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Posted 11 Feb 2012

Quickly Check whether C++ Template Instances have the Same Parameters

, 11 Feb 2012 CPOL
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How to quickly check whether C++ Template instances have the same parameters


To assign the permanent type-defined indexes to the derived classes inside the given hierarchy, use the TypeList of all allowed for this hierarchy type and IndexOf template. The base class has a data member int m_ID. In this code, the class constructor is protected, so only descendants of this class can instantiate it as a base class inside the descendant. The virtual function of the descendant compares the m_ID set at the construction of the descendant to the m_ID of the parameter passed to the virtual function.

// This is code from Andrei Alexandresku book "Modern C++ Design", ch. 2

// Copyright (c) 2001 by Andrei Alexandrescu

// This code accompanies the book:

// Alexandrescu, Andrei. "Modern C++ Design: Generic Programming and Design 

// Patterns Applied". Copyright (c) 2001. Addison-Wesley.

// Permission to use, copy, modify, distribute and sell this software for any 

// purpose is hereby granted without fee, provided that the above copyright 

// notice appear in all copies and that both that copyright notice and this 

// permission notice appear in supporting documentation.

class NullType;
// Typelist

template <class T, class U>>
struct TypeList
  typedef T Head;
  typedef U Tail;
#define TYPELIST_1(T1) TypeList<T1, NullType>
#define TYPELIST_2(T1, T2) TypeList<T1, TYPELIST_1(T2)>
#define TYPELIST_3(T1, T2, T3) TypeList<T1, TYPELIST_2(T2, T3)>
#define ..........................................................
// IndexOf

template <class TList, typename T> struct IndexOf;
template <typename T>
struct IndexOf<NullType, T>
  enum { value = -1};
template <class Tail, typename T>
struct IndexOf<TypeList<T, Tail>, T>
  enum {value = 0};
template <class Head, class Tail, typename T>
struct IndexOf<TypeList<Head, Tail>, T>
  enum {temp = IndexOf<Tail, T>::value};
  enum {value = temp == -1 ? -1 : 1 +temp};
// Typedef the typelist for all allowed types, e.g.

typedef TYPELIST_3(short, int, double) SL_TYPELIST;
// Base class

class Base
private:    // Read only

  int m_ID;
protected: // Nobody to play with it

  Base(int ID = -1) : m_ID(ID) {}
public:   // Must

  virtual ~Base() {}
  int GetID(void) const {return m_ID;}
  virtual bool IsRightType(Base* basePtr) = 0;
// Derived classes

template <typename T>
class Child : public Base
  Child():Base(IndexOf<SL_TYPELIST, T>::value) {}
  virtual ~Child() {}

  virtual bool IsRightType(Base* basePtr) 
  {return (basePtr->GetID() == IndexOf<SL_TYPELIST, T>::value);}
// Use it in app:

int _tmain(int argc, _TCHAR* argv[])
  Base* baseInt = new Child<int>;
  Base* baseDbl = new Child<double>;
  bool bChecInt    = baseInt->IsRightType(baseInt); // Return true

  bool bCheckDbl   = baseInt->IsRightType(baseDbl); // Returns false

// Now do as you want: you know types

  return 0;


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


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