namespace First
namespace Second {
void foo()
{
printf("This is Second::foo\n");
}
void bar()
{
printf("This is Second::bar\n");
}
}
void foo()
{
printf("This is global foo\n");
}
int main()
{
// Includes all symbols from namespace Second into the current scope. Note
// that while bar() works, simply using foo() no longer works, since it is
// now ambiguous whether we're calling the foo in namespace Second or the
// top level.
using namespace Second;
bar(); // prints "This is Second::bar"
Second::foo(); // prints "This is Second::foo"
First::Nested::foo(); // prints "This is First::Nested::foo"
::foo(); // prints "This is global foo"
}
///////////////
// Input/Output
///////////////
// C++ input and output uses streams
// cin, cout, and cerr represent stdin, stdout, and stderr.
// << is the insertion operator and >> is the extraction operator.
#include <iostream> // Include for I/O streams
int main()
{
int myInt;
// Prints to stdout (or terminal/screen)
// std::cout referring the access to the std namespace
std::cout << "Enter your favorite number:\n";
// Takes in input
std::cin >> myInt;
// cout can also be formatted
std::cout << "Your favorite number is " << myInt << '\n';
// prints "Your favorite number is <myInt>"
std::cerr << "Used for error messages";
// flush string stream buffer with new line
std::cout << "I flushed it away" << std::endl;
}
//////////
// Strings
//////////
// Strings in C++ are objects and have many member functions
#include <string>
std::string myString = "Hello";
std::string myOtherString = " World";
// + is used for concatenation.
std::cout << myString + myOtherString; // "Hello World"
std::cout << myString + " You"; // "Hello You"
// C++ string length can be found from either string::length() or string::size()
cout << myString.length() + myOtherString.size(); // Outputs 11 (= 5 + 6).
// C++ strings are mutable.
myString.append(" Dog");
std::cout << myString; // "Hello Dog"
// C++ can handle C-style strings with related functions using cstrings
#include <cstring>
char myOldString[10] = "Hello CPP";
cout << myOldString;
cout << "Length = " << strlen(myOldString); // Length = 9
/////////////
// References
/////////////
// In addition to pointers like the ones in C,
// C++ has _references_.
// These are pointer types that cannot be reassigned once set
// and cannot be null.
// They also have the same syntax as the variable itself:
// No * is needed for dereferencing and
// & (address of) is not used for assignment.
std::string foo = "I am foo";
std::string bar = "I am bar";
std::string& fooRef = foo; // This creates a reference to foo.
fooRef += ". Hi!"; // Modifies foo through the reference
std::cout << fooRef; // Prints "I am foo. Hi!"
std::cout << &fooRef << '\n'; // Prints the address of foo
// Doesn't reassign "fooRef". This is the same as "foo = bar", and
// foo == "I am bar"
// after this line.
fooRef = bar;
std::cout << &fooRef << '\n'; // Still prints the address of foo
std::cout << fooRef << '\n'; // Prints "I am bar"
// The address of fooRef remains the same, i.e. it is still referring to foo.
const std::string& barRef = bar; // Create a const reference to bar.
// Like C, const values (and pointers and references) cannot be modified.
barRef += ". Hi!"; // Error, const references cannot be modified.
// Sidetrack: Before we talk more about references, we must introduce a concept
// called a temporary object. Suppose we have the following code:
std::string tempObjectFun() { ... }
std::string retVal = tempObjectFun();
// What happens in the second line is actually:
// - a string object is returned from tempObjectFun
// - a new string is constructed with the returned object as argument to the
// constructor
// - the returned object is destroyed
// The returned object is called a temporary object. Temporary objects are
// created whenever a function returns an object, and they are destroyed at the
// end of the evaluation of the enclosing expression (Well, this is what the
// standard says, but compilers are allowed to change this behavior. Look up
// "return value optimization" if you're into this kind of details). So in this
// code:
foo(bar(tempObjectFun()))
// assuming foo and bar exist, the object returned from tempObjectFun is
// passed to bar, and it is destroyed before foo is called.
// Now back to references. The exception to the "at the end of the enclosing
// expression" rule is if a temporary object is bound to a const reference, in
// which case its life gets extended to the current scope:
void constReferenceTempObjectFun() {
// constRef gets the temporary object, and it is valid until the end of this
// function.
const std::string& constRef = tempObjectFun();
...
