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Posts Tagged ‘c++’

Of the languages I know, emacs-lisp is unusual in that when you need to access the field of a structure, you need to know the name of the type1.

(defstruct person age name)
(defvar dave (make-person))
(setf (person-age dave) 20) ;; getter specifies the type!
(setf (person-name dave) "David Jones")
(message (person-name dave)) ;; -- David Jones

Notice I can’t say (age dave) here, it has to be (person-age ...).

The underlying reason for this is that structures are really vectors. (person-age object) translates into (aref object 1) which should hopefully be pretty fast. Another structure doesn’t need to keep its age member at an offset of 1, so I can’t say it is (generic-structure-age object).

In basic perl objects, because they are really just a hash reference, I don’t need to specify the type. When it is needed, the VM figures it out at runtime.

package Person;

sub new
{
    bless {}, $_[0];
}

package main;

my $dave = Person->new(); # Type specified here
$dave{'age'} = 20;        # but not here
$dave{'name'} = 'David Jones';

print $dave{'name'}, "\n";
$ perl t.pl
David Jones

In many statically typed languages, the fact that the compiler knows what the type is enables it to generate efficient code. I still don’t need to specify it explicitly.

#include 
#include 

using namespace std;

struct person
{
    int age;
    string name;
};

int main()
{
    person dave;

    dave.age = 20;             // No type explicitly specified here
    dave.name = "David Jones"; // or here

    cout << "Name: " << dave.name << "\n";
}
$ g++ t.cpp
$ ./a.exe
Name: David Jones
$

For objects created using the new version of defstruct* I’ve defunned a get-field and set-field that don’t require the type to be specified. Error checking is, as usual, elided

(defsubst get-index (object field)
  (cdr (assoc field (symbol-value (aref object 1)))))

(defun get-field (object field)
  (aref object (get-index object field)))

(defun set-field (object field value)
  (setf (aref object (get-index object field)) value))

(get-field dave 'name)
(set-field dave 'name "Simon Smith")

Although by this time we’re probably both wondering why I didn’t just use a hash like all the (other) scripting languages.


1. To be more accurate, you need to tell the computer what the type is. Obviously, if you want to do something useful with an object, you generally need to know what type it is anyway.

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Multiple Build Commands For Emacs

Alex Bennee left a comment on my dollar editor post:

I then open a number of separate emacs processes for each project I’m actively working on. I mainly do this as each project typically has it’s own make invocation.

I have a similar issue myself where my work projects require different commands but I prefer to avoid running multiple emacs instances if possible. It is not too difficult to fix. The idea is to detect the name of the file and run a different make command depending on which directory the file is in. This can be extended to as many different directories as necessary.

(setq compilation-scroll-output t)

(defun make-lib ()
  (compile "make-lib-cmd"))

(defun make-app ()
  (compile "make-app-cmd"))

(defun run-compile ()
  (interactive)
  (let ((file (buffer-file-name (current-buffer))))
    (cond ((not (stringp file))
           (error "[%s] is not a string.  Invalid buffer %s ?"
                  f (buffer-name)))
          ((string-match "/src/lib/" file) (make-lib))
          ((string-match "/src/app/" file) (make-app))
          (t (error "Invalid location %s" file)))))

(define-key c-mode-base-map [f7] 'run-compile)

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