Merge pull request #800 from mrkline/master

C++ Improvements

c++.html.markdown

@@ -3,326 +3,414 @@ language: c++
 filename: learncpp.cpp
 contributors:
     - ["Steven Basart", "http://github.com/xksteven"]
+    - ["Matt Kline", "https://github.com/mrkline"]
 lang: en
 ---
 
-I am writing this to highlight the differences and 
+C++ is a systems programming language that,
-additions that C++ has with respect to C. My 
+[according to its inventor Bjarne Stroustrup](http://channel9.msdn.com/Events/Lang-NEXT/Lang-NEXT-2014/Keynote),
-suggestion would be to follow the C tutorial first
+was designed to
-then look here for the additions and differences.
+
+- 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.
 
 ```c++
-///////////////////////////////////////   
+//////////////////
-// C++ differences   
+// Comparison to C
-///////////////////////////////////////  
+//////////////////
 
+// C++ is _almost_ a superset of C and shares its basic syntax for
+// variable declarations, primitive types, and functions.
+// However, C++ varies in some of the following ways:
 
-//In C++   
+// A main() function in C++ should return an int,
-//cannot use void main()   
+// though void main() is accepted by most compilers (gcc, clang, etc.)
-int main() { //or int main(int argc, char **argv)   
+// This value serves as the program's exit status.
-    //cannot end with return;   
+// See http://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;
-    //Can also end without return statement   
 }
 
-//In C++  
+// In C++, character literals are one byte.
-/*    
-  //This could lead to compiler errors and is discouraged    
-  //#if 0 #endif pairs are encouraged instead   
-*/   
-   
-//In C++    
-sizeof(10) //Typically 4   
 sizeof('c') == 1
 
-//In C   
+// In C, character literals are the same size as ints.
-sizeof('c') == sizeof(10) //true chars are passed as ints   
+sizeof('c') == sizeof(10)
 
 
-//In C++ strict prototyping   
+// C++ has strict prototyping
-void func(); //function which accepts no arguments   
+void func(); // function which accepts no arguments
 
-//In C   
+// In C
-void func(); //function which may accept arguments    
+void func(); // function which may accept any number of arguments
 
+// Use nullptr instead of NULL in C++
+int* ip = nullptr;
 
-//In C++   
+// C standard headers are available in C++,
-for(int i = 0; i < 10; i++) {;}   
+// but are prefixed with "c" and have no .h suffix.
-//In C must int i must be declared before   
+#include <cstdio>
-
-
-//C++ Supports Function overloading   
-//Provided each function takes different   
-//parameters    
-
-void printing(char const *myString)    
-{printf("String %s\n",myString);} //Hello   
-
-void printing(int myInt)    
-{printf("My int is %d",myInt);} //15   
-
-int main ()    
-{   
-    printing("Hello");   
-    printing(15);   
-}   
-   
-
-
-//C++ Default Function Arguments   
-void two_ints(int a = 1, int b = 4);   
 
 int main()
 {
-    two_ints();            // arguments:  1, 4   
+    printf("Hello, world!\n");
-    two_ints(20);          // arguments: 20, 4   
+    return 0;
-    two_ints(20, 5);       // arguments: 20, 5   
 }
 
+///////////////////////
+// Function overloading
+///////////////////////
 
-//C++ added the nullptr which is different from 0   
+// C++ supports function overloading
-int *ip = nullptr;      // OK   
+// provided each function takes different parameters.
-int value = nullptr;    // error: value is no pointer  
 
+void print(char const* myString)
+{
+    printf("String %s\n", myString);
+}
 
-///////////////////////////////////////   
+void print(int myInt)
-// C++ Additions ontop of C    
+{
-///////////////////////////////////////   
+    printf("My int is %d", myInt);
-
+}
-
-///////////////////////////////////////   
-// C++ Namespace   
-///////////////////////////////////////   
-
-//Namespaces allow you to define your own    
-//functions and variables for use    
-
-// Use '::' to change variable (or function) scope   
-// Putting '::' before a function or variable will   
-// reference a global scope    
-
-// This allows you to make normal c library calls   
-// std is for standard library    
-using namespace std;   
-
-#include <stdio.h>    
-
-int counter = 50;                // global variable    
 
 int main()
 {
-    for (int counter = 1;        // this refers to the   
+    print("Hello"); // Resolves to void print(const char*)
-    counter < 2;                 // local variable   
+    print(15); // Resolves to void print(int)
-    counter++)   
+}
+
+/////////////////////////////
+// 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("Global var %d local var %d\n",    
+            printf("This is First::Nested::foo\n");
-            ::counter,           // global variable    
+        }
-            counter);            // local variable    
+    } // end namespace Nested
-        // => Global var 50 local var 1   
+} // end namespace First
+
+namespace Second {
+    void foo()
+    {
+        printf("This is Second::foo\n")
     }
 }
 
-// Namespaces can be nested   
+void foo()
-
-
-namespace myFirstNameSpace   
 {
-    namespace myInnerSoul   
+    printf("This is global foo\n");
-    {   
-        cos(int x)   
-        {   
-            printf("My inner soul was made to program.");   
-        }   
-    }   
-}   
-
-namespace anotherNameSpace  
-{   
-    cos(int x) {;} //does nothing   
 }
 
 int main()
 {
-    //Specify the full path because main is outside of both namespaces.    
