c++11 plain old object default value

Question

How can I initialize a member variable of a POD (Plain Old Data) in C++11?

class A {
public:
    int theAnswer; // this is not initialized
};

static_assert(std::is_pod<A>::value, "A must be a plain old object");

class B {
public:
    int theAnswer { 42 }; //  this should initialize to 42
};

static_assert(std::is_pod<B>::value, "B must be a plain old object"); // ERROR

class C {
public:
    C() : theAnswer { 42 } { } // Obviously, this is not a trivial default constructor, so it does not work
    int theAnswer;
};

static_assert(std::is_pod<C>::value, "C must be a plain old object"); // ERROR

Nitpicky: POD stands for Plain Old Data (http://stackoverflow.com/questions/146452/what-are-pod-types-in-c) — Cassio Neri, Nov 06 '14 at 16:40

score 10 · Accepted Answer · answered Nov 05 '14 at 09:54

10

You do it where you initialize the whole object. A plain old data object is just that: plain old data, with no invariants, initialization, or any of that fancy stuff. If you want initialization, then it's not POD.

But maybe you don't actually need a POD. Maybe trivially copyable is enough? If all you want to do is memcpy between objects, trivially copyable is the trait you're looking for, not POD.

answered Nov 05 '14 at 09:54

Sebastian Redl

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Yes, you are right. Trivially copyable with memcpy is good enough for my case. – Alessandro Pezzato Nov 05 '14 at 10:02
Unforunately, my compiler (gcc 4.7.3) does not support std::is_trivially_copyable – Alessandro Pezzato Nov 05 '14 at 10:13

score 2 · Answer 2 · answered Nov 05 '14 at 10:01

Here is some standardese explaining why you can't initialize POD members inside the class itself.

[class]/10:

A POD struct is a non-union class that is both a trivial class and a standard-layout class

[class]/6:

A trivial class is a class that has a default constructor (12.1), has no non-trivial default constructors, and is trivially copyable.

[class.ctor]/4:

A default constructor is trivial if it is not user-provided and if:
— its class has no virtual functions (10.3) and no virtual base classes (10.1), and
— no non-static data member of its class has a brace-or-equal-initializer, and
— all the direct base classes of its class have trivial default constructors, and
— for all the non-static data members of its class that are of class type (or array thereof), each such class has a trivial default constructor.

c++11 plain old object default value

2 Answers2