Scope of Variables

Scope determines where a variable is visible and accessible in your code. A variable exists only within the block where it was declared. Step outside that block and the variable ceases to exist.

Local vs Global Scope

A variable declared inside a function is local — only that function can see it. A variable declared outside all functions is global — every function in the file can access it. Global variables are tempting but they make code harder to reason about.

#include <iostream>

int globalCount = 100;

void show() {
  int localNum = 42;
  std::cout << "Global: " << globalCount << "\n";
  std::cout << "Local: " << localNum << "\n";
}

int main() {
  show();
  globalCount = 200;
  std::cout << "Updated global: " << globalCount << "\n";
  return 0;
}

globalCount is accessible from both show() and main(). But localNum only exists inside show() — trying to use it in main() would cause a compile error.

Block Scope

In C++, a block is anything inside curly braces {}. A variable declared inside a block is only visible within that block. This includes if, for, and while blocks:

#include <iostream>

int main() {
  int x = 10;

  if (x > 5) {
    int y = 20;
    std::cout << "Inside block: x=" << x << ", y=" << y << "\n";
  }

  std::cout << "Outside block: x=" << x << "\n";
  return 0;
}

y is created when the if block starts and destroyed when it ends. This is useful for keeping temporary variables contained and avoiding accidental reuse.

Scope Resolution

If a local variable has the same name as a global variable, the local one takes precedence inside that function. You can still access the global one using the scope resolution operator :::

#include <iostream>

int value = 50;

int main() {
  int value = 10;

  std::cout << "Local value: " << value << "\n";
  std::cout << "Global value: " << ::value << "\n";
  return 0;
}

The ::value syntax tells the compiler to look for the global value, ignoring the local one. This is a good trick to know, but shadowing variables like this usually confuses readers — it is better to just use different names.