}
// Another kind of reference introduced in C++11 is specifically for temporary
// objects. You cannot have a variable of its type, but it takes precedence in
// overload resolution:
void someFun(std::string& s) { ... } // Regular reference
void someFun(std::string&& s) { ... } // Reference to temporary object
std::string foo;
someFun(foo); // Calls the version with regular reference
someFun(tempObjectFun()); // Calls the version with temporary reference
// For example, you will see these two versions of constructors for
// std::basic_string:
std::basic_string(const basic_string& other);
std::basic_string(basic_string&& other);
// Idea being if we are constructing a new string from a temporary object (which
// is going to be destroyed soon anyway), we can have a more efficient
// constructor that "salvages" parts of that temporary string. You will see this
// concept referred to as "move semantics".
/////////////////////
// Enums
/////////////////////
// Enums are a way to assign a value to a constant most commonly used for
// easier visualization and reading of code
enum ECarTypes
{
Sedan,
Hatchback,
SUV,
Wagon
};
ECarTypes GetPreferredCarType()
{
return ECarTypes::Hatchback;
}
// As of C++11 there is an easy way to assign a type to the enum which can be
// useful in serialization of data and converting enums back-and-forth between
// the desired type and their respective constants
enum ECarTypes : uint8_t
{
Sedan, // 0
Hatchback, // 1
SUV = 254, // 254
Hybrid // 255
};
void WriteByteToFile(uint8_t InputValue)
{
// Serialize the InputValue to a file
}
void WritePreferredCarTypeToFile(ECarTypes InputCarType)
{
// The enum is implicitly converted to a uint8_t due to its declared enum type
WriteByteToFile(InputCarType);
}
// On the other hand you may not want enums to be accidentally cast to an integer
// type or to other enums so it is instead possible to create an enum class which
// won't be implicitly converted
enum class ECarTypes : uint8_t
{
Sedan, // 0
Hatchback, // 1
SUV = 254, // 254
Hybrid // 255
};
void WriteByteToFile(uint8_t InputValue)
{
// Serialize the InputValue to a file
}
void WritePreferredCarTypeToFile(ECarTypes InputCarType)
{
// Won't compile even though ECarTypes is a uint8_t due to the enum
// being declared as an "enum class"!
WriteByteToFile(InputCarType);
}
//////////////////////////////////////////
// Classes and object-oriented programming
//////////////////////////////////////////
// First example of classes
#include <iostream>
// Declare a class.
// Classes are usually declared in header (.h or .hpp) files.
class Dog {
// Member variables and functions are private by default.
std::string name;
int weight;
// All members following this are public
// until "private:" or "protected:" is found.
public:
// Default constructor
Dog();
// Member function declarations (implementations to follow)
// Note that we use std::string here instead of placing
// using namespace std;
// above.
// Never put a "using namespace" statement in a header.
void setName(const std::string& dogsName);
void setWeight(int dogsWeight);
// Functions that do not modify the state of the object
// should be marked as const.
// This allows you to call them if given a const reference to the object.
// Also note the functions must be explicitly declared as _virtual_
// in order to be overridden in derived classes.
// Functions are not virtual by default for performance reasons.
virtual void print() const;
// Functions can also be defined inside the class body.
// Functions defined as such are automatically inlined.
void bark() const { std::cout << name << " barks!\n"; }
// Along with constructors, C++ provides destructors.
// These are called when an object is deleted or falls out of scope.
// This enables powerful paradigms such as RAII
// (see below)
// The destructor should be virtual if a class is to be derived from;
// if it is not virtual, then the derived class' destructor will
// not be called if the object is destroyed through a base-class reference
// or pointer.
virtual ~Dog();
}; // A semicolon must follow the class definition.
// Class member functions are usually implemented in .cpp files.
