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Posted 23 May 2004

Member Function Pointers and the Fastest Possible C++ Delegates

, 5 Apr 2005 CPOL
A comprehensive tutorial on member function pointers, and an implementation of delegates that generates only two ASM opcodes!
// HOPTER.EXE -- A simple header generator for vararg templates
 C++ doesn't have a vararg feature for templates. This can be a
 nuisance when writing class libraries. There are a few options:
 (a) manually repeat code. This is tedious and error prone.
 (b) Use macros. Messy, can't cope with template
   arguments, and users get hard-to-interpret error messages.
 (c) Use the boost preprocessor library. This slows compilation,
   and introduces a dependency on boost.
 (d) Write a program to automatically generate the code.
   I've never seen a general program to do this job, so this is 
   a very simple command-line program to do it.

 Why is it called Hopter?
 I wanted a word that started with h, to be reminiscent of header
 files. At the time, my 2yo son kept talking about 'Hopter Copters'
 (he couldn't pronounce 'helicopter').
 It also reflects the level of sophistication of this program --
 it was named by a two year old.


 When you analyze the problem, you find that the requirements
 are very simple.
 Varargs are needed in two places: templates and functions.
 You want to use them in two ways: when declaring the template
 or function, and when invoking the function / selecting the 

 It's a brain-dead implementation, which just does a search-and-replace
 of specific strings. It could be done with a few lines using sed on
 a *NIX platform. Because I use Windows, I've done a quick and dirty
 implementation using CString.

In a file, @VARARGS at the start of a line indicates the start of the code
to be expanded. @ENDVAR at the start of a line marks the end of the 

 declare template  --  template <@CLASSARGS> class SomeClass
 template<class Param1, class Param2>
 class SomeClass

 declare function  -- somefunc(@FUNCARGS)
 somefunc(Param1 p1, Param2 p2)

 select template -- SomeClass<@SELARGS>
 SomeClass<Param1, Param2>

 invoke function -- somefunc(@INVOKEARGS)
 somefunc(p1, p2)

 I also provide @NUM which is the number of arguments.
 The case where @NUM is zero is a special case: for template declarations,
 the entire template<> bit needs to disappear. When other arguments are 
 involved, either at the beginning or end of the list, there a commas 
 which might need to be removed.

 I've used .hxx as the extension for these files. This enables C++ 
 I wanted to use an unusual file extension (.HOH) for these files, because they
 aren't real header files. But I also like to use the C++ syntax highlighting 
 in Visual Studio. I chose .hxx because MSVC treats it as C++, but it 
 seems to very rarely used.

 Installation (for VC6, if you want to use .HOH as the extension instead):
 Go into the registry and change 
 HKEY_CURRENT_USER\Software\Microsoft\DevStudio\6.0\Text Editor\Tabs/Language Settings\C/C++\FileExtensions from cpp;cxx;c;h;hxx;hpp;inl;tlh;tli;rc;rc2 
 to cpp;cxx;c;h;hxx;hpp;inl;tlh;tli;rc;rc2;hoh by adding �;hoh� to the end.
 Then add this as a custom build step to the main file of your project:
 hopter $(InputDir)\*.hoh $(InputDir)\*.h


#if defined(_MSC_VER)
#  if (_MSC_VER <1300)
#    include <afx.h>
#  else
#    include "atlstr.h"
#  endif
#  include "CString.hpp"

/* Example: This is an excerpt from boost::function_template.hpp


        typename FunctionPtr,
        typename R BOOST_FUNCTION_COMMA
        typedef typename ct_if<(is_void<R>::value),
                            R BOOST_FUNCTION_COMMA
                            R BOOST_FUNCTION_COMMA
                       >::type type;

Using HOPTER, this can be written as:

    template< typename FunctionPtr, typename R, @CLASSARGS>
      struct get_function_invoker@NUM
        typedef typename ct_if<(is_void<R>::value),
					void_function_invoker@NUM<FunctionPtr, R, @SELARGS>,
					function_invoker@NUM<FunctionPtr,R, @SELARGS>
                       >::type type;

const int MaxNumArgs=8;

void showusage()
	printf("Usage: HOPTER infile.hxx outfile.h\n Reads infile.hxx, creates outfile.h\n");

