C++11: How to initialize array of pointers of some class with nullptr?

Question

I am new to C++. I have class named MyDate. In addition I have a class named Calendar which has a member type of array of pointers to MyDate objects. How should I declare and initialize members of the array to nullptr in the constructor of Calendar?

• M.M: See if you can avoid having this array of pointers in the first place

Answer 1

Smart pointers default-initialize to nullptr:

class Calendar
{
 std::array<std::unique_ptr<Date>, 42> m_dates;
};

Otherwise, std::array is an aggregate, so an empty braced init list will zero-initialize all scalar fields:

class Calendar
{
 std::array<Date *, 42> m_dates {};
};

• jotik: Indeed, you seem to be correct according to Clang and GCC. However, to work around a -Wmissing-field-initializers warning it were perhaps better to use std::array<Date *, 42> m_dates {{nullptr}};

Answer 2

Personally, I'd probably do it like this:

class Calendar {
/* Methods: */
 Calendar() noexcept
 { for (auto & ptr: m_dates) ptr = nullptr; }
/* Fields: */
 /* Array of 42 unique pointers to MyDate objects: */
 std::array<MyDate *, 42> m_dates;
};

PS: You might want to consider using smart pointers like std::unique_ptr or std::shared_ptr instead of raw pointers. Then you wouldn't need to explicitly initialize these in the Calendar constructor at all:

class Calendar {
/* Fields: */
 /* Array of 42 pointers to MyDate objects: */
 std::array<std::unique_ptr<MyDate>, 42> m_dates;
};

EDIT: Without C++11 features, I'd do this:

class Calendar {
/* Methods: */
 Calendar()
 { for (std::size_t i = 0u; i < 42u; ++i) m_dates[i] = NULL; }
/* Fields: */
 /* Array of 42 unique pointers to MyDate objects: */
 MyDate * m_dates[42];
};

• M.M: std::array<MyDate *, 42> m_dates {}; is much simpler
• sarit rotshild: @jotik- should I include some special library for the nullptr?
• jotik: @saritrotshild No, its a C++11 feature. But you need #include <array> for std::array and #include <memory> for std::unique_ptr/std::shared_ptr.
• sarit rotshild: @jotik- I get 2 errors.error:1. #error This file requires compiler and library support for the ISO C++ 2011 standard. This support is currently experimental, and must be enabled with the -std=c++11 or -std=gnu++11 compiler options. 2.error:'array' in namespace 'std' does not name a type.
• jotik: Seems that you have an old compiler which does not fully support C++11. Since nullptr, std::array, and std::unique_ptr/std::shared_ptr are C++11 features, you must use a compiler supporting C++11. If you want to use C++03 or earlier, you should not have used nullptr in the original question, but NULL. In this case the question must be re-tagged etc. I added to my answer an example for earlier versions of C++.