Scope

Scope

Every name in C++ has a scope: the region of the program where it can be seen and used. Where a variable is declared decides where you can reach it.

Local scope

A variable declared inside a function or block is local. It only exists from its declaration to the end of that block:

int main() {
    int x = 5;          // x is local to main
    cout << x;          // fine, x is in scope
    return 0;
}
// x no longer exists out here

A local variable cannot be used outside its block - the compiler will complain that the name is unknown.

Global scope

A variable declared outside all functions (at the top of the file) is global. It can be seen and used by every function in the file:

int x = 5;              // global: visible everywhere below

int main() {
    cout << x;          // fine, global x is in scope
    return 0;
}

Block scoping with braces

Any pair of curly braces { } creates a new scope, even a standalone block. A variable declared inside a block is limited to that block:

int main() {
    {
        int y = 3;      // y lives only inside this block
        cout << y;      // fine
    }
    cout << y;          // ERROR: y is out of scope here
    return 0;
}

The name visibility rules

• A local variable shadows (hides) a global with the same name inside that block.
• A variable is usable only after its declaration point, within its scope.
• Inner scopes can see outer scopes, but outer scopes cannot see inner ones.

int x = 10;             // global x

int main() {
    int x = 20;         // local x shadows the global
    cout << x;          // prints 20
    return 0;
}

TL;DR

• Local scope: declared inside a function/block, visible only there.
• Global scope: declared outside all functions, visible everywhere below.
• Braces { } create a scope; a block's variables end with its closing brace.
• Inner scopes see outer names; outer scopes never see inner ones.
• A local with the same name hides the global in that block.