How to default-initialize std::vector with Base and Derived objects?

Question

Consider the following:

#include <vector>

class Base 
{
public:
    Base() : x(0) {}
    int x;
};

class Derived : public Base 
{
public:
    Derived(double z0) : Base(), z(z0) {}
    double z;
};

class Foo 
{
public:
    // How to specify a default value for parameter vec0,
    // consisting of two Base objects?
    Foo(std::vector<Base> vec0 = ? ? ? ) : vec(vec0) {}
    std::vector<Base> vec;
};

class Bar 
{
public:
    // foo1.vec needs to be initialized with two Base objects.
    // foo2.vec needs to be initialized with two Derived objects.
    Bar() : foo1(? ? ? ), foo2(? ? ? ) {}
    Foo foo1;
    Foo foo2;
};

int main() 
{
    Bar bar;
    // Code here will want to use Base class pointers to access the elements
    // in bar.foo1.vec and bar.foo2.vec.
}

1. How to specify a default parameter in the Foo constructor?

2. In the Bar constructor initializer list, how to specify a vector of Base objects for foo1, and a vector of Derived objects for foo2?

3. How to title this question so others needing to solve this problem can find it?

Answer 1

How to specify a default parameter in the Foo constructor?

Just use initializer list syntax, either explicitly declaring containing elements:

class Foo {
public:
    Foo(std::vector<Base> vec0 = {Base(), Base()}) : vec(vec0) {}
    std::vector<Base> vec;
};

or implicitly declaring with yet another initializer list:

class Foo {
public:
    Foo(std::vector<Base> vec0 = {{}, {}}) : vec(vec0) {}
    std::vector<Base> vec;
};

But if your intention is to create by default list with size 2, I think it's better to do so this way:

class Foo {
public:
    Foo() : vec(2) {}
    std::vector<Base> vec;
};

In the Bar constructor initializer list, how to specify a vector of Base objects for foo1, and a vector of Derived objects for foo2?

You can't do this. You need to either use templates or vector of pointers.

Template example:

template<class V>
class Foo {
public:
    Foo(std::vector<V> vec0 = {{}, {}}) : vec(vec0) {}
    std::vector<V> vec;
};

class Bar {
public:
    // No need to explicitly initialize foo1
    Bar() : foo2({{1.},{2.}}) {}
    Foo<Base> foo1;
    Foo<Derived> foo2;
};

Answer 2

The @sklott 's answer explains well, how the initialization can be done using the initializer list. I would like to focus on the solution in-which Foo has a vector of pointers to Base. Using std::vector<std::shared_ptr<Base>>, you could rewrite @sklott 's solution as follows:

#include <memory>   // std::shared_ptr
#include <utility>  //  std::move

class Foo 
{
public:
    Foo(std::vector<std::shared_ptr<Base>> vec0 = { 
              { std::make_shared<Base>(), std::make_shared<Base>() } 
           }
        )
        : vec{ std::move(vec0) } 
    {}
    std::vector<std::shared_ptr<Base>> vec;
};

class Bar 
{
public:
    Bar() 
        : foo1{}  // calls Foo's default cont'r: initialized with two Base objects.
        , foo2{   // initialized with two Derived objects.
            { std::make_shared<Derived>(1.0), std::make_shared<Derived>(1.0) } 
          }
    {}
    Foo foo1;
    Foo foo2;
};

---

The above code would have an undefied behaviour, if we do not provide a virtual destructor to the Base class. Therefore, to make it complete solution:

class Base 
{
public:
    Base() : x(0) {}
    int x;
    virtual ~Base() = default;   // don't forget to provide a virtual destructor
};