Construction of child class with added member variable

Question

In my program I have an abstract class from which several classes inherit. Each child class introduces a member variable to store data. I noticed that when attempting to initialize the child class, whether using aggregate initialization or initializer list, I get errors, shown below.

struct FooBase {};
struct Foo : FooBase { int value; };

int main()
{
    Foo f = {8}; // "initializer for aggregate with no elements requires explicit braces"
    Foo f{8};    // same error as above
}

I figured this is because Foo inherits the constructors from FooBase, but I have a couple questions about the behavior.

Since the type is specified at compile time, why doesn't the aggregate constructor of Foo take precedence?
Is there a way to force precedence of the default constructor so that above initialization is possible?
(Bit more broad) In general, how are (standard and aggregate) constructors of classes passed on to their children?

As I understand, the options would be setting the data after initialization (or making a setter method) or explicitly defining a constructor for Foo. However, specifically in the context of the last question, what happens with move and copy constructors? (Is there good practice for ensuring classes are well behaved under inheritance?)

r3mus n0x · Accepted Answer · 2018-06-27T18:58:11.250

4

In your example you need to provide additional empty braces to initialize base class:

Foo f = {{}, 8};
Foo f{{}, 8};

Generally speaking though, not every class can be aggregate-initialized. The class is considered to be an aggregate if it has (see the source):

no private or protected non-static data members
no user-provided, inherited, or explicit constructors (explicitly defaulted or deleted constructors are allowed)
no virtual, private, or protected base classes
no virtual member functions

Also, there is no such thing as an "aggregate constructor" and a default constructor is not related to aggregate initialization in any way.

Constructors of base classes are not inherited unless you use using Base::Base to do so. Copy and move constructors are also not inherited. They are automatically generated by the compiler for each class unless they are explicitly defined or implicitly deleted.

edited Jun 27 '18 at 18:58

answered Jun 27 '18 at 18:44

r3mus n0x

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Thank you for clarifying that; I didn't know there is no "aggregate constructor". When I was tinkering with my code to figure out the answer, I explicitly defined a constructor in `Foo` to initialize `value`, and it worked with aggregate initialization. Is this actually C++ using the initializer list? – Riddick Jun 27 '18 at 19:03
2

@Riddick, it's called [list initialization](https://en.cppreference.com/w/cpp/language/list_initialization) which considers class' constructors in a certain order and actually is **not the same as initializer list**. Initializer list is usually refers to `std::initializer_list` which can be used to simulate aggregate initialization of array-like classes (e.g. `std::vector`). – r3mus n0x Jun 27 '18 at 19:10

score 2 · Answer 2 · answered Jun 27 '18 at 18:45

The default constructor is a constructor which takes no arguments and value initializes all the members of a class; a constructor which takes an int and uses it to initialize value would have to be a user-defined constructor.

Constructors are not passed onto their children. If you add the using declaration using FooBase::FooBase in the definition of Foo then all of the constructors of FooBase will become visible in Foo (although in this example this would have no effect as FooBase only contains the constructors that are provided by default).

Construction of child class with added member variable

2 Answers2