How to Generate C++ Class Template Definitions

++ class template header and implementation (skeleton) definitions are often hard to read, let alone to write. Especially when defining template classes that are derived from base template classes, I often find myself struggling for the correct syntax.

In this article, I will present a template-based source code generator to produce C++ class template header implementation (skeleton) definitions in .hpp and .cpp files, based on a minimal yet functional set of methods.

A template is a way to specify generic code, with a placeholder for the type. Note that the type is the only “parameter” of a template, but a very powerful one, since anything from a function to a class (or a routine) can be specified in “general” terms without concerning yourself about the specific type. Yet. These details are postponed until you start to use the template. You can consider templates to be compile-time polymorphic, yet typesafe (in contrast to C MACROs).

Function vs. Class
When talking about C++ templates, one should realize that there are, in fact, two kinds of templates: function templates and class templates. The former are quite easy to implement, because they usually only contain the template(s) in their definition. As an example of a function template, here is a function that produces the minimum of two arguments, without specifying the actual type of the arguments:

template T max(const T &X, const T &Y){  if (X > Y)     return X;  else    return Y;}

T is the usual template character that is used to specify the typename, which?at the time of definition?is unknown, and will be determined when you actually use the template in your source code. Here is an example:

int x = max(6, 42); // compiler determines T = intfloat y = max(3.1415927, 2.20371); // compiler determines T = float

Or explicitly, as follows:

int x = max (6, 42); // explicit template syntax

The C++ compiler will be able to determine?at compile time?where the calls to this function template are made, which argument types are used, and hence which “expansions” of this function template have to be generated (like a MACRO expansion) and then compiled and linked into an executable. All this is happening behind the scenes, of course, although template expansion can take a lot of compiler and linker resource (as you may find out when you start to use them more often).

Class templates are similar to function templates in that the compiler will determine at compile-time which expansions (or instantions) of the class template are needed. The fact that they are classes and not merely functions, makes the syntax a bit more difficult, however.

Pop Quiz
Even if you’re an experienced C++ class template user, could you tell me from the top of your head what the syntax would be of the implementation skeleton for the copy constructor of a template class TDerived, which is derived from a template class TBase? You have 10 seconds …

It turns out to be as follows:

template  TDerived::TDerived(const TDerived& copy): TBase(copy)

But I don’t blame you if you couldn’t come up with that right away. If you did know the answer, then you probably don’t need to read the remainder of this article, unless you’re also interested in a template-based template header/source generator. That’s what I made for myself, to help me remember.

Canonical Class
But before I want to continue with class templates, let’s first talk about a minimum useful class, sometimes also called a canonical class. By this I mean a class definition which is minimal (only a few key methods), but still complete and useful. This means that the class should at least contain the default constructor (without an argument), the destructor, a copy constructor, the assignment operator, the compare operator and last?optionally?the stream operator (always useful when debugging). When a class contains at least these methods, we can call it a canonical class.

Since I’m planning to produce a template header and source generator, it’s important to realise what I should generate and what not. Making sure that I produce a canonical class?especially when it comes to templates, can make the difference between a nice but useless or an actual useful tool.As an example, here is the canonical class definition (header file) of a class TBase:

class TBase{  public:    // Constructors & Destructors    TBase(void);    TBase(const TBase& copy);    virtual ~TBase(void);    // Operator overloading    TBase& operator = (const TBase& other);    int operator == (const TBase& other) const;    // Output    friend ostream& operator << (ostream& os, const TBase& other);};

Canonical Class Template
We can modify the listing above to turn it into a canonical template class definition. Just like function templates, this means we have to use the template syntax in a few places, and sometimes in more than a few.Luckily, it's not that hard, and the result can be seen in the following listing:

template  class TBase{  public:    // Constructors & Destructors    TBase(void);    TBase(const TBase& copy);    virtual ~TBase(void);    // Operator overloading    TBase& operator = (const TBase& other);    int operator == (const TBase& other) const;    // Output    friend ostream& operator << (ostream& os, const TBase& other);};

Just to let you know what the implementation looks like (the empty skeletons, that is), take a look at the following listing:

// Constructors & Destructorstemplate  TBase::TBase(void) {}template  TBase::TBase(const TBase& copy) {}template  TBase::~TBase(void) {}// Operator overloadingtemplate  TBase& TBase::operator = (const TBase& other) {}template  int TBase::operator == (const TBase& other) const {}// Outputtemplate  ostream& operator << (ostream& os, const TBase& other) {}

This is usually the place where I could do with a little help or support to get the class template syntax right.

