Modern C++: Temp Vars in If/Switch

#cpp #tutorial #beginners

Following my series from less known features of Modern C++.

Modern C++: An Introduction

Swastik Baranwal ・ Dec 23 '19

#cpp #beginners #tutorial #codenewbie

This feature is small and minor one so this article is small but an important for sure.

From C++ 17 you can declare temporary variables on if and switch statements just like loops. As the variables are temporary, you can't access those variables as they are only declared in the if or switch block.

If Statement

#include <iostream>

int main()
{
  if (int i = 0; i > 0)
    {
      std::cout << "i is positive" << std::endl;
    }
  else if (i < 0)
    {
      std::cout << "i is negative" << std::endl;
    }
  else
    {
      std::cout << "i is zero" << std::endl;
    }
 // std::cout << i << std::endl;    // i will not be available here
}

Switch Statement

#include <iostream>

int main()
{
  switch (int i = 0)
    {
    case 0:
      std::cout << "i is negative";
      break;

    case 1:
      std::cout << "i is one";

    default:
      std::cout << "invalid";
      break;
    }
  //std::cout << i << std::endl;        // i will not be available here
}

If we comment out the last line of both of the examples then an error will be raised.

main.cpp: In function ‘int main()’:
main.cpp:26:17: error: ‘i’ was not declared in this scope
    std::cout << i << std::endl;        // i will not be available here

This feature is useful when you need to temporary declare variables for checking and minor comparisons.