Dog::Dog()
{
std::cout << "A dog has been constructed\n";
}
// Objects (such as strings) should be passed by reference
// if you are modifying them or const reference if you are not.
void Dog::setName(const std::string& dogsName)
{
name = dogsName;
}
void Dog::setWeight(int dogsWeight)
{
weight = dogsWeight;
}
// Notice that "virtual" is only needed in the declaration, not the definition.
void Dog::print() const
{
std::cout << "Dog is " << name << " and weighs " << weight << "kg\n";
}
Dog::~Dog()
{
std::cout << "Goodbye " << name << '\n';
}
int main() {
Dog myDog; // prints "A dog has been constructed"
myDog.setName("Barkley");
myDog.setWeight(10);
myDog.print(); // prints "Dog is Barkley and weighs 10 kg"
return 0;
} // prints "Goodbye Barkley"
// Inheritance:
// This class inherits everything public and protected from the Dog class
// as well as private but may not directly access private members/methods
// without a public or protected method for doing so
class OwnedDog : public Dog {
public:
void setOwner(const std::string& dogsOwner);
// Override the behavior of the print function for all OwnedDogs. See
// https://en.wikipedia.org/wiki/Polymorphism_(computer_science)#Subtyping
// for a more general introduction if you are unfamiliar with
// subtype polymorphism.
// The override keyword is optional but makes sure you are actually
// overriding the method in a base class.
void print() const override;
private:
std::string owner;
};
// Meanwhile, in the corresponding .cpp file:
void OwnedDog::setOwner(const std::string& dogsOwner)
{
owner = dogsOwner;
}
void OwnedDog::print() const
{
Dog::print(); // Call the print function in the base Dog class
std::cout << "Dog is owned by " << owner << '\n';
// Prints "Dog is <name> and weights <weight>"
// "Dog is owned by <owner>"
}
//////////////////////////////////////////
// Initialization and Operator Overloading
//////////////////////////////////////////
// In C++ you can overload the behavior of operators such as +, -, *, /, etc.
// This is done by defining a function which is called
// whenever the operator is used.
#include <iostream>
using namespace std;
class Point {
public:
// Member variables can be given default values in this manner.
double x = 0;
double y = 0;
// Define a default constructor which does nothing
// but initialize the Point to the default value (0, 0)
Point() { };
// The following syntax is known as an initialization list
// and is the proper way to initialize class member values
Point (double a, double b) :
x(a),
y(b)
{ /* Do nothing except initialize the values */ }
// Overload the + operator.
Point operator+(const Point& rhs) const;
// Overload the += operator
Point& operator+=(const Point& rhs);
// It would also make sense to add the - and -= operators,
// but we will skip those for brevity.
};
Point Point::operator+(const Point& rhs) const
{
// Create a new point that is the sum of this one and rhs.
return Point(x + rhs.x, y + rhs.y);
}
// It's good practice to return a reference to the leftmost variable of
// an assignment. `(a += b) == c` will work this way.
Point& Point::operator+=(const Point& rhs)
{
x += rhs.x;
y += rhs.y;
// `this` is a pointer to the object, on which a method is called.
return *this;
}
int main () {
Point up (0,1);
Point right (1,0);
// This calls the Point + operator
// Point up calls the + (function) with right as its parameter
Point result = up + right;
// Prints "Result is upright (1,1)"
std::cout << "Result is upright (" << result.x << ',' << result.y << ")\n";
return 0;
}
/////////////////////
// Templates
/////////////////////
// Templates in C++ are mostly used for generic programming, though they are
// much more powerful than generic constructs in other languages. They also
// support explicit and partial specialization and functional-style type
// classes; in fact, they are a Turing-complete functional language embedded
// in C++!
// We start with the kind of generic programming you might be familiar with. To
// define a class or function that takes a type parameter:
template<class T>
class Box {
public:
// In this class, T can be used as any other type.
void insert(const T&) { ... }
};
// During compilation, the compiler actually generates copies of each template
// with parameters substituted, so the full definition of the class must be
// present at each invocation. This is why you will see template classes defined
// entirely in header files.
// To instantiate a template class on the stack:
Box<int> intBox;
// and you can use it as you would expect:
intBox.insert(123);
// You can, of course, nest templates:
Box<Box<int> > boxOfBox;
boxOfBox.insert(intBox);
// Until C++11, you had to place a space between the two '>'s, otherwise '>>'
// would be parsed as the right shift operator.
// You will sometimes see
// template<typename T>
// instead. The 'class' keyword and 'typename' keywords are _mostly_
// interchangeable in this case. For the full explanation, see
// https://en.wikipedia.org/wiki/Typename
// (yes, that keyword has its own Wikipedia page).
// Similarly, a template function:
template<class T>
void barkThreeTimes(const T& input)
{
input.bark();
input.bark();
input.bark();
}
// Notice that nothing is specified about the type parameters here. The compiler
// will generate and then type-check every invocation of the template, so the
// above function works with any type 'T' that has a const 'bark' method!
