Names in inner scopes can hide names in outer scopes. : block scope variable « Language Basics « C++ Tutorial

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2. Language Basics
3. block scope variable

#include <iostream>
#include <ostream>

int main()
{
  for (int i = 0; i < 10; ++i)
  {
    int x = 2;
    if (x < i)
    {
      double x = 3.4;
      std::cout << x; 
    }
    std::cout << x;   
  }
  //std::cout << x;     // Error: no x declared in this scope
}

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