Derived Templates
If you've been able to keep up with me so far, then let's get to the final round: templates derived from other templates. Sometimes you just have to derive your own custom class template TDerived from a base template class TBase (sound familiar?).And just for your amusement (and mine), I've included the header listing for the derived canonical class template definition below:

template  class TDerived: public TBase{  public:    // Constructors & Destructors    TDerived(void);    TDerived(const TDerived& copy);    virtual ~TDerived(void);    // Operator overloading    TDerived& operator = (const TDerived& other);    int operator == (const TDerived& other) const;    // Output    friend ostream& operator << (ostream& os, const TDerived& other);};

Certainly this TDerived class template definition needs a list of empty implementation skeletons, which are defined as follows (empty because they're skeletons, but they still need to be implemented by the programmer, of course).

// Constructors & Destructorstemplate  TDerived::TDerived(void): TBase() {}template  TDerived::TDerived(const TDerived& copy): TBase(copy) {}template  TDerived::~TDerived(void) {}// Operator overloadingtemplate  TDerived& TDerived::operator = (const TDerived& other) {}template  int TDerived::operator == (const TDerived& other) const {}// Outputtemplate  ostream& operator << (ostream& os, const TDerived& other) {}

OK, who could already produce the above listing without a second thought? If you could, then you probably didn't need to read this article, because the fun stuff is over. What remains is the description of a little tool that I made for myself to actually produce and generate the output listings that we've seen so far.

Template Template
If you look closely at the listings presented so far, you can see a pattern (believe me, there is logic behind this class template syntax). In fact, I have been able to produce two template files that can be used to generate the template listings we've seen in this article. The template file for the class definition (typically inside a header file) is defined as follows:

//    File: <#class>.hpp//  Author: drs. Robert E. Swart>//    Date: <#date>//    Time: <#time>// Version: 0.01// Generated by: HeadGen (c) 1995-2001 by Bob Swart
(aka Dr.Bob - www.drbob42.com)// Changes://#ifndef <#class>_hpp#define <#class>_hpp#include <#includebase>template > class <#class> <#publicbase>{ public: // Constructors & Destructors <#class>(void); <#class>(const <#class><#template>& copy); virtual ~<#class>(void);> // Accessing functions // Modifier functions // Operator overloading <#class><#template>& operator = (const <#class><#template>& other); int operator == (const <#class><#template>& other) const; // Streaming output friend ostream& operator << (ostream& os, const <#class><#template>& other); protected: private:};#endif

Note the special #-tags. WebBroker developers may recognize these as tags used in the PageProducer components. That's actually the case, since I'm using a TPageProducer component (from the Internet tab) to expand the above template into a true class template definition header?with or without a template base class. The same technique can be applied to the following template listing, that can be used to produce the empty template skeleton implementations:

//    File: <#class>.cpp//  Author: drs. Robert E. Swart//    Date: <#date>//    Time: <#time>// Version: 0.01// Generated by: HeadGen (c) 1995-2001 by Bob Swart
(aka Dr.Bob - www.drbob42.com)// Changes://#include "<#include>.hpp"// Constructors & Destructorstemplate > <#class><#template>::<#class>(void) <#base><#body>template > <#class><#template>::<#class>(const<#class><#template>& copy) <#basecopy><#body>template > <#class><#template>::~<#class>(void)<#body>// Operator overloadingtemplate > <#class><#template>& <#class><#template>::operator = (const <#class><#template>& other)<#body>template > int <#class><#template>::operator == (const<#class><#template>& other) const<#body>// Streaming outputtemplate > ostream& operator << (ostream&os, const <#class><#template>& other)<#body>

Again, the above listing can be used to produce a stand-alone class template as well as a derived class template. We only need to specify three options: the class name, the (optional) base class name, and the template character.

HeadGen

?
Figure 1Get a close-up view of the HeadGen application.

The utility HeadGen only requires the class name (the template character is T by default), as can be seen in Figure 1.

For the base class, specify Base in the Class Name box leave the Ancestor type box empty, and click on Generate. For the derived class, specify Derived in the Class Name box, Base in the Ancestor Type box and then click on Generate again. In both cases, the T will be added as prefix automatically (files Base.hpp and Base.cpp will contain the definition for TBase).

A very simple Borland C++Builder example program (to test the syntax of the generated files) can be seen below:

//---------------------------------------------------------#pragma hdrstop#include "Base.cpp" // TBase#include "Derived.cpp"; // TDerived//--------------------------------------------------------typedef TDerived TintClass;#pragma argsusedint main(int argc, char* argv[]){  TintClass* Bob = new TintClass();  TintClass Swart = TintClass(*Bob);  if (*Bob == Swart) { *Bob = Swart; }  return 0;}//--------------------------------------------------------

Note that I needed to include the .cpp files of the templates, and not only (or just) the .hpp files. That's because the .cpp files are "expanded" (like MACROs) to the compiler, which must find them in order to be able to use them.

External Templates
The two template files are external files HeadGen.h (for the .hpp header) and HeadGen.c (for the .cpp source file). As an additional benefit, you can edit these templates and make sure your own copyright statements appear in them. Make sure to keep all #-tags intact, though, otherwise the template PageProducer won't be able to work correctly anymore.

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