Dog fluffy;
fluffy.setName("Fluffy")
barkThreeTimes(fluffy); // Prints "Fluffy barks" three times.
// Template parameters don't have to be classes:
template<int Y>
void printMessage() {
std::cout << "Learn C++ in " << Y << " minutes!\n";
}
// And you can explicitly specialize templates for more efficient code. Of
// course, most real-world uses of specialization are not as trivial as this.
// Note that you still need to declare the function (or class) as a template
// even if you explicitly specified all parameters.
template<>
void printMessage<10>() {
std::cout << "Learn C++ faster in only 10 minutes!\n";
}
printMessage<20>(); // Prints "Learn C++ in 20 minutes!"
printMessage<10>(); // Prints "Learn C++ faster in only 10 minutes!"
/////////////////////
// Exception Handling
/////////////////////
// The standard library provides a few exception types
// (see https://en.cppreference.com/w/cpp/error/exception)
// but any type can be thrown as an exception
#include <exception>
#include <stdexcept>
// All exceptions thrown inside the _try_ block can be caught by subsequent
// _catch_ handlers.
try {
// Do not allocate exceptions on the heap using _new_.
throw std::runtime_error("A problem occurred");
}
// Catch exceptions by const reference if they are objects
catch (const std::exception& ex)
{
std::cout << ex.what();
}
// Catches any exception not caught by previous _catch_ blocks
catch (...)
{
std::cout << "Unknown exception caught";
throw; // Re-throws the exception
}
///////
// RAII
///////
// RAII stands for "Resource Acquisition Is Initialization".
// It is often considered the most powerful paradigm in C++
// and is the simple concept that a constructor for an object
// acquires that object's resources and the destructor releases them.
// To understand how this is useful,
// consider a function that uses a C file handle:
void doSomethingWithAFile(const char* filename)
{
// To begin with, assume nothing can fail.
FILE* fh = fopen(filename, "r"); // Open the file in read mode.
if (fh == NULL) {
// Handle possible error
}
doSomethingWithTheFile(fh);
doSomethingElseWithIt(fh);
fclose(fh); // Close the file handle.
}
// Unfortunately, things are quickly complicated by error handling.
// Suppose fopen can fail, and that doSomethingWithTheFile and
// doSomethingElseWithIt return error codes if they fail.
// (Exceptions are the preferred way of handling failure,
// but some programmers, especially those with a C background,
// disagree on the utility of exceptions).
// We now have to check each call for failure and close the file handle
// if a problem occurred.
bool doSomethingWithAFile(const char* filename)
{
FILE* fh = fopen(filename, "r"); // Open the file in read mode
if (fh == nullptr) // The returned pointer is null on failure.
return false; // Report that failure to the caller.
// Assume each function returns false if it failed
if (!doSomethingWithTheFile(fh)) {
fclose(fh); // Close the file handle so it doesn't leak.
return false; // Propagate the error.
}
if (!doSomethingElseWithIt(fh)) {
fclose(fh); // Close the file handle so it doesn't leak.
return false; // Propagate the error.
}
fclose(fh); // Close the file handle so it doesn't leak.
return true; // Indicate success
}
// C programmers often clean this up a little bit using goto:
bool doSomethingWithAFile(const char* filename)
{
FILE* fh = fopen(filename, "r");
if (fh == nullptr)
return false;
if (!doSomethingWithTheFile(fh))
goto failure;
if (!doSomethingElseWithIt(fh))
goto failure;
fclose(fh); // Close the file
return true; // Indicate success
failure:
fclose(fh);
return false; // Propagate the error
}
// If the functions indicate errors using exceptions,
// things are a little cleaner, but still sub-optimal.
void doSomethingWithAFile(const char* filename)
{
FILE* fh = fopen(filename, "r"); // Open the file in shared_ptrread mode
if (fh == nullptr)
throw std::runtime_error("Could not open the file.");
try {
doSomethingWithTheFile(fh);
doSomethingElseWithIt(fh);
}
catch (...) {
fclose(fh); // Be sure to close the file if an error occurs.
throw; // Then re-throw the exception.
}
fclose(fh); // Close the file
// Everything succeeded
}
// Compare this to the use of C++'s file stream class (fstream)
// fstream uses its destructor to close the file.
