How can i use member initialization list to initialize an array?

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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 for T is called (and the initialization is ill-formed if T 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 of T 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 of 0 (zero) converted to T;
  • 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.

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