The only sensible thing you can do with a C-array in C++03 is value-initialize it (in C++11 and beyond it can be list-initialized).
From the C++03 standard, §8.5/7:
An object whose initializer is an empty set of parentheses, i.e., (), shall be value-initialized.
And from §8.5/5:
To value-initialize an object of type
T
means:
- if
T
is a class type with a user-declared constructor, then the default constructor forT
is called (and the initialization is ill-formed ifT
has no accessible default constructor);- if
T
is a non-union class type without a user-declared constructor, then every non-static data member and base-class component ofT
is value-initialized;- if
T
is an array type, then each element is value-initialized;- otherwise, the object is zero-initialized
To zero-initialize an object of type
T
means:
- if
T
is a scalar type, the object is set to the value of0
(zero) converted toT
;- if
T
is a non-union class type, each nonstatic data member and each base-class subobject is zero-initialized;- if
T
is a union type, the object’s first named data member) is zero-initialized;- if
T
is an array type, each element is zero-initialized;- if
T
is a reference type, no initialization is performed.
So, if your constructor definition is changed to
A::A() : a(), ptr() { }
then you are guaranteed that post-construction, all 5 elements of A::a
will have the value '\0'
and A::ptr
will be null.