// Recall from above that destructors are automatically called
// whenever an object falls out of scope.
void doSomethingWithAFile(const std::string& filename)
{
// ifstream is short for input file stream
std::ifstream fh(filename); // Open the file
// Do things with the file
doSomethingWithTheFile(fh);
doSomethingElseWithIt(fh);
} // The file is automatically closed here by the destructor
// This has _massive_ advantages:
// 1. No matter what happens,
// the resource (in this case the file handle) will be cleaned up.
// Once you write the destructor correctly,
// It is _impossible_ to forget to close the handle and leak the resource.
// 2. Note that the code is much cleaner.
// The destructor handles closing the file behind the scenes
// without you having to worry about it.
// 3. The code is exception safe.
// An exception can be thrown anywhere in the function and cleanup
// will still occur.
// All idiomatic C++ code uses RAII extensively for all resources.
// Additional examples include
// - Memory using unique_ptr and shared_ptr
// - Containers - the standard library linked list,
// vector (i.e. self-resizing array), hash maps, and so on
// all automatically destroy their contents when they fall out of scope.
// - Mutexes using lock_guard and unique_lock
/////////////////////
// Smart Pointer
/////////////////////
// Generally a smart pointer is a class which wraps a "raw pointer" (usage of "new"
// respectively malloc/calloc in C). The goal is to be able to
// manage the lifetime of the object being pointed to without ever needing to explicitly delete
// the object. The term itself simply describes a set of pointers with the
// mentioned abstraction.
// Smart pointers should be preferred over raw pointers, to prevent
// risky memory leaks, which happen if you forget to delete an object.
// Usage of a raw pointer:
Dog* ptr = new Dog();
ptr->bark();
delete ptr;
// By using a smart pointer, you don't have to worry about the deletion
// of the object anymore.
// A smart pointer describes a policy, to count the references to the
// pointer. The object gets destroyed when the last
// reference to the object gets destroyed.
// Usage of "std::shared_ptr":
void foo()
{
// It's no longer necessary to delete the Dog.
std::shared_ptr<Dog> doggo(new Dog());
doggo->bark();
}
// Beware of possible circular references!!!
// There will be always a reference, so it will be never destroyed!
std::shared_ptr<Dog> doggo_one(new Dog());
std::shared_ptr<Dog> doggo_two(new Dog());
doggo_one = doggo_two; // p1 references p2
doggo_two = doggo_one; // p2 references p1
// There are several kinds of smart pointers.
// The way you have to use them is always the same.
// This leads us to the question: when should we use each kind of smart pointer?
// std::unique_ptr - use it when you just want to hold one reference to
// the object.
// std::shared_ptr - use it when you want to hold multiple references to the
// same object and want to make sure that it's deallocated
// when all references are gone.
// std::weak_ptr - use it when you want to access
// the underlying object of a std::shared_ptr without causing that object to stay allocated.
// Weak pointers are used to prevent circular referencing.
/////////////////////
// Containers
/////////////////////
// Containers or the Standard Template Library are some predefined templates.
// They manage the storage space for its elements and provide
// member functions to access and manipulate them.
// Few containers are as follows:
// Vector (Dynamic array)
// Allow us to Define the Array or list of objects at run time
#include <vector>
std::string val;
std::vector<string> my_vector; // initialize the vector
std::cin >> val;
my_vector.push_back(val); // will push the value of 'val' into vector ("array") my_vector
my_vector.push_back(val); // will push the value into the vector again (now having two elements)
// To iterate through a vector we have 2 choices:
// Either classic looping (iterating through the vector from index 0 to its last index):
for (int i = 0; i < my_vector.size(); i++) {
std::cout << my_vector[i] << '\n'; // for accessing a vector's element we can use the operator []
}
// or using an iterator:
vector<string>::iterator it; // initialize the iterator for vector
for (it = my_vector.begin(); it != my_vector.end(); ++it) {
std::cout << *it << '\n';
}
// Set
// Sets are containers that store unique elements following a specific order.
// Set is a very useful container to store unique values in sorted order
// without any other functions or code.
