Template Aliases (C++11)
It can be convenient, especially in template design, to create aliases for types. You can use typedef to create aliases for template specializations:
// define three typedef aliases
typedef std::array<double, 12> arrd;
typedef std::array<int, 12> arri;
typedef std::array<std::string, 12> arrst;
arrd gallons; // gallons is type std::array<double, 12>
arri days; // days is type std::array<int, 12>
arrst months; // months is type std::array<std::string, 12>
But if you find yourself writing code similar to the preceding typedefs, over and over, you might wonder if you’ve forgotten some language feature that simplifies the task or if the language has forgotten to supply such a feature. In this case, C++11 provides a feature previously missing—a way to use a template to provide a family of aliases. Here’s what the approach looks like:
template<typename T>
using arrtype = std::array<T,12>; // template to create multiple aliases
This makes arrtype a template alias that can be used as a type, as follows:
arrtype<double> gallons; // gallons is type std::array<double, 12>
arrtype<int> days; // days is type std::array<int, 12>
arrtype<std::string> months; // months is type std::array<std::string, 12>
In short, arrtype<T> means type std::array<T,12>.
C++11 extends the using = syntax to non-templates too. In that case, it becomes equivalent to an ordinary typedef:
typedef const char * pc1; // typedef syntax
using pc2 = const char *; // using = syntax
typedef const int *(*pa1)[10]; // typedef syntax
using pa2 = const int *(*)[10]; // using = syntax
As you get used to it, you may find the new form more readable because it separates the type name from type information more clearly.
Another C++11 addition to templates is the variadic template, which allows you to define a template class or function that can take a variable number of initializers. Chapter 18, “Visiting with the New C++ Standard,” looks into this topic.
Summary
C++ provides several means for reusing code. Public inheritance, described in Chapter 13, “Class Inheritance,” enables you to model is-a relationships, with derived classes being able to reuse the code of base classes. Private and protected inheritance also let you reuse base-class code, this time modeling has-a relationships. With private inheritance, public and protected members of the base class become private members of the derived class. With protected inheritance, public and protected members of the base class become protected members of the derived class. Thus, in either case, the public interface of the base class becomes an internal interface for the derived class. This is sometimes described as inheriting the implementation but not the interface because a derived object can’t explicitly use the base-class interface. Thus, you can’t view a derived object as a kind of base object. Because of this, a base-class pointer or reference is not allowed to refer to a derived object without an explicit type cast.
You can also reuse class code by developing a class with members that are themselves objects. This approach, called containment, layering, or composition, also models the has-a relationship. Containment is simpler to implement and use than private or protected inheritance, so it is usually preferred. However, private and protected inheritance have slightly different capabilities. For example, inheritance allows a derived class access to protected members of a base class. Also it allows a derived class to redefine a virtual function inherited from the base class. Because containment is not a form of inheritance, neither of these capabilities are options when you reuse class code via containment. On the other hand, containment is more suitable if you need several objects of a given class. For example, a State class could contain an array of County objects.
Multiple inheritance (MI) allows you to reuse code for more than one class in a class design. Private or protected MI models the has-a relationship, and public MI models the is-a relationship. MI can create problems with multidefined names and multi-inherited bases. You can use class qualifiers to resolve name ambiguities and virtual base classes to avoid multi-inherited bases. However, using virtual base classes introduces new rules for writing initialization lists for constructors and for resolving ambiguities.
Class templates let you create a generic class design in which a type, usually a member type, is represented by a type parameter. A typical template looks like this:
template <class T>
class Ic
{
T v;
...
public:
Ic(const T & val) : v(val) { }
...
};
Here T is the type parameter, and it acts as a stand-in for a real type to be specified at a later time. (This parameter can have any valid C++ name, but T and Type are common choices.) You can also use typename instead of class in this context:
template <typename T> // same as template <class T>
class Rev {...} ;
Class definitions (instantiations) are generated when you declare a class object and specify a particular type. For example, the following declaration causes the compiler to generate a class declaration in which every occurrence of the type parameter T in the template is replaced by the actual type short in the class declaration:
class Ic<short> sic; // implicit instantiation
In this case, the class name is Ic<short>, not Ic. Ic<short> is termed a specialization of the template. In particular, it is an implicit instantiation.
