diff --git a/CppCoreGuidelines.md b/CppCoreGuidelines.md
index 9969b9d..bb4f659 100644
--- a/CppCoreGuidelines.md
+++ b/CppCoreGuidelines.md
@@ -4409,7 +4409,7 @@ Constructor rules:
* [C.40: Define a constructor if a class has an invariant](#Rc-ctor)
* [C.41: A constructor should create a fully initialized object](#Rc-complete)
* [C.42: If a constructor cannot construct a valid object, throw an exception](#Rc-throw)
-* [C.43: Ensure that a value type class has a default constructor](#Rc-default0)
+* [C.43: Ensure that a copyable (value type) class has a default constructor](#Rc-default0)
* [C.44: Prefer default constructors to be simple and non-throwing](#Rc-default00)
* [C.45: Don't define a default constructor that only initializes data members; use member initializers instead](#Rc-default)
* [C.46: By default, declare single-argument constructors `explicit`](#Rc-explicit)
@@ -5096,17 +5096,16 @@ Another reason has been to delay initialization until an object is needed; the s
???
-### C.43: Ensure that a value type class has a default constructor
+### C.43: Ensure that a copyable (value type) class has a default constructor
##### Reason
Many language and library facilities rely on default constructors to initialize their elements, e.g. `T a[10]` and `std::vector v(10)`.
-A default constructor often simplifies the task of defining a suitable [moved-from state](#???).
+A default constructor often simplifies the task of defining a suitable [moved-from state](#???) for a type that is also copyable.
##### Note
-We have not (yet) formally defined [value type](#SS-concrete), but think of it as a class that behaves much as an `int`:
-it can be copied using `=` and usually compared using `==`.
+A [value type](#SS-concrete) is a class that is copyable (and usually also comparable).
It is closely related to the notion of Regular type from [EoP](http://elementsofprogramming.com/) and [the Palo Alto TR](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3351.pdf).
##### Example
@@ -5178,41 +5177,47 @@ Assuming that you want initialization, an explicit default initialization can he
int i {}; // default initialize (to 0)
};
-##### Example
+##### Notes
-There are classes that simply don't have a reasonable default.
+Classes that don't have a reasonable default construction are usually not copyable either, so they don't fall under this guideline.
-A class designed to be useful only as a base does not need a default constructor because it cannot be constructed by itself:
+For example, a base class is not a value type (base classes should not be copyable) and so does not necessarily need a default constructor:
- struct Shape { // pure interface: all members are pure virtual functions
- void draw() = 0;
- void rotate(int) = 0;
- // ...
+ // Shape is an abstract base class, not a copyable value type.
+ // It may or may not need a default constructor.
+ struct Shape {
+ virtual void draw() = 0;
+ virtual void rotate(int) = 0;
+ // =delete copy/move functions
+ // ...
};
-A class that must acquire a resource during construction:
+A class that must acquire a caller-provided resource during construction often cannot have a default constructor, but it does not fall under this guideline because such a class is usually not copyable anyway:
+ // std::lock_guard is not a copyable value type.
+ // It does not have a default constructor.
lock_guard g {mx}; // guard the mutex mx
lock_guard g2; // error: guarding nothing
-##### Note
-
A class that has a "special state" that must be handled separately from other states by member functions or users causes extra work
-(and most likely more errors). For example
+(and most likely more errors). Such a type can naturally use the special state as a default constructed value, whether or not it is copyable:
+ // std::ofstream is not a copyable value type.
+ // It does happen to have a default constructor
+ // that goes along with a special "not open" state.
ofstream out {"Foobar"};
// ...
out << log(time, transaction);
-If `Foobar` couldn't be opened for writing and `out` wasn't set to throw exceptions upon errors, the output operations become no-ops.
-The implementation must take care of that case, and users must remember to test for success.
+Similar special-state types that are copyable, such as copyable smart pointers that have the special state "==nullptr", should use the special state as their default constructed value.
-Pointers, even smart pointers, that can point to nothing (null pointers) are an example of this.
-Having a default constructor is not a panacea; ideally it defaults to a meaningful state such as `std::string`s `""` and `std::vector`s `{}`.
+However, it is preferable to have a default constructor default to a meaningful state such as `std::string`s `""` and `std::vector`s `{}`.
##### Enforcement
-* Flag classes that are copyable by `=` or comparable with `==` without a default constructor
+* Flag classes that are copyable by `=` without a default constructor
+* Flag classes that are comparable with `==` but not copyable
+
### C.44: Prefer default constructors to be simple and non-throwing