Perl 中 bless 的意義和用法

时间：2010-08-11 来源：bigluo

程序说明

网上的很多教程都没有把bless讲清楚，我通过摸索和实验，终于明白bless是什么意思了，简单的讲：

bless有两个参数：对象的引用、类的名称。
类的名称是一个字符串，代表了类的类型信息，这是理解bless的关键。
所谓bless就是把类型信息赋予实例变量。

程序包括5个文件：
person.pm ：实现了person类
dog.pm ：实现了dog类
bless.pl ：正确的使用bless
bless.wrong.pl ：错误的使用bless
bless.cc ：使用C++语言实现了与bless.pl相同功能的代码

person.pm

#!/usr/bin/perl -w

package person;
use strict;

sub sleep() {
        my ($self) = @_;
        my $name = $self->{"name"};

        print("$name is person, he is sleeping\n");
}

sub study() {
        my ($self) = @_;
        my $name = $self->{"name"};

        print("$name is person, he is studying\n");
}
return 1;

dog.pm

#!/usr/bin/perl -w

package dog;
use strict;

sub sleep() {
        my ($self) = @_;
        my $name = $self->{"name"};

        print("$name is dog, he is sleeping\n");
}

sub bark() {
        my ($self) = @_;
        my $name = $self->{"name"};

        print("$name is dog, he is barking\n");
}

return 1;

bless.pl

#!/usr/bin/perl =w

use strict;
use person;
use dog;

sub main()
{
        my $object = {"name" => "tom"};

        # 先把"tom"变为人

        bless($object, "person");
        $object->sleep();
        $object->study();

        # 再把"tom"变为狗

        bless($object, "dog");
        $object->sleep();
        $object->bark();

        # 最后，再把"tom"变回人

        bless($object, "person");
        $object->sleep();
        $object->study();
}

&main();

# 程序运行时输出：
# tom is person, he is sleeping
# tom is person, he is studying
# tom is dog, he is sleeping
# tom is dog, he is barking
# tom is person, he is sleeping

bless.wrong.pl

#!/usr/bin/perl =w

use strict;
use person;
use dog;

sub main()
{
        my $object = {"name" => "tom"};

        # 没有把类型信息和$object绑定，因此无法获知$object有sleep方法

        $object->sleep();
        $object->study();
}

&main();

# 程序运行输出为：

使用c++实现bless的功能

c中的代码

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

struct object {
        char name[16];
};

struct person {
        char name[16];

        void sleep() { printf("%s is person, he is sleeping\n", this->name); }
        void study() { printf("%s is person, he is studying\n", this->name); }
};

struct dog {
        char name[16];

        void sleep() { printf("%s is dog, he is sleeping\n", this->name); }
        void bark() { printf("%s is dog, he is barking\n", this->name); }
};

#define bless(object, type) ((type*) object)

int main()
{
        struct object * o = (struct object *) malloc(sizeof(struct object));
        strcpy(o->name, "tom");

        // 先把"tom"变为人

        bless(o, person)->sleep();
        bless(o, person)->study();

        // 再把"tom"变为狗

        bless(o, dog)->sleep();
        bless(o, dog)->bark();

        // 最后，再把"tom"变回人

        bless(o, person)->sleep();
        bless(o, person)->study();
        return 0;
}

// 程序运行时输出：

// tom is person, he is sleeping
// tom is person, he is studying
// tom is dog, he is sleeping
// tom is dog, he is barking
// tom is person, he is sleeping

关键的地方就是把对象o的类型转变为person类型和dog类型