December 2010 – vilimblog

Turns out you can also do decorators in C, this came in handy for something else I had to work on recently. It’s not quite as nice, because functions aren’t first-class, but handy nonetheless.

Here’s what that looks like:

#include <stdio.h>

typedef int (* const decorableFnType)(int paramOne, int paramTwo);

int functionOne(int paramOne, int paramTwo)
{
	printf("functionOne: %d, %dn", paramOne, paramTwo);
	return 0;
}

int functionTwo(int paramOne, int paramTwo)
{
	printf("functionTwo: %d, %dn", paramOne, paramTwo);
	return 0;
}

int functionThree(int paramOne, int paramTwo)
{
	printf("functionThree: %d, %dn", paramOne, paramTwo);
	return 0;
}

int decorator(int paramOne, int paramTwo, decorableFnType originalFn)
{
	printf("decorator precondition!n");
	originalFn(paramOne, paramTwo);
	return 0;
}

int main(int argc, char **argv)
{
	printf("hello, C decorators!n");
	
	functionOne(1, 2);
	
	decorator(1, 2, functionOne);
	decorator(1, 2, functionTwo);
	decorator(1, 2, functionThree);
	
	return 0;
}

#include <stdio.h>

typedef int (* const decorableFnType)(int paramOne, int paramTwo);

int functionOne(int paramOne, int paramTwo)

{

printf("functionOne: %d, %dn", paramOne, paramTwo);

return 0;

}

int functionTwo(int paramOne, int paramTwo)

{

printf("functionTwo: %d, %dn", paramOne, paramTwo);

return 0;

}

int functionThree(int paramOne, int paramTwo)

{

printf("functionThree: %d, %dn", paramOne, paramTwo);

return 0;

}

int decorator(int paramOne, int paramTwo, decorableFnType originalFn)

{

printf("decorator precondition!n");

originalFn(paramOne, paramTwo);

return 0;

}

int main(int argc, char **argv)

{

printf("hello, C decorators!n");

functionOne(1, 2);

decorator(1, 2, functionOne);

decorator(1, 2, functionTwo);

decorator(1, 2, functionThree);

return 0;

}

The output of which is:

vilimpoc@funky:~$ ./c-decorators
hello, C decorators!
functionOne: 1, 2
decorator precondition!
functionOne: 1, 2
decorator precondition!
functionTwo: 1, 2
decorator precondition!
functionThree: 1, 2
vilimpoc@funky:~$

This might be a bit simplistic, in reality, you’d probably want to decorate a function with a signature like the following, so that you can just change the structure and not bother w/a bajillion function declarations:

typedef int (* const decorableFnType)(SomeStruct * data);

1	typedef int (* const decorableFnType)(SomeStruct * data);

Again, not super elegant, but handy.

One reason why Javascript rocks:

As a use case, imagine you want to restrict a certain set of functions to only run if you are logged in. Doing stuff like this is ridiculously easy with first-class functions.

Here’s a generic decoration example:

(function()
{
	var original = function(paramOne, paramTwo) {
		console.log('original: ' + paramOne + ' ' + paramTwo);
	};

	var decorator = function(originalFn) {
		return function(paramOne, paramTwo) {
			console.log('decorator: ' + paramOne + ' ' + paramTwo);
			originalFn(paramOne, paramTwo);
		};
	};

	var fnTable = {};
	var registerCallback = function(url, callback) {
		fnTable[url] = callback;
	};

	registerCallback('/abc', original);
	registerCallback('/def', decorator(original));

	fnTable['/abc']('one', 'two');
	fnTable['/def']('three', 'four');
})();

(function()

{

var original = function(paramOne, paramTwo) {

console.log('original: ' + paramOne + ' ' + paramTwo);

};

var decorator = function(originalFn) {

return function(paramOne, paramTwo) {

console.log('decorator: ' + paramOne + ' ' + paramTwo);

originalFn(paramOne, paramTwo);

};

var fnTable = {};

var registerCallback = function(url, callback) {

fnTable[url] = callback;

};

registerCallback('/abc', original);

registerCallback('/def', decorator(original));

fnTable['/abc']('one', 'two');

fnTable['/def']('three', 'four');

})();

Update: Here’s the specific way you’d do this with node.js/Express:

Since all of the URL handlers you register have the same signature, it’s easy to add precondition checks to the handlers via decorators.

var decorator = function(originalFn) {
    return function(req, res) {
        if (req.session.userLoggedIn) {
            originalFn(req, res);
        } else {
            redirectSomewhere(res);
        }
    }
}

app.get('/url', decorator(original));
app.post('/other', decorator(original));

var decorator = function(originalFn) {

return function(req, res) {

if (req.session.userLoggedIn) {

originalFn(req, res);

} else {

redirectSomewhere(res);

}

app.get('/url', decorator(original));

app.post('/other', decorator(original));

You get preconditions essentially for free, which is a damn sight better than adding the if() block to each and every handler function. Also, if you need more preconditions in the future, you can just stack them.

Month: December 2010

Decorators In C

Javascript Decorators