int i;

does not initialize the value.

It's a gotcha to be sure. Sometimes it does, sometimes it doesn't. From a reference[0]:

  #include <string>
  
  struct T1 { int mem; };
  
  struct T2
  {
      int mem;
      T2() {} // “mem” is not in the initializer list
  };
  
  int n; // static non-class, a two-phase initialization is done:
  // 1) zero-initialization initializes n to zero
  // 2) default-initialization does nothing, leaving n being zero
  
  int main()
  {
      [[maybe_unused]]
      int n;            // non-class, the value is indeterminate
      std::string s;    // class, calls default constructor, the value is ""
      std::string a[2]; // array, default-initializes the elements, the value is {"", ""}
      //  int& r;           // Error: a reference
      //  const int n;      // Error: a const non-class
      //  const T1 t1;      // Error: const class with implicit default constructor
      [[maybe_unused]]
      T1 t1;            // class, calls implicit default constructor
      const T2 t2;      // const class, calls the user-provided default constructor
      // t2.mem is default-initialized
  }

That `int n;` on the 11th line is initialized to 0 per standard. `int n;` on line 18, inside a function, is not. And `struct T1 { int mem; };` on line 3 will have `mem` initialized to 0 if `T1` is instantiated like `T1 t1{};`, but not if it's instantiated like `T1 t1;`. There's no way to tell from looking at `struct T1{...}` how the members will be initialized without knowing how they'll be called.

C++ is fun!

[0]https://en.cppreference.com/w/cpp/language/default_initializ...

Stroustrup once said

> "There's a great language somewhere deep inside of C++"

or something to that effect.

Yes, it's called Simula-67.

(the original pre-ISO C++ is basically C with Simula bolted onto it.)

Unless `i` is global…

Most cases, e.g. local var declaration. `int i` does not initialize i.