#include<set>
std::set<int> ST; // Will initialize the set of int data type
ST.insert(30); // Will insert the value 30 in set ST
ST.insert(10); // Will insert the value 10 in set ST
ST.insert(20); // Will insert the value 20 in set ST
ST.insert(30); // Will insert the value 30 in set ST
// Now elements of sets are as follows
// 10 20 30
// To erase an element
ST.erase(20); // Will erase element with value 20
// Set ST: 10 30
// To iterate through Set we use iterators
std::set<int>::iterator it;
for (it = ST.begin(); it != ST.end(); it++) {
std::cout << *it << '\n';
}
// Output:
// 10
// 30
// To clear the complete container we use Container_name.clear()
ST.clear();
std::cout << ST.size(); // will print the size of set ST
// Output: 0
// NOTE: for duplicate elements we can use multiset
// NOTE: For hash sets, use unordered_set. They are more efficient but
// do not preserve order. unordered_set is available since C++11
// Map
// Maps store elements formed by a combination of a key value
// and a mapped value, following a specific order.
#include<map>
std::map<char, int> mymap; // Will initialize the map with key as char and value as int
mymap.insert(pair<char,int>('A',1));
// Will insert value 1 for key A
mymap.insert(pair<char,int>('Z',26));
// Will insert value 26 for key Z
// To iterate
std::map<char,int>::iterator it;
for (it = mymap.begin(); it != mymap.end(); ++it)
std::cout << it->first << "->" << it->second << '\n';
// Output:
// A->1
// Z->26
// To find the value corresponding to a key
it = mymap.find('Z');
std::cout << it->second;
// Output: 26
// NOTE: For hash maps, use unordered_map. They are more efficient but do
// not preserve order. unordered_map is available since C++11.
// Containers with object keys of non-primitive values (custom classes) require
// compare function in the object itself or as a function pointer. Primitives
// have default comparators, but you can override it.
class Foo {
public:
int j;
Foo(int a) : j(a) {}
};
struct compareFunction {
bool operator()(const Foo& a, const Foo& b) const {
return a.j < b.j;
}
};
// this isn't allowed (although it can vary depending on compiler)
// std::map<Foo, int> fooMap;
std::map<Foo, int, compareFunction> fooMap;
fooMap[Foo(1)] = 1;
fooMap.find(Foo(1)); //true
///////////////////////////////////////
// Lambda Expressions (C++11 and above)
///////////////////////////////////////
// lambdas are a convenient way of defining an anonymous function
// object right at the location where it is invoked or passed as
// an argument to a function.
// For example, consider sorting a vector of pairs using the second
// value of the pair
std::vector<pair<int, int> > tester;
tester.push_back(make_pair(3, 6));
tester.push_back(make_pair(1, 9));
tester.push_back(make_pair(5, 0));
// Pass a lambda expression as third argument to the sort function
// sort is from the <algorithm> header
std::sort(tester.begin(), tester.end(), [](const pair<int, int>& lhs, const pair<int, int>& rhs) {
return lhs.second < rhs.second;
});
// Notice the syntax of the lambda expression,
// [] in the lambda is used to "capture" variables
// The "Capture List" defines what from the outside of the lambda should be available inside the function body and how.
// It can be either:
// 1. a value : [x]
// 2. a reference : [&x]
// 3. any variable currently in scope by reference [&]
// 4. same as 3, but by value [=]
// Example:
std::vector<int> dog_ids;
// number_of_dogs = 3;
for (int i = 0; i < 3; i++) {
dog_ids.push_back(i);
}
int weight[3] = {30, 50, 10};
// Say you want to sort dog_ids according to the dogs' weights
// So dog_ids should in the end become: [2, 0, 1]
// Here's where lambda expressions come in handy
sort(dog_ids.begin(), dog_ids.end(), [&weight](const int &lhs, const int &rhs) {
return weight[lhs] < weight[rhs];
});
// Note we captured "weight" by reference in the above example.
// More on Lambdas in C++ : https://stackoverflow.com/questions/7627098/what-is-a-lambda-expression-in-c11
///////////////////////////////
// Range For (C++11 and above)
///////////////////////////////
// You can use a range for loop to iterate over a container
int arr[] = {1, 10, 3};
for (int elem: arr) {
cout << elem << endl;
}
// You can use "auto" and not worry about the type of the elements of the container
// For example:
for (auto elem: arr) {
// Do something with each element of arr
}
/////////////////////
// Fun stuff
/////////////////////
// Aspects of C++ that may be surprising to newcomers (and even some veterans).
// This section is, unfortunately, wildly incomplete; C++ is one of the easiest
// languages with which to shoot yourself in the foot.
// You can override private methods!
class Foo {
virtual void bar();
};
class FooSub : public Foo {
virtual void bar(); // Overrides Foo::bar!
