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Compile-time array initialization

Started by Opwiz Jul 6, 2004 at 8:27 AM 20 replies 4.3k views

Original Post

Jul 6, 2004 at 8:27 AM

Is there a way to initialize an array of size N at compile time (where N is a constant integer specified as a template parameter)? E.g. if I wish the elements to be initialized to N^2/i (where i is the array index).

Zahlman

Jul 6, 2004 at 8:55 AM

Wrap it in a templated class?
Create a templated function that returns such an array?

Opwiz

Jul 6, 2004 at 9:43 AM

I'm not sure how to do it. Can you give an example?

Remember I want the initialized array available at compile-time.

aboeing

Jul 6, 2004 at 10:30 AM

edit: missread the question.
I belive the book accompanying loki explains how to do stuff like this.
Check his Andrei Alexandrescu website

rypyr

Jul 6, 2004 at 11:14 AM

Quote:
Original post by Opwiz
Is there a way to initialize an array of size N at compile time (where N is a constant integer specified as a template parameter)? E.g. if I wish the elements to be initialized to N^2/i (where i is the array index).

So you want the 0th element to hold infinity? ;)

Opwiz

Jul 6, 2004 at 11:17 AM

Quote:
have a look at this: http://www.gamedev.net/community/forums/topic.asp?topic_id=252723&whichpage=1

Nice :). Thank you.

Opwiz

Jul 6, 2004 at 11:18 AM

Quote:
So you want the 0th element to hold infinity? ;)

Ok, that was not a very good example :)

Opwiz

Jul 6, 2004 at 2:05 PM

This is what I've managed to do:

template<int N>class ClassWithStaticArray{public:	void PrintElems()	{		std::cout << dummy << std::endl;			for(int i = 0; i < N; ++i)		{			std::cout << i << ": " << elem << std::endl;		}	}private:	template<int D>	static int Init()	{		return elem[D] = Init<D - 1>() - 1;	}	template<>	static int Init<0>()	{		return elem[0] = N;	}	static int elem[N];	static const int dummy;};template<int N>int ClassWithStaticArray<N>::elem[N];template<int N>const int ClassWithStaticArray<N>::dummy = Init<N>();

I have to use a static array but that happens to be what I need so that is not really a problem. The static array is initialized at run-time but only if the "dummy" variable is referenced. I want it to be generated at compile-time, or atleast without having to reference a dummy variable. Any ideas?

Opwiz

Jul 6, 2004 at 2:23 PM

Slightly better:

template<int N>class ClassWithStaticArray{public:	void PrintElems()	{		for(int i = 0; i < N; ++i)		{			std::cout << i << ": " << ar << std::endl;		}	}private:	template<int D>	static int& Init()	{		return elem[D] = Init<D - 1>() - 1;	}	template<>	static int& Init<0>()	{		return elem[0] = N;	}	static int elem[N];	static int* ar;};template<int N>int ClassWithStaticArray<N>::elem[N];template<int N>int* ClassWithStaticArray<N>::ar = &Init<N>() - N;

Whenever I need to reference the array I use the pointer "ar" that gets initialized when first used. Still generated at run-time though. A cleaner run-time solution would be to initialize the array at the constructor and use a flag to ensure that it only gets initialized once.

DigitalDelusion

Jul 6, 2004 at 3:27 PM

this is both, horrible and beautiful ;) but works for me, to use it simply substitue the fib meta function template.

#include <cstdio>#include <cstdlib>template <template<unsigned> class F, unsigned N>class value_array{	template <template<unsigned> class F, unsigned N>	class value_holder	{		value_holder<F,N-1> base;		const unsigned val;	public:		value_holder(void): val( F<N>::val){}	};	template <template<unsigned> class F>	class value_holder<F,0>	{		const unsigned val;	public:		value_holder(void): val( F<0>::val){}	};	static const value_holder<F,N> values;public:	static const unsigned size = N;	unsigned operator[](unsigned n){ return *(reinterpret_cast<const unsigned*>( &values)+n);}};template <template<unsigned> class F, unsigned N>const typename value_array<F,N>::value_holder<F,N> value_array<F,N>::values;//need a test "function"template <unsigned N>struct fib{	enum {val = fib<N-1>::val + fib<N-2>::val};};template <>struct fib<0>{ enum {val = 1};};template <>struct fib<1>{ enum {val = 1};};int main(void){	static value_array<fib, 10> fibo;	for(unsigned n = 0; n != 10; ++n)		printf("%d ", fibo[n]);	puts("");	system("pause");}