An explicit instantiation occurs when you declare a specific specialization of the class, using the keyword template:
template class IC<int>; // explicit instantiation
In this situation, the compiler uses the general template to generate an int specialization Ic<int>, even though no objects have yet been requested of that class.
You can provide explicit specializations, which are specialized class declarations that override a template definition. You just define the class, starting with template<>, and then you use the template class name, followed by angle brackets containing the type for which you want a specialization. For example, you could provide a specialized Ic class for character pointers as follows:
template <> class Ic<char *>.
{
char * str;
...
public:
Ic(const char * s) : str(s) { }
...
};
Then a declaration of the following form would use the specialized definition for chic rather than using the general template:
class Ic<char *> chic;
A class template can specify more than one generic type and can also have non-type parameters:
template <class T, class TT, int n>
class Pals {...};
The following declaration would generate an implicit instantiation using double for T, string for TT, and 6 for n:
Pals<double, string, 6> mix;
A class template can also have parameters that are templates:
template < template <typename T> class CL, typename U, int z>
class Trophy {...};
Here z stands for an int value, U stands for the name of a type, and CL stands for a class template declared using template <typename T>.
Class templates can be partially specialized:
template <class T> Pals<T, T, 10> {...};
template <class T, class TT> Pals<T, TT, 100> {...};
template <class T, int n> Pals <T, T*, n> {...};
The first example here creates a specialization in which both types are the same and n has the value 6. Similarly, the second creates a specialization for n equal to 100, and the third creates a specialization for which the second type is a pointer to the first type.
Template classes can be members of other classes, structures, and templates.
The goal of all these methods is to allow programmers to reuse tested code without having to copy it manually. This simplifies the programming task and makes programs more reliable.
Chapter Review
1. For each of the following sets of classes, indicate whether public or private derivation is more appropriate for Column B:
2. Suppose you have the following definitions:
class Frabjous {
private:
char fab[20];
public:
Frabjous(const char * s = "C++") : fab(s) { }
virtual void tell() { cout << fab; }
};
class Gloam {
private:
int glip;
Frabjous fb;
public:
Gloam(int g = 0, const char * s = "C++");
Gloam(int g, const Frabjous & f);
void tell();
};
Given that the Gloam version of tell() should display the values of glip and fb, provide definitions for the three Gloam methods.
3. Suppose you have the following definitions:
class Frabjous {
private:
char fab[20];
public:
Frabjous(const char * s = "C++") : fab(s) { }
virtual void tell() { cout << fab; }
};
class Gloam : private Frabjous{
private:
int glip;
public:
Gloam(int g = 0, const char * s = "C++");
Gloam(int g, const Frabjous & f);
void tell();
};
Given that the Gloam version of tell() should display the values of glip and fab, provide definitions for the three Gloam methods.
4. Suppose you have the following definition, based on the Stack template of Listing 14.13 and the Worker class of Listing 14.10:
Stack<Worker *> sw;
Write out the class declaration that will be generated. Just do the class declaration, not the non-inline class methods.
5. Use the template definitions in this chapter to define the following:
• An array of string objects
• A stack of arrays of double
• An array of stacks of pointers to Worker objects
How many template class definitions are produced in Listing 14.18?
