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How to override trait function and call it from the overridden function?

Posted by: admin November 30, 2021 Leave a comment

Questions:

Scenario:

trait A {
    function calc($v) {
        return $v+1;
    }
}

class MyClass {
    use A;

    function calc($v) {
        $v++;
        return A::calc($v);
    }
}

print (new MyClass())->calc(2); // should print 4

This code doesn’t work, and I cannot find a way to call a trait function like it was inherited. I tried calling self::calc($v), static::calc($v), parent::calc($v), A::calc($v) and the following:

trait A {
    function calc($v) {
        return $v+1;
    }
}

class MyClass {
    use A {
        calc as traitcalc;
    }

    function calc($v) {
        $v++;
        return traitcalc($v);
    }
}

Nothing works.

Is there a way to make it work or must I override completely the trait function which is much more complex than this 🙂

Answers:

Your last one was almost there:

trait A {
    function calc($v) {
        return $v+1;
    }
}

class MyClass {
    use A {
        calc as protected traitcalc;
    }

    function calc($v) {
        $v++;
        return $this->traitcalc($v);
    }
}

The trait is not a class. You can’t access its members directly. It’s basically just automated copy and paste…

###

If the class implements the method directly, it will not use the traits version. Perhaps what you are thinking of is:

trait A {
    function calc($v) {
        return $v+1;
    }
}

class MyClass {
    function calc($v) {
        return $v+2;
    }
}

class MyChildClass extends MyClass{
}

class MyTraitChildClass extends MyClass{
    use A;
}

print (new MyChildClass())->calc(2); // will print 4

print (new MyTraitChildClass())->calc(2); // will print 3

Because the child classes do not implement the method directly, they will first use that of the trait if there otherwise use that of the parent class.

If you want, the trait can use method in the parent class (assuming you know the method would be there) e.g.

trait A {
    function calc($v) {
        return parent::calc($v*3);
    }
}
// .... other code from above
print (new MyTraitChildClass())->calc(2); // will print 8 (2*3 + 2)

You can also provide for ways to override, but still access the trait method as follows:

trait A {
    function trait_calc($v) {
        return $v*3;
    }
}

class MyClass {
    function calc($v) {
        return $v+2;
    }
}


class MyTraitChildClass extends MyClass{
    use A {
      A::trait_calc as calc;
    }
}


class MySecondTraitChildClass extends MyClass{
    use A {
      A::trait_calc as calc;
    }

    public function calc($v) {
      return $this->trait_calc($v)+.5;
    }
}


print (new MyTraitChildClass())->calc(2); // will print 6
echo "\n";
print (new MySecondTraitChildClass())->calc(2); // will print 6.5

You can see it work at http://sandbox.onlinephpfunctions.com/code/e53f6e8f9834aea5e038aec4766ac7e1c19cc2b5

###

An alternative approach if interested – with an extra intermediate class to use the normal OOO way. This simplifies the usage with parent::methodname

trait A {
    function calc($v) {
        return $v+1;
    }
}

// an intermediate class that just uses the trait
class IntClass {
    use A;
}

// an extended class from IntClass
class MyClass extends IntClass {
    function calc($v) {
        $v++;
        return parent::calc($v);
    }
}

###

Using another trait:

trait ATrait {
    function calc($v) {
        return $v+1;
    }
}

class A {
    use ATrait;
}

trait BTrait {
    function calc($v) {
        $v++;
        return parent::calc($v);
    }
}

class B extends A {
    use BTrait;
}

print (new B())->calc(2); // should print 4

###

Another variation: Define two functions in the trait, a protected one that performs the actual task, and a public one which in turn calls the protected one.

This just saves classes from having to mess with the ‘use’ statement if they want to override the function, since they can still call the protected function internally.

trait A {
    protected function traitcalc($v) {
        return $v+1;
    }

    function calc($v) {
        return $this->traitcalc($v);
    }
}

class MyClass {
    use A;

    function calc($v) {
        $v++;
        return $this->traitcalc($v);
    }
}

class MyOtherClass {
    use A;
}


print (new MyClass())->calc(2); // will print 4

print (new MyOtherClass())->calc(2); // will print 3