The C++ function and template parameter lists are special syntactic constructs, and it is impossible to directly
manipulate or generate them using C++ constructs.
This leads to unnecessary code repetition.
</div>
<div>
Consider the implementation of the <code>is_function<></code> metafunction is Boost.
The implementation uses an overloaded <code>is_function_tester()</code> function that is used for testing if a type is convertible
to a pointer to a function.
Because of the special treatment of parameter lists, it is not possible to directly match a function with an arbitrary parameter list.
Instead, the <code>is_function_tester()</code> must be overloaded for every distinct number of parameters that is to be supported.
For example:
</div>
<divclass="code"><pre>
template<class R>
yes_type is_function_tester(R (*)());
template<class R, class A0>
yes_type is_function_tester(R (*)(A0));
template<class R, class A0, class A1>
yes_type is_function_tester(R (*)(A0, A1));
template<class R, class A0, class A1, class A2>
yes_type is_function_tester(R (*)(A0, A1, A2));
// ...
</pre></div>
<div>
The need for this kind of repetition occurs particularly frequently while implementing generic components or metaprogramming facilities,
but the need also manifests itself in many far simpler situations.
</div>
<h4>Typical Solutions</h4>
<div>
Typically the repetition is done manually.
Manual code repetition is highly unproductive, but sometimes more readable to the untrained eye.
</div>
<div>
Another solution is to write an external program for generating the repeated code or use some other extra linguistic means such as a smart editor.
Unfortunately, using external code generators has many disadvantages:
<ul>
<li>Writing the generator takes time. (This could be helped by using a standard generator.)</li>
<li>It is no longer productive to manipulate C++ code directly.</li>
<li>Invoking the generator may be difficult.</li>
<li>Automating the invocation of the generator can be difficult in certain environments. (Automatic invocation is desirable for active libraries.)</li>
<li>Porting and distributing the generator may be difficult or simply takes precious time.</li>
</ul>
</div>
<h4>What about the preprocessor?</h4>
<div>
Because C++ comes with a preprocessor, one would assume that it would support these kinds of needs directly.
Using the preprocessor in this case is highly desirable because:
<ul>
<li>The preprocessor is highly portable.</li>
<li>The preprocessor is automatically invoked as part of the compilation process.</li>
<li>Preprocessor metacode can be directly embedded into the C++ source code.</li>
<li>Compilers generally allow viewing or outputting the preprocessed code, which can be used for debugging or to copy and paste the generated code.</li>
</ul>
</div>
<div>
Most unfortunately, the preprocessor is a very low level preprocessor that specifically does not support repetition or recursive macros.
<i>For detailed information on the capabilities and limitations of the preprocessor, please refer to the C++ standard <ahref="../bibliography.html#std">[Std]</a>.</i>
In order to change the maximum number of function parameters supported, you now simply change the <code>MAX_IS_FUNCTION_TESTER_PARAMS</code> definition and recompile.
