C++ и его система типов: Как иметь дело с данными с несколькими типами?

Что касается скорости, вы скажете:

Было очень медленно передавать этот класс в качестве распределителя строк {{1 }} приходилось вызывать все время.

Вы знаете, что в большинстве случаев вы должны передавать объекты по ссылке? Ваше решение выглядит работоспособным для простого интерпретатора.

Одним из очевидных решений является определение иерархии типов:

class Type
{
};

class Int : public Type
{
};

class String : public Type
{
};

и так далее. В качестве полного примера давайте напишем интерпретатор для крошечного языка. Язык позволяет объявлять переменные следующим образом:

var a 10

Это создаст объект Int , присвоит ему значение 10 и сохранит его в таблице переменных под именем a . Операции можно вызывать с переменными. Например, операция сложения двух значений Int выглядит так:

+ a b

Вот полный код интерпретатора:

#include <iostream>
#include <string>
#include <vector>
#include <sstream>
#include <cstdlib>
#include <map>

// The base Type object from which all data types are derived.
class Type
{
public:
  typedef std::vector<Type*> TypeVector;
  virtual ~Type () { }

  // Some functions that you may want all types of objects to support:

  // Returns the string representation of the object.
  virtual const std::string toString () const = 0;
  // Returns true if other_obj is the same as this.
  virtual bool equals (const Type &other_obj) = 0;
  // Invokes an operation on this object with the objects in args
  // as arguments.
  virtual Type* invoke (const std::string &opr, const TypeVector &args) = 0;
};

// An implementation of Type to represent an integer. The C++ int is
// used to actually store the value.  As a consequence this type is
// machine dependent, which might not be what you want for a real
// high-level language.
class Int : public Type
{
public:
  Int () : value_ (0), ret_ (NULL) { }
  Int (int v) : value_ (v), ret_ (NULL) { }
  Int (const std::string &v) : value_ (atoi (v.c_str ())), ret_ (NULL) { }
  virtual ~Int ()
  {
    delete ret_;
  }
  virtual const std::string toString () const
  {
    std::ostringstream out;
    out << value_;
    return out.str ();
  }
  virtual bool equals (const Type &other_obj)
  {    
    if (&other_obj == this) 
      return true;
    try
      {
        const Int &i = dynamic_cast<const Int&> (other_obj);
        return value_ == i.value_;
      }
    catch (std::bad_cast ex)
      {
        return false;
      }
  }
  // As of now, Int supports only addition, represented by '+'.
  virtual Type* invoke (const std::string &opr, const TypeVector &args)    
  {
    if (opr == "+")
      {
        return add (args);
      }
    return NULL;
  }
private:
  Type* add (const TypeVector &args)
  {
    if (ret_ == NULL) ret_ = new Int;
    Int *i = dynamic_cast<Int*> (ret_);
    Int *arg = dynamic_cast<Int*> (args[0]);
    i->value_ = value_ + arg->value_;
    return ret_;
  }
  int value_;
  Type *ret_;
};

// We use std::map as a symbol (or variable) table.
typedef std::map<std::string, Type*> VarsTable;
typedef std::vector<std::string> Tokens;

// A simple tokenizer for our language. Takes a line and
// tokenizes it based on whitespaces.  
static void
tokenize (const std::string &line, Tokens &tokens)
{
  std::istringstream in (line, std::istringstream::in);
  while (!in.eof ())
    {
      std::string token;
      in >> token;
      tokens.push_back (token);
    }
}

// Maps varName to an Int object in the symbol table.  To support
// other Types, we need a more complex interpreter that actually infers
// the type of object by looking at the format of value.
static void
setVar (const std::string &varName, const std::string &value,
        VarsTable &vars)
{
  Type *t = new Int (value);
  vars[varName] = t;
}

// Returns a previously mapped value from the symbol table.
static Type *
getVar (const std::string &varName, const VarsTable &vars)
{
  VarsTable::const_iterator iter = vars.find (varName);
  if (iter == vars.end ())
    {
      std::cout << "Variable " << varName 
                << " not found." << std::endl;
      return NULL;
    }
  return const_cast<Type*> (iter->second);
}

// Invokes opr on the object mapped to the name var01.
// opr should represent a binary operation. var02 will
// be pushed to the args vector. The string represenation of
// the result is printed to the console.
static void
invoke (const std::string &opr, const std::string &var01,
        const std::string &var02, const VarsTable &vars)
{
  Type::TypeVector args;
  Type *arg01 = getVar (var01, vars);
  if (arg01 == NULL) return;
  Type *arg02 = getVar (var02, vars);
  if (arg02 == NULL) return;
  args.push_back (arg02);
  Type *ret = NULL;
  if ((ret = arg01->invoke (opr, args)) != NULL)
    std::cout << "=> " << ret->toString () << std::endl;
  else
    std::cout << "Failed to invoke " << opr << " on " 
              << var01 << std::endl;
}

