Question

Is there a way to provide default parameter values for methods of a template class? For example I have the following:

template<class T>
class A
{
public:
    A foo(T t);
};

How should I modify this to give foo a default parameter of type T? For example: T is int then a default value of -23, or T is char* then default value of "something", etc. Is this even possible?

Answer 1

If you want the default parameter to be just the default value (zero, usually), then you can write A foo(T t = T()). Otherwise, I suggest a trait class:

template <typename T> struct MyDefaults
{
  static const T value = T();
};

template <> struct MyDefaults<int>
{
  static const int value = -23;
};


template<class T>
class A
{
public:
    A foo(T t = MyDefaults<T>::value);
};

Writing the constant value inside the class definition only works for integral types, I believe, so you may have to write it outside for all other types:

template <> struct MyDefaults<double>
{
  static const double value;
};
const double MyDefaults<double>::value = -1.5;

template <> struct MyDefaults<const char *>
{
  static const char * const value;
};
const char * const MyDefaults<const char *>::value = "Hello World";

In C++11, you could alternatively say static constexpr T value = T(); to make the template work for non-integral values, provided that T has a default constructor that is declared constexpr:

template <typename T> struct MyDefaults
{
  static constexpr T value = T();
};

template <> struct MyDefaults<const char *>
{
  static constexpr const char * value = "Hello World";
};