looking at the asm output for it you can find this gem

_DATA	SEGMENT?values@?$value_array@Ufib@@$09@@0V?$value_holder@Ufib@@$09@1@B DD 01H ; value_array<fib,10>::values	DD	01H	DD	02H	DD	03H	DD	05H	DD	08H	DD	0dH	DD	015H	DD	022H	DD	037H	DD	059H_DATA	ENDS

that's compile values =)

and yes, Im insane...

Fruny

Jul 6, 2004 at 4:20 PM

Have you thought about code generation? It is probably the simplest solution...

// gen_array_cpp.cpp#include <iostream>#include <fstream>int main(int argc, char* argv[]){   if(argc<2)   {      std::cerr << "Usage: " << argv[0] << " <array size>" << std::endl;      return 1;   }   size_t array_size = atoi(argv[1]);   std::ofstream array_h("array.h");   if(!array_h)   array_h << "// AUTOMATICALLY GENERATED FILE, DO NOT MODIFY.\n"           << "#ifndef INCLUDE_ARRAY_H\n"           << "#define INCLUDE_ARRAY_H\n"           << std::endl;;   array_h << "extern int array[" << array_size << "];" << std::endl;   array_h << "#endif // INCLUDE_ARRAY_H" << std::endl;   array_h.close();   std::ofstream array_cpp("array.cpp");   if(!array_cpp)   {      std::cerr << "Could not open file array.cpp" << std::endl;      return 1;   }   array_cpp << "// AUTOMATICALLY GENERATED FILE, DO NOT MODIFY.\n"   array_cpp << "int array[" << array_size << "] = {" << std::endl;   for(size_t i=0; i<array_size; ++i)         array_cpp << "  " << array_size*array_size / (i+1) << "," << std::endl;   array_cpp << "};" << std::endl;   array_cpp.close();}

or in python

import systry:   array_size = int(sys.argv[1])except:   print >> sys.stderr, "Usage: %s <array size>" % sys.argv[0]   exit(1)array_h = file("array.h","w")print >> array_h, "// AUTOMATICALLY GENERATED FILE, DO NOT MODIFY."print >> array_h, "#ifndef INCLUDE_ARRAY_H"print >> array_h, "#define INCLUDE_ARRAY_H"print >> array_hprint >> array_h, "extern int array[%d];" % array_sizeprint >> array_hprint >> array_h, "#endif // INCLUDE_ARRAY_H"array_h.close()array_cpp = file("array.cpp", "w")print >> array_cpp, "// AUTOMATICALLY GENERATED FILE, DO NOT MODIFY."print >> array_cppprint >> array_cpp, "array[%d] = {" % array_sizefor i in xrange(array_size):   print >> array_cpp, "  %d," % (array_size**2 / (i+1))print >> array_cpp, "};"array_cpp.close()

You add those programs to your build process, and use the files they generate as source files in your project. Simple, maintainable, and not relying on any complex language tricks that may or may not be supported by your compiler.

Opwiz

Jul 6, 2004 at 4:27 PM

Quote:
this is both, horrible and beautiful ;) but works for me, to use it simply substitue the fib meta function template.

I'm amazed it compiles :) very nice. Your example worked fine. Got internal compiler error though when I tried to implement it inside my class (my table value algorithm needs to use a template parameter specified for the class so I can't put the algorithm outside the class).

DigitalDelusion

Jul 6, 2004 at 4:33 PM

Sounds odd, would you mind posting code for that Im curious.

Opwiz

Jul 6, 2004 at 4:38 PM

Quote:
Have you thought about code generation? It is probably the simplest solution...

