5.5 Zero Initialization

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For fundamental types such as int , double , or pointer types, there is no default constructor that initializes them with a useful default value. Instead, any noninitialized local variable has an undefined value:

 void foo()  {      int x;  //  x  has undefined value  int* ptr;  //  ptr  points to somewhere (instead of nowhere)  } 

Now if you write templates and want to have variables of a template type initialized by a default value, you have the problem that a simple definition doesn't do this for built-in types:

 template <typename T>  void foo()  {      T x;  //  x  has undefined value if  T  is built-in type  } 

For this reason, it is possible to call explicitly a default constructor for built-in types that initializes them with zero (or false for bool ). That is, int() yields zero. As a consequence you can ensure proper default initialization even for built-in types by writing the following:

 template <typename T>  void foo()  {      T x = T();  //  x  is zero (or  false  )if  T  is a built-in type  } 

To make sure that a member of a class template, for which the type is parameterized, gets initialized, you have to define a default constructor that uses an initializer list to initialize the member:

 template <typename T>  class MyClass {    private:      T x;    public:      MyClass() : x() {  // ensures that  x  is initialized even for built-in types  }   }; 

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