Imperfect C++ Practical Solutions for Real-Life Programming

Chapter 23. Template Constructors

An obvious disadvantage of bolt-ins, veneers, and other templates that derive from their parameterizing type(s), is that they hide that type's constructors. Consider the hypothetical classes in Listing 23.1.

Example 23.1. 

class Double
{
public:
  typedef double  value_type;
public:
  Double();
  explicit Double(double d);
public:
  double GetValue() const;
  . . .
};

template <typename T>
class Wrapper
  : public T
{
  . . .
};

typedef Wrapper<Double>  Double_t;

Double_t  d1;
Double_t  d2(12.34); // Error!

Wrapper has effectively hidden the second constructor of Double, so the attempt to nondefault construct it is an error.

The method of solving this problem for Microsoft's Active Template Library (ATL) is to avoid it. CComObject ...