Inheriting Constructors means just that. A derived class can implicitly inherit constructors from its base class(es).
The syntax is as follows:
struct B
{
B(int); // normal constructor 1
B(string); // normal constructor 2
};
struct D : B
{
using B::B; // inherit constructors from B
};
So now D has the following constructors implicitly defined:
D::D(int); // inherited
D::D(string); // inherited
Ds members are default constructed by these inherited constructors.
It is as though the constructors were defined as follows:
D::D(int x) : B(x) {}
D::D(string s) : B(s) {}
The feature isn't anything special. It is just a shorthand to save typing boilerplate code.
Here are the gory details:
12.9 Inheriting Constructors
1) A using-declaration that names a constructor implicitly declares a
set of inheriting constructors. The candidate set of inherited
constructors from the class X named in the using-declaration consists
of actual constructors and notional constructors that result from the
transformation of defaulted parameters as follows:
- all non-template constructors of X, and
- for each non-template constructor of X that has at least one parameter with a default argument, the set of constructors that
results from omitting any ellipsis parameter specification and
successively omitting parameters with a default argument from the end
of the parameter-type-list, and
- all constructor templates of X, and
- for each constructor template of X that has at least one parameter with a default argument, the set of constructor templates that results
from omitting any ellipsis parameter specification and successively
omitting parameters with a default argument from the end of the
parameter-type-list
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