It has crossed my mind.. But my first thought was that it would be complicated and hard to maintain. You would have to change the input parameters to the code generator each time you change the size of the static array. Also if you plan to use several instances of the static array (initialized differently depending on some template parameters) it becomes impossible to implement with code generation.

Fruny

Jul 6, 2004 at 4:43 PM

Quote:
Original post by Opwiz
It has crossed my mind.. But my first thought was that it would be complicated and hard to maintain.

It's not that bad, really. :)

Quote:
You would have to change the input parameters to the code generator each time you change the size of the static array.

It can be done via a command-line parameter to the code generator. If it's part of your build process, it'll be transparent.

Quote:
Also if you plan to use several instances of the static array (initialized differently depending on some template parameters) it becomes impossible to implement with code generation.

Ok, it does get a bit more complicated, though still feasible ;)

Opwiz

Jul 6, 2004 at 4:55 PM

Quote:
Sounds odd, would you mind posting code for that Im curious.

This generates
fatal error C1001: INTERNAL COMPILER ERROR
(compiler file 'msc1.cpp', line 2701)

I'm using MSVC++ .NET 7.1

template<unsigned T>class CompilerErrorExample{public:	CompilerErrorExample()	{		for(unsigned i = 0; i < T; ++i)		{			std::cout << i << ": " << fibo << std::endl;		}	}	~CompilerErrorExample()	{	}	template<unsigned N>	struct fib	{		enum {val = fib<N-1>::val + fib<N-2>::val};	};	template <>	struct fib<0>{ enum {val = 1};};	template <>	struct fib<1>{ enum {val = 1};};	static value_array<fib, T> fibo;};template<unsigned T>value_array<CompilerErrorExample<T>::fib, T> CompilerErrorExample<T>::fibo;int main(void){	CompilerErrorExample<10> e;	return 0;}

I'm not sure if the declaration of fibo is correct but the compiler generates internal error with or without.

[Edit: corrected code]

DigitalDelusion

Jul 6, 2004 at 5:00 PM

I just noticed that, just putting the generative templte inside the class somehow confuses the compiler, thinking 'bout a general workaroudn for your scenario.

C-Junkie

Jul 6, 2004 at 5:02 PM

Since msvc crapped out on you, here are the errors gcc gives me:

$ g++ er.cc -o er
er.cc:26: error: explicit specialization in non-namespace scope

class
 CompilerErrorExample'
er.cc:26: error: enclosing class templates are not explicitly specialized
er.cc:27: error: template parameters not used in partial specialization:
er.cc:27: error:

T'
er.cc:29: error: explicit specialization in non-namespace scope

class
 CompilerErrorExample'
er.cc:29: error: enclosing class templates are not explicitly specialized
er.cc:30: error: template parameters not used in partial specialization:
er.cc:30: error:

T'
er.cc:32: error: ISO C++ forbids declaration of

value_array' with no type
er.cc:32: error: template-id

value_array
template struct CompilerErrorExample::fib, T>' used as a
declarator
er.cc:32: error: parse error before

;' token
er.cc:37: error: syntax error before

<' token
er.cc:37: error:

T' was not declared in this scope
er.cc:37: error: template argument 1 is invalid
er.cc:37: error: ISO C++ forbids declaration of

fibo' with no type
er.cc: In constructor

CompilerErrorExample::CompilerErrorExample() [with
 unsigned int T = 10]':
er.cc:41: instantiated from here
er.cc:12: error: invalid types

int[unsigned int]' for array subscript

This might help find out what's wrong.

Fruny

Jul 6, 2004 at 5:05 PM

gcc says:

foo.cc:25: error: explicit specialization in non-namespace scope `class   CompilerErrorExample<T>'foo.cc:25: error: enclosing class templates are not explicitly specializedfoo.cc:26: error: template parameters not used in partial specialization:foo.cc:26: error:         `T'

idem on 28/29.

It doesn't like your nested specializations. Take fib out of CompilerErrorExample.

Opwiz

Jul 6, 2004 at 5:05 PM

Quote:
Since msvc crapped out on you, here are the errors gcc gives me:

Did you include DigitalDelusions value_array code?

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