// A simple REPL for our language. Type 'quit' to exit
// the loop.
int 
main (int argc, char **argv)
{
  VarsTable vars;
  std::string line;
  while (std::getline (std::cin, line))
    {
      if (line == "quit")
        break;
      else
        {
          Tokens tokens;
          tokenize (line, tokens);
          if (tokens.size () != 3)
            {
              std::cout << "Invalid expression." << std::endl;
              continue;
            }
          if (tokens[0] == "var")
            setVar (tokens[1], tokens[2], vars);
          else
            invoke (tokens[0], tokens[1], tokens[2], vars);
        }
    }  
  return 0;
}

Пример взаимодействия с интерпретатором:

/home/me $ ./mylang

var a 10
var b 20
+ a b
30
+ a c
Variable c not found.
quit

Согласно решению Виджая, реализация будет такой:

Type* array;
// to initialize the array
array = new Type(size_of_array);
// when you want to add values
array[0] = new Int(42);
// to add another string value
array[1] = new String("fourty two");

В его коде отсутствует бит, КАК извлечь эти значения ... Вот моя версия (на самом деле Я узнал об этом от Ogre и модифицировал по своему вкусу).

Пример использования:

Any array[4];
// Automatically understands it's an integer
array[0] = Any(1);
// But let's say you want the number to be thought of as float
array[1] = Any<float>(2);
// What about string?
array[2] = Any<std::string>("fourty two");
// Note that this gets the compiler thinking it's a char*
// instead of std::string
array[3] = Any("Sometimes it just turns out to be what you don't want!");

Хорошо, теперь посмотрим, является ли конкретный элемент строкой:

if(array[2].isType<std::string>()
{
   // Extract the string value.
   std::string val = array[2].cast<std::string>();
   // Make the string do your bidding!!!... /evilgrin
   // WAIT! But what if you want to directly manipulate
   // the value in the array?
   std::string& val1 = array[2].cast<std::string>();
   // HOHOHO... now any changes to val1 affects the value
   // in the array ;)
}

Код для класса Any приведен ниже. Не стесняйтесь использовать его как хотите :). Надеюсь это поможет!

В заголовочном файле ... скажем Any.h

    #include <typeinfo>
    #include <exception>

    /*
     *    \class Any
     *    \brief A variant type to hold any type of value.
     *    \detail This class can be used to store values whose types are not 
     *        known before hand, like to store user-data.
     */
    class Any
    {
    public:
        /*!
         *    \brief Default constructor. 
         */

    Any(void);

    /*!
     *    \brief Constructor that accepts a default user-defined value.
     *    \detail This constructor copies that user-defined value into a 
     *        place holder. This constructor is explicit to avoid the compiler
     *        to call this constructor implicitly when the user didn't want
     *        the conversion to happen.
     *    \param val const reference to the value to be stored.
     */
    template <typename ValueType>
    explicit Any(const ValueType& val);

    /*!
     *    \brief Copy constructor.
     *    \param other The \c Any variable to be copied into this.
     */
    Any(const Any& other);

    /*!
     *    \brief Destructor, does nothing other than destroying the place holder.
     */
    ~Any(void);

    /*!
     *    \brief Gets the type of the value stored by this class.
     *    \detail This function uses typeid operator to determine the type
     *        of the value it stores.
     *    \remarks If the place holder is empty it will return Touchscape::VOID_TYPE.
     *        It is wise to check if this is empty by using the function Any::isEmpty().
     */
    const std::type_info& getType() const;

    /*!
     *    \brief Function to verify type of the stored value.
     *    \detail This function can be used to verify the type of the stored value.
     *    Usage:
     *    \code
     *    int i;
     *    Touchscape::Any int_any(i);
     *    // Later in your code...
     *    if (int_any.isType<int>())
     *    {
     *        // Do something with int_any.
     *    }
     *    \endcode
     *    \return \c true if the type matches, false otherwise.
     */
    template <typename T>
    bool isType() const;

    /*!
     *    \brief Checks if the type stored can be converted 'dynamically'
     *        to the requested type.
     *    \detail This would useful when the type stored is a base class
     *        and you would like to verify if it can be converted to type
     *        the user wants.
     *    Example:
     *    \code
     *    class Base
     *    {
     *        // class implementation.
     *    };
     *    class Derived : public Base
     *    {
     *        // class implementation.
     *    };
     *
     *    // In your implementation function.
     *    {
     *        //...
     *        // Somewhere in your code.
     *        Base* a = new Derived();
     *        Touchscape::Any user_data(a);
     *        my_object.setUserData(user_data);
     *        // Then when you need to know the user-data type
     *        if(my_object.getUserData().isDynamicType<Derived>())
     *        {
     *            // Do something with the user data
     *        }
     *    }
     *    \endcode
     *    \return \c true if the value stored can be dynamically casted to the target type.
     *    \deprecated This function will be removed and/or changed in the future.
     */
    template <typename T>
    bool isDynamicType() const;

