Thinking In C++. Volume 2: Practical Programming

Without understanding the likes of the previous examples in this section, novices find themselves frustrated because they can’t get a simple stream output inserter to work. If you don’t define your operators inside the definition of Box, you must provide the forward declarations we showed earlier:^.

//: C05:Box1.cpp

// Defines template operators

#include

using namespace std;

// Forward declarations

template

class Box;

template

Box operator+(const Box&, const Box&);

template

ostream& operator<<(ostream&, const Box&);

template

class Box {

  T t;

public:

  Box(const T& theT) : t(theT) {}

  friend Box operator+<>(const Box&, const Box&);

  friend ostream& operator<< <>(ostream&, const Box&);

};

template

Box operator+(const Box& b1, const Box& b2) {

  return Box(b1.t + b2.t);

}

template

ostream& operator<<(ostream& os, const Box& b) {

  return os << '[' << b.t << ']';

}

int main() {

  Box b1(1), b2(2);

  cout << b1 + b2 << endl;  // [3]

//  cout << b1 + 2 << endl; // no implicit conversions!

} ///:~

Here we are defining both an addition operator and an output stream operator. The main program reveals a disadvantage of this approach: you can’t depend on implicit conversions (see the expression b1 + 2) because templates do not provide them. Using the in-class, non-template approach is shorter and more robust:^.

//: C05:Box2.cpp

// Defines non-template operators

#include

using namespace std;

template

class Box {

  T t;

public:

  Box(const T& theT) : t(theT) {}

  friend Box operator+(const Box& b1,

          const Box& b2) {

    return Box(b1.t + b2.t);

  }

  friend ostream& operator<<(ostream& os,

                const Box& b) {

    return os << '[' << b.t << ']';

  }

};

int main() {

  Box b1(1), b2(2);

  cout << b1 + b2 << endl;  // [3]

  cout << b1 + 2 << endl;   // [3]

} ///:~

Because the operators are normal functions (overloaded for each specialization of Box—just int in this case, of course), implicit conversions are applied as normal; so the expression b1 + 2 is valid^.

Friend templates

You can be precise as to which specializations of a template are friends of a class. In the examples in the previous section, only the specialization of the function template f with the same type that specialized Friendly was a friend. For example, only the specialization f(const Friendly&) is a friend of the class Friendly. This was accomplished by using the template parameter for Friendly to specialize f in its friend declaration. If we had wanted to, we could have made a particular, fixed specialization of f a friend to all instances of Friendly, like this:^.

// Inside Friendly:

  friend void f<>(const Friendly&);

By using double instead of T, the double specialization of f has access to the non-public members of any Friendly specialization. The specialization f( ) still isn’t instantiated unless it is explicitly called, of course^.

Likewise, if you were to declare a non-template function with no parameters dependent on T, that single function would be a friend to all instances of Friendly:^.

// Inside of Friendly:

  friend void g(int);  // g(int) befriends all Friendly’s

As always, since g(int) is unqualified, it must be defined at file scope (the namespace scope containing Friendly)^.

It is also possible to arrange for all specializations of f to be friends for all specializations of Friendly, with a so-called friend template, as follows:

template

class Friendly {

  template friend void f<>(const Friendly&);

Since the template argument for the friend declaration is independent of T, any combination of T and U is allowed, achieving the friendship objective. Like member templates, friend templates can appear within non-template classes as well^.

Template programming idioms

Since language is a tool of thought, new language features tend to spawn new programming techniques. In this section we cover some commonly-used template programming idioms that have emerged in the years since templates were added to the C++ language.^[62] 

Traits

The traits template technique, pioneered by Nathan Myers, is a means of bundling type-dependent declarations together. In essence, using traits allows you to "mix and match" certain types and values with contexts that use them in a flexible manner, while keeping your code readable and maintainable^.

The simplest example of a traits template is the numeric_limits class template defined in . The primary template is defined as follows:

template class numeric_limits {

public:

  static const bool is_specialized = false;

  static T min() throw();

  static T max() throw();

  static const int digits = 0;

  static const int digits10 = 0;

  static const bool is_signed = false;

  static const bool is_integer = false;

  static const bool is_exact = false;

  static const int radix = 0;