+    // Assume everything is from the namespace "Second"
-    //Will print out My inner soul was made to program.    
+    // unless otherwise specified.
-    myFirstNameSpace::myInnerSoul::cos(60);   
+    using namespace 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
-// C++ Strings   
+// cin, cout, and cerr represent stdin, stdout, and stderr.
-///////////////////////////////////////   
+// << is the insertion operator and >> is the extraction operator.
 
-//Strings in C++ are Objects and have many functions  
+#include <iostream> // Include for I/O streams
-myString = "Hello";   
-myOtherString = " World";   
 
-myString + myOtherString; // => "Hello World"    
+using namespace std; // Streams are in the std namespace (standard library)
-
-myString + ' You'; // => "Hello You"   
-
-myString != myOtherString; //True   
-
-//An example of a string method   
-myString.append(" Dog"); // => "Hello Dog"   
-
-
-///////////////////////////////////////   
-// C++ Input Output   
-///////////////////////////////////////   
-
-//C++ input and output streams   
-//cin, cout, cerr, << is insertion and >> is extraction operator   
-#include <iostream>   
-
-using namespace std;   
 
 int main()
 {
-
    int myInt;
 
-   //Prints to stdout (or terminal/screen)   
+   // Prints to stdout (or terminal/screen)
-   cout << "Enter your fav number:\n";   
+   cout << "Enter your favorite number:\n";
-   //Takes in input   
+   // Takes in input
    cin >> myInt;
 
-   //cout can also be formatted   
+   // cout can also be formatted
-   cout << "Your fav number is " << myInt << "\n";   
+   cout << "Your favorite number is " << myInt << "\n";
-   //Your fav number is ##  
+   // prints "Your favorite number is <myInt>"
 
     cerr << "Used for error messages";
 }
 
+//////////
+// Strings
+//////////
 
-///////////////////////////////////////   
+// Strings in C++ are objects and have many member functions
-// C++ Classes   
+#include <string>
-///////////////////////////////////////
+
+using namespace std; // Strings are also in the namespace std (standard library)
+
+string myString = "Hello";
+string myOtherString = " World";
+
+// + is used for concatenation.
+cout << myString + myOtherString; // "Hello World"
+
+cout << myString + " You"; // "Hello You"
+
+// C++ strings are mutable and have value semantics.
+myString.append(" Dog");
+cout << myString; // "Hello Dog"
 
 
-//First example of classes   
+/////////////
+// 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.
+
+using namespace std;
+
+string foo = "I am foo";
+string bar = "I am bar";
+
+
+string& fooRef = foo; // This creates a reference to foo.
+fooRef += ". Hi!"; // Modifies foo through the reference
+cout << fooRef; // Prints "I am foo. Hi!"
+
+fooRef = bar; // Error: references cannot be reassigned.
+
+const 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.
+
+//////////////////////////////////////////
+// Classes and object-oriented programming
+//////////////////////////////////////////
+
+// First example of classes
 #include <iostream>
 
-//define a class    
+// Declare a class.
-class Doggie   
+// 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;
 
-   // These are only the declarations   
+// All members following this are public
-   //Can also have private and protected   
+// until "private:" or "protected:" is found.
-   public:     
+public:
-       //The public methods (can also include variables)   
 
     // Default constructor
-   Doggie();   
+    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 setName(std::string dogsName);   
     void setWeight(int dogsWeight);
-   void printDog();   
 
-   //Can define functions within class declaration too   
+    // Functions that do not modify the state of the object
-   void dogBark() {std::cout << "Bark Bark\n"}   
+    // 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;
 
-   //Destructors are methods that free the allocated space   
+    // Functions can also be defined inside the class body.
-   ~doggieDestructor();   
+    // Functions defined as such are automatically inlined.
-   //if no destructor compiler defines the trivial destructor   
+    void bark() const { std::cout << name << " barks!\n" }
 
-//Classes are similar to structs and must close the } with ;    
+    // 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)
+    // Destructors must be virtual to allow classes to be derived from this one.
+    virtual ~Dog();
 
-// This is the implementation of the class methods   
+}; // A semicolon must follow the class definition.
-// Also called the definition   
+
-void Doggie::Doggie () {   
+// Class member functions are usually implemented in .cpp files.
