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Merge pull request #800 from mrkline/master

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C++ Improvements
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Levi Bostian
2014-10-17 20:51:37 -05:00
parent 2c5031dae9 03d1bc5ed9
commit 57b5eb1bac
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@@ -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
additions that C++ has with respect to C. My
suggestion would be to follow the C tutorial first
then look here for the additions and differences.
C++ is a systems programming language that,
[according to its inventor Bjarne Stroustrup](http://channel9.msdn.com/Events/Lang-NEXT/Lang-NEXT-2014/Keynote),
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.
```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++
//cannot use void main()
int main() { //or int main(int argc, char **argv)
//cannot end with return;
// A main() function in C++ should return an int,
// though void main() is accepted by most compilers (gcc, clang, etc.)
// This value serves as the program's exit status.
// 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++
/*
//This could lead to compiler errors and is discouraged
//#if 0 #endif pairs are encouraged instead
*/
//In C++
sizeof(10) //Typically 4
// In C++, character literals are one byte.
sizeof('c') == 1
//In C
sizeof('c') == sizeof(10) //true chars are passed as ints
// In C, character literals are the same size as ints.
sizeof('c') == sizeof(10)
//In C++ strict prototyping
// C++ has strict prototyping
void func(); // function which accepts no arguments
// 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++
for(int i = 0; i < 10; i++) {;}
//In C must int i must be declared before
//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
// C standard headers are available in C++,
// but are prefixed with "c" and have no .h suffix.
#include <cstdio>
int main()
{
printing("Hello");
printing(15);
printf("Hello, world!\n");
return 0;
}
///////////////////////
// Function overloading
///////////////////////
// C++ supports function overloading
// provided each function takes different parameters.
//C++ Default Function Arguments
void two_ints(int a = 1, int b = 4);
int main()
void print(char const* myString)
{
two_ints(); // arguments: 1, 4
two_ints(20); // arguments: 20, 4
two_ints(20, 5); // arguments: 20, 5
printf("String %s\n", myString);
}
//C++ added the nullptr which is different from 0
int *ip = nullptr; // OK
int value = nullptr; // error: value is no pointer
///////////////////////////////////////
// C++ Additions ontop of C
///////////////////////////////////////
///////////////////////////////////////
// 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()
void print(int myInt)
{
for (int counter = 1; // this refers to the
counter < 2; // local variable
counter++)
{
printf("Global var %d local var %d\n",
::counter, // global variable
counter); // local variable
// => Global var 50 local var 1
}
}
// Namespaces can be nested
namespace myFirstNameSpace
{
namespace myInnerSoul
{
cos(int x)
{
printf("My inner soul was made to program.");
}
}
}
namespace anotherNameSpace
{
cos(int x) {;} //does nothing
printf("My int is %d", myInt);
}
int main()
{
//Specify the full path because main is outside of both namespaces.
//Will print out My inner soul was made to program.
myFirstNameSpace::myInnerSoul::cos(60);
print("Hello"); // Resolves to void print(const char*)
print(15); // Resolves to void print(int)
}
/////////////////////////////
// Default function arguments
/////////////////////////////
///////////////////////////////////////
// C++ Strings
///////////////////////////////////////
// You can provide default arguments for a function
// if they are not provided by the caller.
//Strings in C++ are Objects and have many functions
myString = "Hello";
myOtherString = " World";
myString + myOtherString; // => "Hello World"
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;
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 foo()
{
printf("This is global foo\n");
}
int main()
{
// Assume everything is from the namespace "Second"
// unless otherwise specified.
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
// 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
using namespace std; // Streams are in the std namespace (standard library)
int main()
{
int myInt;
// Prints to stdout (or terminal/screen)
cout << "Enter your fav number:\n";
cout << "Enter your favorite number:\n";
// Takes in input
cin >> myInt;
// cout can also be formatted
cout << "Your fav number is " << myInt << "\n";
//Your fav number is ##
cout << "Your favorite number is " << myInt << "\n";
// prints "Your favorite number is <myInt>"
cerr << "Used for error messages";
}
//////////
// Strings
//////////
///////////////////////////////////////
// C++ Classes
///////////////////////////////////////
// Strings in C++ are objects and have many member functions
#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"
/////////////
// 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
class Doggie
{
// 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;
// These are only the declarations
//Can also have private and protected
// All members following this are public
// until "private:" or "protected:" is found.
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
void dogBark() {std::cout << "Bark Bark\n"}
// 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;
//Destructors are methods that free the allocated space
~doggieDestructor();
//if no destructor compiler defines the trivial destructor
// Functions can also be defined inside the class body.
// Functions defined as such are automatically inlined.
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
// Also called the definition
void Doggie::Doggie () {
std::cout << "A doggie is born. Woof!\n";
}; // A semicolon must follow the class definition.
// Class member functions are usually implemented in .cpp files.
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) {
weight = doggie_weight;
void Dog::setWeight(int dogsWeight)
{
weight = dogsWeight;
}
void Doggie::printDog () {
std::cout << "Dog is " << name << " weighs" << weight << "\n";
// 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";
}
void Doggie::~doggieDestructor () {
delete[] name;
delete weight;
void Dog::~Dog()
{
cout << "Goodbye " << name << "\n";
}
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
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:
//C++ Class inheritance
// This class inherits everything public and protected from the Dog class
class OwnedDog : public Dog {
class German_Sheperd : public Doggie
{
//This class now inherits everything public and protected from Doggie class
void setOwner(const std::string& dogsOwner)
//Good practice to put d_ in front of datatypes in classes
std::string d_type;
// 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;
public:
void dogType() {d_type = "German Sheperd";}
private:
std::string owner;
};
// Meanwhile, in the corresponding .cpp file:
///////////////////////////////////////
// C++ Exception Handling
///////////////////////////////////////
try {
throw 12.25; // throws a double no handler declared
} catch (int errorNum)
void OwnedDog::setOwner(const std::string& dogsOwner)
{
std::cout << "I caught an int " << errorNum << "\n";
//default catcher
} catch (...)
{
std::cout << "I got an error. Not sure what but I can pass it up.";
throw;
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>"
}
///////////////////////////////////////
// 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:
double x,y;
Vector () {};
Vector (double a, double b) : x(a), y(b) {}
Vector operator + (const CVector&);
Vector operator += (const CVector&);
// 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.
};
Vector Vector::operator+ (const Vector& rhs)
Point Point::operator+(const Point& rhs) const
{
Vector temp;
temp.x = x + rhs.x;
temp.y = y + rhs.y;
return temp;
// Create a new point that is the sum of this one and rhs.
return Point(x + rhs.x, y + rhs.y);
}
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);
Vector right (1,0);
Vector result;
// This calls the Vector + operator
// Vector up calls the + (function) with right as its paramater
result = up + right;
// prints out => Result is upright (1,1)
Point up (0,1);
Point right (1,0);
// This calls the Point + operator
// Point up calls the + (function) with right as its paramater
Point result = up + right;
// Prints "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/
for other reference material: http://www.cplusplus.com/doc/tutorial/
// The standard library provides a few exception types
// (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>