Local Scope

Local Scope

Variables are not visible everywhere. A scope is the region where a variable can be referenced - and variables declared inside a block belong to that block only.

Inside the block

Any variable declared in a block { } (including a function body) is local to that block:

int main() {
  int x = 5;          // x lives in main's block
  if (x > 3) {
    int y = 7;        // y lives only inside this if
    cout << x + y;    // ok: x is visible here
  }
  cout << y;          // ERROR: y is not in scope here
  return 0;
}

• x is visible in the whole main body, including inside the if.
• y exists only inside the if braces; outside, the name is gone.

Function variables are local

Parameters and variables inside a function are local: they're born when the function runs and destroyed when it returns. That's why an int x in f() and an int x in g() don't collide - they live in different scopes.

Use before declaration is illegal

A variable only exists from its declaration point onward. Referencing it earlier - or outside its block - is an error. An uninitialized local holds garbage until you give it a value.

TL;DR

• Variables declared in a block are visible only inside that block.
• A function's variables are local to that function.
• Inner blocks can use outer variables; outer blocks can't use inner ones.
• The variable is gone once its block ends.
• Use a variable only after its declaration.