6. Describe the differences between virtual and nonvirtual base classes.
Programming Exercises
1. The Wine class has a string class object member (see Chapter 4) that holds the name of a wine and a Pair object (as discussed in this chapter) of valarray<int> objects (as discussed in this chapter). The first member of each Pair object holds the vintage years, and the second member holds the numbers of bottles owned for the corresponding particular vintage year. For example, the first valarray object of the Pair object might hold the years 1988, 1992, and 1996, and the second valarray object might hold the bottle counts 24, 48, and 144. It may be convenient for Wine to have an int member that stores the number of years. Also some typedefs might be useful to simplify the coding:
typedef std::valarray<int> ArrayInt;
typedef Pair<ArrayInt, ArrayInt> PairArray;
Thus, the PairArray type represents type Pair<std::valarray<int>, std::valarray<int> >. Implement the Wine class by using containment. The class should have a default constructor and at least the following constructors:
// initialize label to l, number of years to y,
// vintage years to yr[], bottles to bot[]
Wine(const char * l, int y, const int yr[], const int bot[]);
// initialize label to l, number of years to y,
// create array objects of length y
Wine(const char * l, int y);
The Wine class should have a method GetBottles() that, given a Wine object with y years, prompts the user to enter the corresponding number of vintage years and bottle counts. A method Label() should return a reference to the wine name. A method sum() should return the total number of bottles in the second valarray<int> object in the Pair object.
The program should prompt the user to enter a wine name, the number of elements of the array, and the year and bottle count information for each array element. The program should use this data to construct a Wine object and then display the information stored in the object. For guidance, here’s a sample test program:
// pe14-1.cpp -- using Wine class with containment
#include <iostream>
#include "winec.h"
int main ( void )
{
using std::cin;
using std::cout;
using std::endl;
cout << "Enter name of wine: ";
char lab[50];
cin.getline(lab, 50);
cout << "Enter number of years: ";
int yrs;
cin >> yrs;
Wine holding(lab, yrs); // store label, years, give arrays yrs elements
holding.GetBottles(); // solicit input for year, bottle count
holding.Show(); // display object contents
const int YRS = 3;
int y[YRS] = {1993, 1995, 1998};
int b[YRS] = { 48, 60, 72};
// create new object, initialize using data in arrays y and b
Wine more("Gushing Grape Red",YRS, y, b);
more.Show();
cout << "Total bottles for " << more.Label() // use Label() method
<< ": " << more.sum() << endl; // use sum() method
cout << "Bye
";
return 0;
}
And here’s some sample output:
Enter name of wine: Gully Wash
Enter number of years: 4
Enter Gully Wash data for 4 year(s):
Enter year: 1988
Enter bottles for that year: 42
Enter year: 1994
Enter bottles for that year: 58
Enter year: 1998
Enter bottles for that year: 122
Enter year: 2001
Enter bottles for that year: 144
Wine: Gully Wash
Year Bottles
1988 42
1994 58
1998 122
2001 144
Wine: Gushing Grape Red
Year Bottles
1993 48
1995 60
1998 72
Total bottles for Gushing Grape Red: 180
Bye
2. This exercise is the same as Programming Exercise 1, except that you should use private inheritance instead of containment. Again, a few typedefs might prove handy. Also you might contemplate the meaning of statements such as the following:
PairArray::operator=(PairArray(ArrayInt(),ArrayInt()));
cout << (const string &)(*this);
The class should work with the same test program as shown in Programming Exercise 1.
3. Define a QueueTp template. Test it by creating a queue of pointers-to-Worker (as defined in Listing 14.10) and using the queue in a program similar to that in Listing 14.12.
4. A Person class holds the first name and the last name of a person. In addition to its constructors, it has a Show() method that displays both names. A Gunslinger class derives virtually from the Person class. It has a Draw() member that returns a type double value representing a gunslinger’s draw time. The class also has an int member representing the number of notches on a gunslinger’s gun. Finally, it has a Show() function that displays all this information.
A PokerPlayer class derives virtually from the Person class. It has a Draw() member that returns a random number in the range 1 through 52, representing a card value. (Optionally, you could define a Card class with suit and face value members and use a Card return value for Draw().) The PokerPlayer class uses the Person show() function. The BadDude class derives publicly from the Gunslinger and PokerPlayer classes. It has a Gdraw() member that returns a bad dude’s draw time and a Cdraw() member that returns the next card drawn. It has an appropriate Show() function. Define all these classes and methods, along with any other necessary methods (such as methods for setting object values) and test them in a simple program similar to that in Listing 14.12.