-    std::cout << "A doggie is born. Woof!\n";   
+void Dog::Dog()
+{
+    std::cout << "A dog has been constructed\n";
 }
 
-void Doggie::setName (std::string doggie_name) {    
+// 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 = doggie_name;
 }
 
-void Doggie::setWeight (int doggie_weight) {   
+void Dog::setWeight(int dogsWeight)
-    weight = doggie_weight;   
-}   
-
-void Doggie::printDog () {   
-    std::cout << "Dog is " << name << " weighs" << weight << "\n";    
-}   
-
-void Doggie::~doggieDestructor () {   
-    delete[] name;   
-    delete weight;    
-}   
-
-int main () {    
-  Doggie deedee; // prints out a doggie is born. Woof!   
-  deedee.setName ("Barkley");    
-  deedee.setWeight(1000000);   
-  deedee.printDog;   
-  //prints => Dog is Barkley weighs 1000000    
-  return 0;   
-}   
-
-
-//C++ Class inheritance   
-
-class German_Sheperd : public Doggie  
 {
-   //This class now inherits everything public and protected from Doggie class
+    weight = dogsWeight;
+}
 
-   //Good practice to put d_ in front of datatypes in classes   
+// Notice that "virtual" is only needed in the declaration, not the definition.
-   std::string d_type;   
+void Dog::print() const
+{
+    std::cout << "Dog is " << name << " and weighs " << weight << "kg\n";
+}
 
-   public:  
+void Dog::~Dog()
-      void dogType() {d_type = "German Sheperd";}   
+{
+    cout << "Goodbye " << name << "\n";
+}
+
+int main() {
+    Dog myDog; // prints "A dog has been constructed"
+    myDog.setName("Barkley");
+    myDog.setWeight(10);
+    myDog.printDog(); // 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
+class OwnedDog : public Dog {
+
+    void setOwner(const std::string& dogsOwner)
+
+    // Override the behavior of the print function for all OwnedDogs. See
+    // http://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)
-///////////////////////////////////////   
-// C++ Exception Handling   
-///////////////////////////////////////    
-
-try {   
-   throw 12.25;  // throws a double no handler declared   
-} catch (int errorNum)   
 {
-  std::cout << "I caught an int " << errorNum << "\n";   
+    owner = dogsOwner;
-//default catcher   
-} catch (...)   
-{   
-    std::cout << "I got an error. Not sure what but I can pass it up.";   
-    throw;   
 }
 
+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>"
+}
 
-///////////////////////////////////////    
+//////////////////////////////////////////
-// C++ Operator Overloading   
+// Initialization and Operator Overloading
-///////////////////////////////////////   
+//////////////////////////////////////////
 
-// In C++ you can overload operators such as +, -, new, etc.   
+// 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 Vector {   
+class Point {
-    public:   
+public:
-        double x,y;   
+    // Member variables can be given default values in this manner.
-        Vector () {};   
+    double x = 0;
-        Vector (double a, double b) : x(a), y(b) {}   
+    double y = 0;
-        Vector operator + (const CVector&);   
+
-        Vector operator += (const CVector&);   
+    // 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.
 };
 
-Vector Vector::operator+ (const Vector& rhs)    
+Point Point::operator+(const Point& rhs) const
 {
-    Vector temp;   
+    // Create a new point that is the sum of this one and rhs.
-    temp.x = x + rhs.x;   
+    return Point(x + rhs.x, y + rhs.y);
-    temp.y = y + rhs.y;   
-    return temp;   
 }
 
-Vector Vector::operator+= (const Vector& rhs)   
+Point& Point::operator+=(const Point& rhs)
 {
     x += rhs.x;
     y += rhs.y;
@@ -330,19 +418,173 @@ Vector Vector::operator+= (const Vector& rhs)
 }
 
 int main () {
-    Vector up (0,1);   
+    Point up (0,1);
-    Vector right (1,0);   
+    Point right (1,0);
-    Vector result;    
+    // This calls the Point + operator
-    // This calls the Vector + operator   
+    // Point up calls the + (function) with right as its paramater
-    // Vector up calls the + (function) with right as its paramater   
+    Point result = up + right;
-    result = up + right;   
+    // Prints "Result is upright (1,1)"
-    // prints out => Result is upright (1,1)   
     cout << "Result is upright (" << result.x << ',' << result.y << ")\n";
     return 0;
 }
 
-```
+/////////////////////
-Futher Reading   
+// Exception Handling
+/////////////////////
 
-for more resources see: http://www.icce.rug.nl/documents/cplusplus/   
+// The standard library provides a few exception types
-for other reference material: http://www.cplusplus.com/doc/tutorial/  
+// (see http://en.cppreference.com/w/cpp/error/exception)
+// but any type can be thrown an as exception
+#include <exception>
+
+// 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::exception("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 Allocation 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.
+
+    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.
+        reuturn 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)
+        reuturn 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 read mode
+    if (fh == nullptr)
+        throw std::exception("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
+// whenver 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
+```
+Futher Reading:
+
+An up-to-date language reference can be found at
+<http://cppreference.com/w/cpp>
+
+Additional resources may be found at <http://cplusplus.com>