C++快速学习入门教程

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转自 http://learnxinyminutes.com/docs/c++/

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//////////////////
// 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:

// 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;
}

// In C++, character literals are one byte.
sizeof('c') == 1

// In C, character literals are the same size as ints.
sizeof('c') == sizeof(10)


// C++ has strict prototyping
void func(); // function which accepts no arguments

// In C
void func(); // function which may accept any number of arguments

// Use nullptr instead of NULL in C++
int* ip = nullptr;

// C standard headers are available in C++,
// but are prefixed with "c" and have no .h suffix.
#include <cstdio>

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", 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 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 favorite number:\n";
   // Takes in input
   cin >> myInt;

   // cout can also be formatted
   cout << "Your favorite number is " << myInt << "\n";
   // prints "Your favorite number is <myInt>"

    cerr << "Used for error messages";
}

//////////
// Strings
//////////

// 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>

// 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)
    // Destructors must be virtual to allow classes to be derived from this one.
    virtual ~Dog();

}; // 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";
}

// 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";
}

void Dog::~Dog()
{
    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)
{
    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);
}

Point& Point::operator+=(const Point& rhs)
{
    x += rhs.x;
    y += rhs.y;
    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 paramater
    Point result = up + right;
    // Prints "Result is upright (1,1)"
    cout << "Result is upright (" << result.x << ',' << result.y << ")\n";
    return 0;
}

/////////////////////
// Exception Handling
/////////////////////

// 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

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