how can i short name a compile time state ?

[toton]

Hi,
I want to remember a compile time variable state, just to have some
typing convenience. I am not sure if I can do it using some typedef.
To give a short example,
I have an enum as,

enum dir_type{
dir_x,dir_y,dir_xd
};

and 2 specializations as,

template<dir_type>
dir_type o_dir();

template<>
dir_type o_dir<dir_x>(){
return dir_y;
}
template<>
dir_type o_dir<dir_y>(){
return dir_x;
}

now in a function I want to do,

template<dir_type dt>
void funct(){
std::cout<<dt<<std::endl;
dir_type od = o_dir<dt>(); /// what would be compile time equiv of
this line so than i can call next line?
dir_type d = o_dir<od>();
}
Now for the commented line, I want a compile time typedef , so that od
is a name rather than a runtime state, and I can call the next line.
This is just to remove repetitive writing of o_dir<dt> , in several
places when i can simply write od.

thanks
abir

 

[Michael DOUBEZ]

toton a écrit :

Hi,
I want to remember a compile time variable state, just to have some
typing convenience. I am not sure if I can do it using some typedef.
To give a short example,
I have an enum as,

enum dir_type{
dir_x,dir_y,dir_xd
};

and 2 specializations as,

template<dir_type>
dir_type o_dir();

template<>
dir_type o_dir<dir_x>(){
return dir_y;
}
template<>
dir_type o_dir<dir_y>(){
return dir_x;
}

now in a function I want to do,

template<dir_type dt>
void funct(){
std::cout<<dt<<std::endl;
dir_type od = o_dir<dt>(); /// what would be compile time equiv of
this line so than i can call next line?
dir_type d = o_dir<od>();
}
Now for the commented line, I want a compile time typedef , so that od
is a name rather than a runtime state, and I can call the next line.
This is just to remove repetitive writing of o_dir<dt> , in several
places when i can simply write od.

You cannot use a runtime variable (od) to specialize your template
(o_dir). If your program really has the form you gave (i.e. it returns
fixed values), you can transform your functions into mapping structures:

template<dir_type>
struct o_dir();

template<>
struct o_dir<dir_x>
{
static const dir_type value = dir_y;
}

template<>
struct o_dir<dir_y>
{
static const dir_type value = dir_x;
}

template<dir_type dt>
void funct()
{
std::cout<<dt<<std::endl;
static const dir_type od = o_dir<dt>::value;
/* static const */dir_type d = o_dir<od>::value;
}

Michael

 

[James Kanze]

I want to remember a compile time variable state, just to have some
typing convenience. I am not sure if I can do it using some typedef.
To give a short example,
I have an enum as,

enum dir_type{
dir_x,dir_y,dir_xd
};

and 2 specializations as,

template<dir_type>
dir_type o_dir();

template<>
dir_type o_dir<dir_x>(){
return dir_y;}

template<>
dir_type o_dir<dir_y>(){
return dir_x;
}

now in a function I want to do,

template<dir_type dt>
void funct(){
std::cout<<dt<<std::endl;
dir_type od = o_dir<dt>(); /// what would be compile time equiv of
this line so than i can call next line?
dir_type d = o_dir<od>();}

Now for the commented line, I want a compile time typedef , so
that od is a name rather than a runtime state, and I can call
the next line. This is just to remove repetitive writing of
o_dir<dt> , in several places when i can simply write od.

As you've written it, you can't. od is a value, initialized by
the return value of a function, and as such, is not a compile
time constant (at least not yet---there's some discussion of
allowing such trivial functions to be used in constant
expressions).

I'm not too sure what you're trying to achieve. Perhaps
something like:

template< dir_type >
struct o_dir ;
template<>
struct o_dir< dir_x >
{
static dir_type const other = dir_y ;
} ;
// etc.

could be used, e.g.:

template< dir_type dt >
void
funct()
{
dir_type const od = o_dir:ther ;
dir_type d = o_dir< od >:ther ;
// ...
}

 

[toton]

As you've written it, you can't. od is a value, initialized by
the return value of a function, and as such, is not a compile
time constant (at least not yet---there's some discussion of
allowing such trivial functions to be used in constant
expressions).

I'm not too sure what you're trying to achieve. Perhaps
something like:

template< dir_type >
struct o_dir ;
template<>
struct o_dir< dir_x >
{
static dir_type const other = dir_y ;
} ;
// etc.

could be used, e.g.:

template< dir_type dt >
void
funct()
{
dir_type const od = o_dir:ther ;
dir_type d = o_dir< od >:ther ;
// ...
}

--
James Kanze (GABI Software) email:[email protected]
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Hi thanks for the solution. It is working, and this is what I wanted.
However one problem is that , its name causes problem with one of
runtime function defined as,
dir_type o_dir(dir_type dir);
Can I put same name struct and function definition in same name space,
or I need to rename 2 versions as different name.
(What i wanted to achieve with this is, one version which returns
orthogonal direction in run time using switch case statement, another
compile time version like const std::size_t dir_x =0; const
std::size_t dir_y = 1;
std::size_t o_dir[] = {dir_y,dir_x}; kind of lookup, except, I wanted
it in terms of enum, where values are not sequential (but finite) ,
and safe. In this version I cant make dir_y = 1000; If I make, 1000
size array is required.
another option is to use some kind of static (compile time ) map. But
I found the solution proposed by you a simpler one)

Thanks for responds
abir

 

[James Kanze]

On Feb 25, 1:56 pm, James Kanze <[email protected]> wrote:

[...]

Hi thanks for the solution. It is working, and this is what I wanted.
However one problem is that , its name causes problem with one of
runtime function defined as,
dir_type o_dir(dir_type dir);
Can I put same name struct and function definition in same
name space, or I need to rename 2 versions as different name.

You can use the same name for both a class and a function in the
same scope, but I wouldn't. It's a hack, present only for
reasons of C compatibility, and will confuse the readers. I'd
come up with some different names. (Generally, the name of a
function should be a verb; the name of a class never is.)

(What i wanted to achieve with this is, one version which returns
orthogonal direction in run time using switch case statement, another
compile time version like const std::size_t dir_x =0; const
std::size_t dir_y = 1;
std::size_t o_dir[] = {dir_y,dir_x}; kind of lookup, except, I wanted
it in terms of enum, where values are not sequential (but finite) ,
and safe. In this version I cant make dir_y = 1000; If I make, 1000
size array is required.

I'm not sure I understand completely, but perhaps some sort of
metaprogramming trick could be used.