/// Makes the necessary subsititutions in 'bigblock', and writes to fout.
void PrintVarArgs(FILE *fout, CString bigblock, int num) {
	CString numstr;
	CString invokelist;
	CString funclist;
	CString selectlist;
	CString classlist;
	CString commastr;
	numstr.Format("%d", num);
	if (num==0) {
	}else {
		funclist="Param1 p1";
		selectlist = "Param1";
		classlist = "class Param1";
		commastr=", ";
		CString str;
		for (int k=2; k<=num; k++) {
			str.Format(", p%d", k);
			str.Format(", Param%d p%d", k, k);
			str.Format(", Param%d", k);
			selectlist += str;
			str.Format(", class Param%d", k);
			classlist += str;

	// Simple replacement of number of arguments
	bigblock.Replace("@NUM", numstr);

	// Template declarations
	if (num==0) bigblock.Replace("template<@CLASSARGS>", "");
	else bigblock.Replace("template<@CLASSARGS>", 
		(CString)"template<" + classlist+">");
	if (num==0) bigblock.Replace("template <@CLASSARGS>", "");
	else bigblock.Replace("template <@CLASSARGS>", 
		(CString)"template<" + classlist+">");
	bigblock.Replace(",@CLASSARGS", commastr + classlist);
	bigblock.Replace(", @CLASSARGS", commastr + classlist);
	bigblock.Replace("@CLASSARGS, ", classlist + commastr);
	bigblock.Replace("@CLASSARGS,", classlist + commastr);
	bigblock.Replace("@CLASSARGS", classlist);

	// Template selections
	CString selargstr;
	if (num==0) selargstr = "";
	else selargstr = (CString)"<"+selectlist+">";

	bigblock.Replace("<@SELARGS>", selargstr);
	bigblock.Replace("< @SELARGS >", selargstr);
	bigblock.Replace(",@SELARGS", commastr + selectlist);
	bigblock.Replace(", @SELARGS", commastr + selectlist);
	bigblock.Replace("@SELARGS, ", selectlist + commastr);
	bigblock.Replace("@SELARGS,", selectlist + commastr);
	bigblock.Replace("@SELARGS", selectlist);

	// Function declarations
	bigblock.Replace(",@FUNCARGS", commastr + funclist);
	bigblock.Replace(", @FUNCARGS", commastr + funclist);
	bigblock.Replace("@FUNCARGS, ", funclist + commastr);
	bigblock.Replace("@FUNCARGS,", funclist + commastr);
	bigblock.Replace("@FUNCARGS", funclist);

	// Function invocation
	bigblock.Replace(",@INVOKEARGS", commastr + invokelist);
	bigblock.Replace(", @INVOKEARGS", commastr + invokelist);
	bigblock.Replace("@INVOKEARGS, ", invokelist + commastr);
	bigblock.Replace("@INVOKEARGS,", invokelist + commastr);
	bigblock.Replace("@INVOKEARGS", invokelist);

	fprintf(fout, bigblock);				

int main(int argc, char* argv[])
	if (argc<3) { showusage(); return 2; };
	FILE * fin;
	FILE *fout;
//	CString infilename;
//	infilename=argv[1];
	fin =fopen(argv[1],"rt");
	if (!fin) { printf("Error, can't open %s\n", argv[1]); return 1; };
	fout = fopen (argv[2], "wt");
	if (!fout) { printf("Error, can't open %s\n", argv[2]); return 1; };
	char buf[5000];
	for (;;) {
	 if (!fgets(buf, 5000, fin)) break;
		if (!strncmp(buf, "@VARARGS", 8)) {
			// found a match...
			CString bigblock;
			for (;;) {
				if (feof(fin)) { printf("No matching @ENDVAR !!??\n"); return 1; };
				fgets(buf, 5000, fin);
				if (!strncmp(buf, "@ENDVAR", 7)) break;
//			fprintf(fout, "// Auto-generated code [[[\n");
			for (int k=0; k<=MaxNumArgs; k++) {
				fprintf(fout, "//N=%d\n", k);
				PrintVarArgs(fout, bigblock, k);
//			fprintf(fout, "// ]]] End auto-generated code\n");
		} else {
			fprintf(fout, "%s", buf);
	return 0;

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About the Author

Don Clugston
Germany Germany
I'm an Australian physicist/software engineer living in Leipzig, Germany. I've published papers about windows (the glass kind), semiconductor device physics, environmental philosophy and theology, vacuum systems, and now (at CodeProject) the innards of C++. (Yes, my C.V. is a terrible mess.) I'm a major contributor to the D programming language. I can read New Testament Greek, and can speak German with a dreadful accent.

I have one wife, one son (pictured), and one daughter, who are all much cuter than I am.

"The light shines in the darkness, but the darkness has not overcome it."

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