5. Here are some class declarations:
// emp.h -- header file for abstr_emp class and children
#include <iostream>
#include <string>
class abstr_emp
{
private:
std::string fname; // abstr_emp's first name
std::string lname; // abstr_emp's last name
std::string job;
public:
abstr_emp();
abstr_emp(const std::string & fn, const std::string & ln,
const std::string & j);
virtual void ShowAll() const; // labels and shows all data
virtual void SetAll(); // prompts user for values
friend std::ostream &
operator<<(std::ostream & os, const abstr_emp & e);
// just displays first and last name
virtual ~abstr_emp() = 0; // virtual base class
};
class employee : public abstr_emp
{
public:
employee();
employee(const std::string & fn, const std::string & ln,
const std::string & j);
virtual void ShowAll() const;
virtual void SetAll();
};
class manager: virtual public abstr_emp
{
private:
int inchargeof; // number of abstr_emps managed
protected:
int InChargeOf() const { return inchargeof; } // output
int & InChargeOf(){ return inchargeof; } // input
public:
manager();
manager(const std::string & fn, const std::string & ln,
const std::string & j, int ico = 0);
manager(const abstr_emp & e, int ico);
manager(const manager & m);
virtual void ShowAll() const;
virtual void SetAll();
};
class fink: virtual public abstr_emp
{
private:
std::string reportsto; // to whom fink reports
protected:
const std::string ReportsTo() const { return reportsto; }
std::string & ReportsTo(){ return reportsto; }
public:
fink();
fink(const std::string & fn, const std::string & ln,
const std::string & j, const std::string & rpo);
fink(const abstr_emp & e, const std::string & rpo);
fink(const fink & e);
virtual void ShowAll() const;
virtual void SetAll();
};
class highfink: public manager, public fink // management fink
{
public:
highfink();
highfink(const std::string & fn, const std::string & ln,
const std::string & j, const std::string & rpo,
int ico);
highfink(const abstr_emp & e, const std::string & rpo, int ico);
highfink(const fink & f, int ico);
highfink(const manager & m, const std::string & rpo);
highfink(const highfink & h);
virtual void ShowAll() const;
virtual void SetAll();
};
Note that the class hierarchy uses MI with a virtual base class, so keep in mind the special rules for constructor initialization lists for that case. Also note the presence of some protected-access methods. This simplifies the code for some of the highfink methods. (Note, for example, that if highfink::ShowAll() simply calls fink::ShowAll() and manager::ShowAll(), it winds up calling abstr_emp::ShowAll() twice.) Provide the class method implementations and test the classes in a program. Here is a minimal test program:
// pe14-5.cpp
// useemp1.cpp -- using the abstr_emp classes
#include <iostream>
using namespace std;
#include "emp.h"
int main(void)
{
employee em("Trip", "Harris", "Thumper");
cout << em << endl;
em.ShowAll();
manager ma("Amorphia", "Spindragon", "Nuancer", 5);
cout << ma << endl;
ma.ShowAll();
fink fi("Matt", "Oggs", "Oiler", "Juno Barr");
cout << fi << endl;
fi.ShowAll();
highfink hf(ma, "Curly Kew"); // recruitment?
hf.ShowAll();
cout << "Press a key for next phase:
";
cin.get();
highfink hf2;
hf2.SetAll();
cout << "Using an abstr_emp * pointer:
";
abstr_emp * tri[4] = {&em, &fi, &hf, &hf2};
for (int i = 0; i < 4; i++)
tri[i]->ShowAll();
return 0;
}
Why is no assignment operator defined?
Why are ShowAll() and SetAll() virtual?
Why is abstr_emp a virtual base class?
Why does the highfink class have no data section?
Why is only one version of operator<<() needed?
What would happen if the end of the program were replaced with this code?
abstr_emp tri[4] = {em, fi, hf, hf2};
for (int i = 0; i < 4; i++)
tri[i].ShowAll();