How to write a simple class in C++?

问题:

I have been reading a lot of tutorials on C++ class but they miss something that other tutorials include.

Can someone please show me how to write and use a very simple C++ class that uses visibility, methods and a simple constructor and destructor?

回答1:

Well documented example taken and explained better from Constructors and Destructors in C++:

#include <iostream>            // for cout and cin

class Cat                      // begin declaration of the class
{
  public:                      // begin public section
    Cat(int initialAge);       // constructor
    Cat(const Cat& copy_from); //copy constructor
    Cat& operator=(const Cat& copy_from); //copy assignment
    ~Cat();                    // destructor

    int GetAge() const;        // accessor function
    void SetAge(int age);      // accessor function
    void Meow();
 private:                      // begin private section
    int itsAge;                // member variable
    char * string;
};

// constructor of Cat,
Cat::Cat(int initialAge)
{
  itsAge = initialAge;
  string = new char[10]();
}

//copy constructor for making a new copy of a Cat
Cat::Cat(const Cat& copy_from) {
   itsAge = copy_from.itsAge;
   string = new char[10]();
   std::copy(copy_from.string+0, copy_from.string+10, string);
}

//copy assignment for assigning a value from one Cat to another
Cat& Cat::operator=(const Cat& copy_from) {
   itsAge = copy_from.itsAge;
   std::copy(copy_from.string+0, copy_from.string+10, string);
}

// destructor, just an example
Cat::~Cat()
{
    delete[] string;
}

// GetAge, Public accessor function
// returns value of itsAge member
int Cat::GetAge() const
{
   return itsAge;
}

// Definition of SetAge, public
// accessor function
 void Cat::SetAge(int age)
{
   // set member variable its age to
   // value passed in by parameter age
   itsAge = age;
}

// definition of Meow method
// returns: void
// parameters: None
// action: Prints "meow" to screen
void Cat::Meow()
{
   cout << "Meow.\n";
}

// create a cat, set its age, have it
// meow, tell us its age, then meow again.
int main()
{
  int Age;
  cout<<"How old is Frisky? ";
  cin>>Age;
  Cat Frisky(Age);
  Frisky.Meow();
  cout << "Frisky is a cat who is " ;
  cout << Frisky.GetAge() << " years old.\n";
  Frisky.Meow();
  Age++;
  Frisky.SetAge(Age);
  cout << "Now Frisky is " ;
  cout << Frisky.GetAge() << " years old.\n";
  return 0;
}

回答2:

Even if he is a student, worth trying to answer because it is a complex one not that easy at least for a new Visitor of C++ :)

Classes in C++ serve an intersection of two design paradigms,

1) ADT :: which means basically a new type, something like integers 'int' or real numbers 'double' or even a new concept like 'date'. in this case the simple class should look like this,

class NewDataType
{
public:
// public area. visible to the 'user' of the new data type.
.
.
.
private:
// no one can see anything in this area except you.
.
.
.
};

this is the most basic skeleton of an ADT... of course it can be simpler by ignoring the public area! and erasing the access modifiers (public, private) and the whole thing will be private. but that is just nonsense. Because the NewDataType becomes useless! imagine an 'int' that you can just declare but you CAN NOT do anything with it.

Then, you need some useful tools that are basically not required to the existence of the NewDataType, but you use them to let your type look like any 'primitive' type in the language.

the first one is the Constructor. The constructor is needed in many places in the language. look at int and lets try to imitate its behavior.

int x; // default constructor.

int y = 5; // copy constructor from a 'literal' or a 'constant value' in simple wrods.
int z = y; // copy constructor. from anther variable, with or without the sametype.
int n(z); // ALMOST EXACTLY THE SAME AS THE ABOVE ONE, it isredundant for 'primitive' types, but really needed for the NewDataType.

every line of the above lines is a declaration, the variable gets constructed there.

and in the end imagine the above int variables in a function, that function is called 'fun',

int fun()
{
    int y = 5;
    int z = y;
    int m(z);

    return (m + z + y)
    // the magical line.
}

you see the magical line, here you can tell the compiler any thing you want! after you do every thing and your NewDataType is no more useful for the local scope like in the function, you KILL IT. a classical example would be releasing the memory reserved by 'new'!

so our very simple NewDataType becomes,

class NewDataType
{
public:
// public area. visible to the 'user' of the new data type.
    NewDataType()
    { 
        myValue = new int;
        *myValue = 0;
    }

    NewDataType(int newValue)
    {
        myValue = new int;
        *myValue = newValue;
    }

    NewDataType(const NewDataType& newValue){

        myValue = new int;
        *myValue = newValue.(*myValue);
    }
private:
// no one can see anything in this area except you.
    int* myValue;
};

Now this is the very basic skeleton, to start building a useful class you have to provide public functions.

there are A LOT of tiny tools to consider in building a class in C++,

. . . .

2) Object :: which means basically a new type, but the difference is that it belongs to brothers, sisters, ancestors and descendants. look at 'double' and 'int' in C++, the 'int' is a sun of 'double' because every 'int' is a 'double' at least in concept :)

回答3:

class A
{
  public:
    // a simple constructor, anyone can see this
    A() {}
  protected:
    // a simple destructor. This class can only be deleted by objects that are derived from this class
    // probably also you will be unable to allocate an instance of this on the stack
    // the destructor is virtual, so this class is OK to be used as a base class
    virtual ~A() {}
  private:
    // a function that cannot be seen by anything outside this class
    void foo() {}
};

回答4:

#include <iostream>
#include <string>

class Simple {
public:
  Simple(const std::string& name);
  void greet();
  ~Simple();
private:
  std::string name;
};

Simple::Simple(const std::string& name): name(name) {
  std::cout << "hello " << name << "!" << std::endl;
}

void Simple::greet() {
  std::cout << "hi there " << name << "!" << std::endl;
}

Simple::~Simple() {
  std::cout << "goodbye " << name << "!" << std::endl;
}

int main()
{
  Simple ton("Joe");
  ton.greet();
  return 0;
}

Silly, but, there you are. Note that "visibility" is a misnomer: public and private control accessibility, but even "private" stuff is still "visible" from the outside, just not accessible (it's an error to try and access it).