[C++ switch statements] variable scope question

[Nyap]

My tutorial said this:

switch (x)
{
case 1:
int y;// okay, declaration is allowed
y=4;// okay, this is an assignment
break;

case 2:
y=5;// okay, y was declared above, so we can use it here too
break;

case 3:
int z=4;// illegal, you can't initialize new variables in the case statements
break;

default:
std::cout<<"default case"<<std::endl;
break;
}

Note that although variable y was defined in case 1, it was used in case 2 as well. All cases are considered part of the same scope, so a declaration in one case can be used in subsequent cases.

If "case" executes the statements below it until something like break; stops it, why does case 2 work? if case 2 was executed instead of case 1, then int y; would have never been executed/declared, and therefore it should cause a compile error, right?

 

[Cjuub]

"int y;" is not something that gets executed, it's used by the compiler during compile time to reserve space for y whether case 1 gets executed or not. Since it's declared before all uses of it, and in the same scope, it's perfectly legal.

 

[sarkwalvein]

Declarations are never executed. They are just declared variables with easy memory allocation living only inside a given scope.

 

[Nyap]

so scope is just a high level thing, that prevents you from compiling if the decleration isn't above where it's used? the execution path doesn't matter? im confused

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let me explain what I'm thinking right now

scope isn't a thing during runtime, and is just there to prevent confusion (so that people can't use a variable that's been declared somewhere completely different)
duration is what really matters

 

[Cjuub]

Yes, scope is only checked during compile time to make sure the generated low level code is correct. Scopes do not exist in the actual machine code, it's just rules set up by the language to keep things structured.

 

[Nyap]

ahhhhh ok

 

[Coto]

if you add {} between each case all variables reserved (defined) are pushed to stack, and destroyed when abandoning such scope. So you add sub-scopes

Also in low level assembly, for example ARM generates relative addresses + PC for variables such static definitions(your scope has control over it, not you), or at least it will try, (LDR r0,=0xc070c070) otherwise doing a weird combination of the barrel shifter you can create inmediate values, this for creating compiler optimized code.

mvn pc,#-134217725       @(0xf8000003~0xffffffff) equals  ldr pc,=0x7ffffffc  @just a single #inmediate instruction!

 

[Nyap]

if you add {} between each case all variables reserved (defined) are pushed to stack, and destroyed when abandoning such scope. So you add sub-scopes

but isn't that called duration? according to my tutorial:

Local variables have automatic duration, which means they are created when the block they are part of is entered, and destroyed when the block they are part of is exited

Global variables have static duration, which means they are created when the program starts and are destroyed when it ends

 

[Coto]

no its scope.

 

[Nyap]

D:

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How do you close a thread? I think I'll just close this before I get confused even more

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meh I'll just unwatch the thread