refactor: Add missing default constructor to prevector iterators

Author: MarcoFalke

src/prevector.h

@@ -47,13 +47,14 @@ class prevector {
     typedef const value_type* const_pointer;
 
     class iterator {
-        T* ptr;
+        T* ptr{};
     public:
         typedef Diff difference_type;
         typedef T value_type;
         typedef T* pointer;
         typedef T& reference;
         typedef std::random_access_iterator_tag iterator_category;
+        iterator() = default;
         iterator(T* ptr_) : ptr(ptr_) {}
         T& operator*() const { return *ptr; }
         T* operator->() const { return ptr; }
@@ -77,13 +78,14 @@ class prevector {
     };
 
     class reverse_iterator {
-        T* ptr;
+        T* ptr{};
     public:
         typedef Diff difference_type;
         typedef T value_type;
         typedef T* pointer;
         typedef T& reference;
         typedef std::bidirectional_iterator_tag iterator_category;
+        reverse_iterator() = default;
         reverse_iterator(T* ptr_) : ptr(ptr_) {}
         T& operator*() { return *ptr; }
         const T& operator*() const { return *ptr; }
@@ -98,13 +100,14 @@ class prevector {
     };
 
     class const_iterator {
-        const T* ptr;
+        const T* ptr{};
     public:
         typedef Diff difference_type;
         typedef const T value_type;
         typedef const T* pointer;
         typedef const T& reference;
         typedef std::random_access_iterator_tag iterator_category;
+        const_iterator() = default;
         const_iterator(const T* ptr_) : ptr(ptr_) {}
         const_iterator(iterator x) : ptr(&(*x)) {}
         const T& operator*() const { return *ptr; }
@@ -128,13 +131,14 @@ class prevector {
     };
 
     class const_reverse_iterator {
-        const T* ptr;
+        const T* ptr{};
     public:
         typedef Diff difference_type;
         typedef const T value_type;
         typedef const T* pointer;
         typedef const T& reference;
         typedef std::bidirectional_iterator_tag iterator_category;
+        const_reverse_iterator() = default;
         const_reverse_iterator(const T* ptr_) : ptr(ptr_) {}
         const_reverse_iterator(reverse_iterator x) : ptr(&(*x)) {}
         const T& operator*() const { return *ptr; }