    /*!
     *    \brief Convert the value stored to the required type.
     *    \detail This function is used just like a static-cast to retrieve
     *        the stored value.
     *    \return A reference to the stored value.
     *    \warning This function will throw std::bad_cast exception if it
     *        finds the target type to be incorrect.
     */
    template <typename T>
    T& cast();

    /*!
     *    \brief Convert the value stored to the required type (const version).
     *    \detail This function is used just like static_cast to retrieve
     *        the stored value.
     *    \return A \c const reference to the stored value.
     *    \warning This function will throw std::bad_cast exception if it
     *        finds the target type to be incorrect.
     */
    template <typename T>
    const T& cast() const;

    /*!
     *    \brief Dynamically converts the stored value to the target type
     *    \detail This function is just like dynamic_cast to retrieve
     *        the stored value to the target type.
     *    \return A reference to the stored value.
     *    \warning This function will throw std::bad_cast exception if it
     *        finds that the value cannot be dynamically converted to the target type.
     *    \deprecated This function will be removed and/or changed in the future.
     */
    template <typename T>
    T& dynamicCast();

    /*!
     *    \brief Dynamically converts the stored value to the target type (const version)
     *    \detail This function is just like dynamic_cast to retrieve
     *        the stored value to the target type.
     *    \return A const reference to the stored value.
     *    \warning This function will throw std::bad_cast exception if it
     *        finds that the value cannot be dynamically converted to the target type.
     *    \deprecated This function will be removed and/or changed in the future.
     */
    template <typename T>
    const T& dynamicCast() const;

    /*!
     *    \brief Swaps the contents with another \c Any variable.
     *    \return reference to this instance.
     */
    Any& swap(Any& other);

    /*!
     *    \brief Checks if the place holder is empty.
     *    \return \c true if the the place holder is empty, \c false otherwise.
     */
    bool isEmpty() const;

    /*!
     *    \brief Checks if the place holder is \b not empty.
     *    \return \c true if the the place holder is not empty, \c false otherwise.
     *    \remarks This is just a lazy programmer's attempt to make the code look elegant.
     */
    bool isNotEmpty() const;

    /*!
     *    \brief Assignment operator
     *    \detail Assigns a 'raw' value to this instance.
     *    \return Reference to this instance after assignment.
     */
    template <typename ValueType>
    Any& operator = (const ValueType& rhs);

    /*!
     *    \brief Default assignment operator
     *    \detail Assigns another \c Any type to this one.
     *    \return Reference to this instance after assignment.
     */
    Any& operator = (const Any& rhs);

    /*!
     *    \brief Boolean equality operator
     */
    bool operator == (const Any& other) const;

    /*!
     *    \brief Boolean equality operator that accepts a 'raw' type.
     */
    template<typename ValueType>
    bool operator == (const ValueType& other) const;

    /*!
     *    \brief Boolean inequality operator
     */
    bool operator != (const Any& other) const;

    /*!
     *    \brief Boolean inequality operator that accepts a 'raw' type.
     */


      template<typename ValueType>
        bool operator != (const ValueType& other) const;
    protected:
        /*!
         *    \class PlaceHolder
         *    \brief The place holder base class
         *    \detail The base class for the actual 'type'd class that stores
         *        the value for T

ouchscape::Any.
     */
    class PlaceHolder
    {
    public:

        /*!
         *    \brief Virtual destructor.
         */
        virtual ~PlaceHolder(){}

        /*!
         *    \brief Gets the \c type_info of the value stored.
         *    \return (const std::type_info&) The typeid of the value stored.
         */
        virtual const std::type_info& getType() const = 0;
        /*!
         *    \brief Clones this instance.
         *    \return (PlaceHolder*) Cloned instance.
         */
        virtual PlaceHolder* clone() const = 0;
    };

    /*!
     *    \class PlaceHolderImpl
     *    \brief The class that ultimately keeps hold of the value stored
     *        in Touchscape::Any.
     */
    template <typename ValueType>
    class PlaceHolderImpl : public PlaceHolder
    {
    public:
        /*!
         *    \brief The only constructor allowed.
         *    \param val The value to store.
         */
        PlaceHolderImpl(const ValueType& val)
            :m_value(val){}
        /*!
         *    \brief The destructor.
         *    \detail Does nothing
         */
        ~PlaceHolderImpl(){}