};
// 0 == false == NULL (most of the time)!
bool* pt = new bool;
*pt = 0; // Sets the value points by 'pt' to false.
pt = 0; // Sets 'pt' to the null pointer. Both lines compile without warnings.
// nullptr is supposed to fix some of that issue:
int* pt2 = new int;
*pt2 = nullptr; // Doesn't compile
pt2 = nullptr; // Sets pt2 to null.
// There is an exception made for bools.
// This is to allow you to test for null pointers with if(!ptr),
// but as a consequence you can assign nullptr to a bool directly!
*pt = nullptr; // This still compiles, even though '*pt' is a bool!
// '=' != '=' != '='!
// Calls Foo::Foo(const Foo&) or some variant (see move semantics) copy
// constructor.
Foo f2;
Foo f1 = f2;
// Calls Foo::Foo(const Foo&) or variant, but only copies the 'Foo' part of
// 'fooSub'. Any extra members of 'fooSub' are discarded. This sometimes
// horrifying behavior is called "object slicing."
FooSub fooSub;
Foo f1 = fooSub;
// Calls Foo::operator=(Foo&) or variant.
Foo f1;
f1 = f2;
///////////////////////////////////////
// Tuples (C++11 and above)
///////////////////////////////////////
#include<tuple>
// Conceptually, Tuples are similar to old data structures (C-like structs)
// but instead of having named data members,
// its elements are accessed by their order in the tuple.
// We start with constructing a tuple.
// Packing values into tuple
auto first = make_tuple(10, 'A');
const int maxN = 1e9;
const int maxL = 15;
auto second = make_tuple(maxN, maxL);
// Printing elements of 'first' tuple
std::cout << get<0>(first) << " " << get<1>(first) << '\n'; //prints : 10 A
// Printing elements of 'second' tuple
std::cout << get<0>(second) << " " << get<1>(second) << '\n'; // prints: 1000000000 15
// Unpacking tuple into variables
int first_int;
char first_char;
tie(first_int, first_char) = first;
std::cout << first_int << " " << first_char << '\n'; // prints : 10 A
// We can also create tuple like this.
tuple<int, char, double> third(11, 'A', 3.14141);
// tuple_size returns number of elements in a tuple (as a constexpr)
std::cout << tuple_size<decltype(third)>::value << '\n'; // prints: 3
// tuple_cat concatenates the elements of all the tuples in the same order.
auto concatenated_tuple = tuple_cat(first, second, third);
// concatenated_tuple becomes = (10, 'A', 1e9, 15, 11, 'A', 3.14141)
std::cout << get<0>(concatenated_tuple) << '\n'; // prints: 10
std::cout << get<3>(concatenated_tuple) << '\n'; // prints: 15
std::cout << get<5>(concatenated_tuple) << '\n'; // prints: 'A'
///////////////////////////////////
// Logical and Bitwise operators
//////////////////////////////////
// Most of the operators in C++ are same as in other languages
// Logical operators
// C++ uses Short-circuit evaluation for boolean expressions, i.e, the second argument is executed or
// evaluated only if the first argument does not suffice to determine the value of the expression
true && false // Performs **logical and** to yield false
true || false // Performs **logical or** to yield true
! true // Performs **logical not** to yield false
// Instead of using symbols equivalent keywords can be used
true and false // Performs **logical and** to yield false
true or false // Performs **logical or** to yield true
not true // Performs **logical not** to yield false
// Bitwise operators
// **<<** Left Shift Operator
// << shifts bits to the left
4 << 1 // Shifts bits of 4 to left by 1 to give 8
// x << n can be thought as x * 2^n
// **>>** Right Shift Operator
// >> shifts bits to the right
4 >> 1 // Shifts bits of 4 to right by 1 to give 2
// x >> n can be thought as x / 2^n
~4 // Performs a bitwise not
4 | 3 // Performs bitwise or
4 & 3 // Performs bitwise and
4 ^ 3 // Performs bitwise xor
// Equivalent keywords are
compl 4 // Performs a bitwise not
4 bitor 3 // Performs bitwise or
4 bitand 3 // Performs bitwise and
4 xor 3 // Performs bitwise xor- An up-to-date language reference can be found at CPP Reference.
- A tutorial for beginners or experts, covering many modern features and good practices: LearnCpp.com
- A tutorial covering basics of language and setting up coding environment is available at TheChernoProject - C++.
- Additional resources may be found at CPlusPlus.