## digitalmars.D.learn - Learn template by math vector class

```I am learning about D template details by implementing a static sized vector
class.  Opinions on this code (like how to solve the obvious errors) or
alternatives that would be better practice in D would be appreciated:

class vector( T, size_t size )
{
private:
T p[ size ];

public:
this()
{
}

this( T[] q )
in
{
assert( q.length > 0 );
}
body
{
foreach ( size_t i, T v; q )
{
p[ i ] = v;
}
}

T opIndex( size_t i )
in
{
assert( i >= 0 && i < size );
}
body
{
return p[ i ];
}

T opIndexassign( T v, size_t i )
in
{
assert( i >= 0 && i < size );
}
body
{
p[ i ] = v;
return p[ i ];
}

T x() { return p[ 0 ]; }
T y() { return p[ 1 ]; }
T z() { return p[ 2 ]; }
T w() { return p[ 3 ]; }

void x( T v ) { p[ 0 ] = v; }
void y( T v ) { p[ 1 ] = v; }
void z( T v ) { p[ 2 ] = v; }
void w( T v ) { p[ 3 ] = v; }
}

example:
vector!( float, 2 ) 2d_vector;

So the obvious problem is that upon instantiation in the example, z and w
attributes implicitly generate array bounds errors.  Is there a form of generic
specification in D that can be applied to solve this?
```
Jun 21 2006
```Darren (dgrant at kerberos-productions dot com) wrote:
I am learning about D template details by implementing a static sized vector
class.  Opinions on this code (like how to solve the obvious errors) or
alternatives that would be better practice in D would be appreciated:

class vector( T, size_t size )
{
private:
T p[ size ];

public:
this()
{
}

this( T[] q )
in
{
assert( q.length > 0 );
}
body
{
foreach ( size_t i, T v; q )
{
p[ i ] = v;
}
}

T opIndex( size_t i )
in
{
assert( i >= 0 && i < size );
}
body
{
return p[ i ];
}

T opIndexassign( T v, size_t i )
in
{
assert( i >= 0 && i < size );
}
body
{
p[ i ] = v;
return p[ i ];
}

T x() { return p[ 0 ]; }
T y() { return p[ 1 ]; }
T z() { return p[ 2 ]; }
T w() { return p[ 3 ]; }

void x( T v ) { p[ 0 ] = v; }
void y( T v ) { p[ 1 ] = v; }
void z( T v ) { p[ 2 ] = v; }
void w( T v ) { p[ 3 ] = v; }
}

example:
vector!( float, 2 ) 2d_vector;

So the obvious problem is that upon instantiation in the example, z and w
attributes implicitly generate array bounds errors.  Is there a form of generic
specification in D that can be applied to solve this?

I think you can use static if:

static if( size < 3 ) //no z component
{
pragma( msg, "2d vectors don't have a z component" ); //tell
compiler to print this msg
static assert(false); //halt compiler
}
```
Jun 22 2006
```On Thu, 22 Jun 2006, Hasan Aljudy wrote:

I think you can use static if:

static if( size < 3 ) //no z component
{
pragma( msg, "2d vectors don't have a z component" ); //tell compiler to
print this msg
static assert(false); //halt compiler
}

Or the even simpler:

static assert(size < 3, "2d vectors...");

Later,
```
Jun 22 2006    Darren <Darren_member pathlink.com> writes:
```In article <e7dgpn\$5q\$1 digitaldaemon.com>, Hasan Aljudy says...
I think you can use static if:

static if( size < 3 ) //no z component
{
pragma( msg, "2d vectors don't have a z component" ); //tell
compiler to print this msg
static assert(false); //halt compiler
}

Wow.  Thanks, that just worked, though I did it a bit different...

Like this:

class vector...
{
..
static if ( size >= 1 )
{
T x() { return p[ 0 ]; }
void x( T v ) { p[ 0 ] = v; }
}
..
}

Now is there a way to generalize this pattern with something like mixins?
ie. do something like

template elementAccessor( name, int index )
{
T name() { return p[ index ]; }
void name( T v ) { p[ index ] = v; }
}

mixin elementAccessor!( x, index = 0 );
mixin elementAccessor!( y, index = 1 );
mixin elementAccessor!( z, index = 2 );
mixin elementAccessor!( w, index = 3 );
```
Jun 22 2006