        /*!
         *    \copydoc Touchscape::PlaceHolder::getType()
         */
        const std::type_info& getType() const
        {
            return typeid(ValueType);
        }

        /*!
         *    \copydoc Touchscape::PlaceHolder::clone()
         */
        PlaceHolder* clone() const
        {
            return new PlaceHolderImpl<ValueType>(m_value);
        }

        ValueType m_value;
    };

    PlaceHolder* m_content;
};

/************************************************************************/
/* Template code implementation section                                 */
/************************************************************************/
template <typename ValueType>
Any::Any(const ValueType& val)
    :m_content(new PlaceHolderImpl<ValueType>(val))
{
}
//---------------------------------------------------------------------
template <typename T>
bool Any::isType() const
{
    bool result = m_content?m_content->getType() == typeid(T):false;
    return result;
}
//---------------------------------------------------------------------
template <typename T>
bool Any::isDynamicType() const
{
    bool result = m_content
        ?dynamic_cast<T>(static_cast<PlaceHolderImpl<T>*>(m_content)->m_value)!=NULL
        :false;
    return result;
}
//---------------------------------------------------------------------
template <typename T>
T& Any::cast()
{
    if (getType() != VOID_TYPE && isType<T>())
    {
        T& result = static_cast<PlaceHolderImpl<T>*>(m_content)->m_value;
        return result;
    }
    StringStream ss;
    ss<<"Cannot convert '"<<getType().name()<<"' to '"<<typeid(T).name()<<"'. Did you mean to use dynamicCast() to cast to a different type?";
    throw std::bad_cast(ss.str().c_str());
}
//---------------------------------------------------------------------
template <typename T>
const T& Any::cast() const
{
    Any& _this = const_cast<Any&>(*this);
    return _this.cast<T>();
}
//---------------------------------------------------------------------
template <typename T>
T& Any::dynamicCast()
{
    T* result = dynamic_cast<T>(static_cast<PlaceHolderImpl<T>*>(m_content)->m_value);
    if (result == NULL)
    {
        StringStream ss;
        ss<<"Cannot convert '"<<getType().name()<<"' to '"<<typeid(T)<<"'.";
        throw std::bad_cast(ss.str().c_str());
    }
    return *result;
}
//---------------------------------------------------------------------
template <typename T>
const T& Any::dynamicCast() const
{
    Any& _this = const_cast<Any&>(*this);
    return _this.dynamicCast<T>();
}
//---------------------------------------------------------------------
template <typename ValueType>
Any& Any::operator = (const ValueType& rhs)
{
    Any(rhs).swap(*this);
    return *this;
}
//---------------------------------------------------------------------
template <typename ValueType>
bool Any::operator == (const ValueType& rhs) const
{
    bool result = m_content == rhs;
    return result;
}
//---------------------------------------------------------------------
template <typename ValueType>
bool Any::operator != (const ValueType& rhs) const
{
    bool result = m_content != rhs;
    return result;
}

Теперь в файле CPP ... Any.cpp

#include "Any.h"

static const std::type_info& VOID_TYPE(typeid(void));

Any::Any( void )
    :m_content(NULL)
{
}
//---------------------------------------------------------------------
Any::Any( const Any& other )
    :m_content(other.m_content?other.m_content->clone():NULL)
{
}
//---------------------------------------------------------------------
Any::~Any( void )
{
    SafeDelete(m_content);
}
//---------------------------------------------------------------------
const std::type_info& Any::getType() const
{
    return m_content?m_content->getType():VOID_TYPE;
}
//---------------------------------------------------------------------
Any& Any::swap( Any& other )
{
    std::swap(m_content, other.m_content);
    return *this;
}
//---------------------------------------------------------------------
Any& Any::operator=( const Any& rhs )
{
    Any(rhs).swap(*this);
    return *this;
}
//---------------------------------------------------------------------
bool Any::isEmpty() const
{
    bool is_empty = m_content == NULL;
    return is_empty;
}
//---------------------------------------------------------------------
bool Any::isNotEmpty() const
{
    bool is_not_empty = m_content != NULL;
    return is_not_empty;
}
//---------------------------------------------------------------------
bool Any::operator==( const Any& other ) const
{
    bool result = m_content == other.m_content;
    return result;
}
//---------------------------------------------------------------------
bool Any::operator!=( const Any& other ) const
{
    bool result = m_content != other.m_content;
    return result;
}

C ++ - язык со строгой типизацией. Я вижу, что вы пришли из нетипизированного языка и все еще думаете в этих терминах.

Если вам действительно нужно хранить несколько типов в переменной, взгляните на boost :: any .

Однако, если вы реализуете интерпретатор, вы должны использовать наследование и классы, представляющие определенный тип.