Proxy in C++
Why read if you can watch?
Watch Proxy's video tutorialBefore and after
BEFORE - TDirect coupling, lots of start-up and shut-down overhead.
class Image
{
int m_id;
static int s_next;
public:
Image()
{
m_id = s_next++;
cout << " $$ ctor: " << m_id << '\n';
}
~Image()
{
cout << " dtor: " << m_id << '\n';
}
void draw()
{
cout << " drawing image " << m_id << '\n';
}
};
int Image::s_next = 1;
int main(void)
{
Image images[5];
for (int i; true;)
{
cout << "Exit[0], Image[1-5]: ";
cin >> i;
if (i == 0)
break;
images[i - 1].draw();
}
}
$$ ctor: 1
$$ ctor: 2
$$ ctor: 3
$$ ctor: 4
$$ ctor: 5
Exit[0], Image[1-5]: 2
drawing image 2
Exit[0], Image[1-5]: 4
drawing image 4
Exit[0], Image[1-5]: 2
drawing image 2
Exit[0], Image[1-5]: 0
dtor: 5
dtor: 4
dtor: 3
dtor: 2
dtor: 1
AFTER - Initialization on first use
- Design an "extra level of indirection" wrapper class
- The wrapper class holds a pointer to the real class
- The pointer is initialized to null
- When a request comes in, the real object is created "on first use" (aka lazy intialization)
- The request is always delegated
class RealImage
{
int m_id;
public:
RealImage(int i)
{
m_id = i;
cout << " $$ ctor: " << m_id << '\n';
}
~RealImage()
{
cout << " dtor: " << m_id << '\n';
}
void draw()
{
cout << " drawing image " << m_id << '\n';
}
};
// 1. Design an "extra level of indirection" wrapper class
class Image
{
// 2. The wrapper class holds a pointer to the real class
RealImage *m_the_real_thing;
int m_id;
static int s_next;
public:
Image()
{
m_id = s_next++;
// 3. Initialized to null
m_the_real_thing = 0;
}
~Image()
{
delete m_the_real_thing;
}
void draw()
{
// 4. When a request comes in, the real object is
// created "on first use"
if (!m_the_real_thing)
m_the_real_thing = new RealImage(m_id);
// 5. The request is always delegated
m_the_real_thing->draw();
}
};
int Image::s_next = 1;
int main(void)
{
Image images[5];
for (int i; true;)
{
cout << "Exit[0], Image[1-5]: ";
cin >> i;
if (i == 0)
break;
images[i - 1].draw();
}
}
Exit[0], Image[1-5]: 2
$$ ctor: 2
drawing image 2
Exit[0], Image[1-5]: 4
$$ ctor: 4
drawing image 4
Exit[0], Image[1-5]: 2
drawing image 2
Exit[0], Image[1-5]: 0
dtor: 4
dtor: 2
