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
CODE:
#!/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
CODE:
#!/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
CODE:
#!/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
# tom is person, he is studying

bless.wrong.pl
CODE:
#!/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：
# Can't call method "sleep" on unblessed reference at bless.wrong.pl line 10.

使用c++实现bless的功能

c中的代码
CODE:
#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
// tom is person, he is studying

本文来自CSDN博客，转载请标明出处：http://blog.csdn.net/liangrockman/archive/2010/06/16/5674168.aspx