compiler gives uninitialized local variable error despite initialized variable

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I'm studying the Declarations in Conditions topics in C++ and faced the below problem.

#include <iostream>
int main() {
  int x;
  std::cin >> x;
  if(int a = 4 && a != x) {
    std::cout << "Bug fixed!" << std::endl;
  }
}

I declared and then initialized the variable a. In the The C++ Programming Language by Bjarne Stroustrup Ed.2011, it is said:

The scope of variable declared in if statement extends from its point of declaration to the end of the statement that the condition controls.

That's what I did, I declared and initialized the variable a, but when I try to compare it against x, compiler gives uninitialized local variable a used error. Why, what's the problem?

I can do

int a = 4; 
if (a != x)
  // ...

but I would like to do it in one line if possible.

if-statement initialization c++
1 Answers

In the expression inside the if condition

int a = 4 && a != x

what the compiler actually sees is

int a = (4 && a != x)

where the value of a is clearly being used before it's initialized (which is what the error is saying), and is not the intent of the code.

From C++17, you can use if-with-initializer syntax to achieve the effect you want

if (int a = 4; a != x)
  // ...
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