2
Replacing with a higher-level abstraction:
Singleton*
instance;
Singleton* GetInstance() {
once {
instance = new T;
}
return instance;
}
once {
instance = new T;
}
return instance;
}
And
allowing this variant:
Singleton*
instance = new T;
Singleton* GetInstance() {
return instance;
}
return instance;
}
•Variables should only be initialized once, so we should require the compiler to implicitly Do the Right Thing.
Example: Double-Checked Locking (DCL).
volatile Singleton* instance;
Singleton* GetInstance() {
if( !instance ) {
// acquire lock
if( !instance ) {
instance = new T;
}
// release lock
}
return instance;
}
if( !instance ) {
// acquire lock
if( !instance ) {
instance = new T;
}
// release lock
}
return instance;
}
•Works on some platforms (including VC++ 2005).
Read the manual carefully.
Read the manual carefully.
•Error-prone. Omit volatile, program compiles & “works.”
•Too low-level. Like coding your own vtables…
Illustrating
a Principle: Codifying Idioms