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Dynamical scope advantages

marhuum (181)
What are dynamical scope advantages in compilation, runtime and others low-level efficiencies respect? Would please get it clarified with the examples ?
TheIdeasMan (6817)
What is dynamical scope, I have never heard of it ?
helios (17574)
I know what dynamic lifetime is. Dynamic/al scope, I have no idea.
Ganado (6810)
Maybe it's dynamical [scope advantages] and not [dynamical scope] advantages? 🤔
brb having a stroke
mbozzi (3932)
Dynamic scope exists but it's not seen very frequently nowadays.

Dynamic scope can be explained by analogy with C++'s exception handling.
When an exception is thrown, the computer must search the call stack for a matching handler.
To emphasize, the set of handlers that are considered depends on the current call stack.

In an environment that employs dynamic scoping:
When a variable is named, the computer must search the call stack for an variable with a matching name. To emphasize, the set of variables that are considered depends on the current call stack.

For example, consider this program. 
1
2
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#include <iostream>

int x = 1;
void g() { x = 3; } 
void f() { int x = 2; g(); std::cout << x << ' '; } 

int main() 
{
  f(); std::cout << x << '\n'; 
  g(); std::cout << x << '\n';
}

Now consider g. In a lexically-scoped language like C++20, the name x refers to the nearest x declared in a textually enclosing scope. That's the global one, ::x, a fact which can be determined by looking at the program's source code.

In a dynamically-scoped language like C++2077, the name x refers to the first x that is found by searching the call stack. That is, the callers are checked in order for a matching declaration. So if f is g's caller, then the environments of g, f, and then finally the global environment are searched in order for a matching declaration.

So if we run this code in C++20, which has lexical scope rules, we get 
2 3
3

But if we write code against the upcoming C++2077 standard, which has dynamic scope rules, then we get
3 1
3
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helios (17574)
You almost had me going. Well played, sir. Well played.
Ganado (6810)
It's about time they fixed this oversight. I just hope it doesn't cause another "Python 2 vs. 3" schism.
jonnin (11437)
Lol!
Ganado, you ok there? No paralysis or anything?
Ganado (6810)
I might be okay but I'm not fine at all!
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