Implicit declaration of special member-func and their standard behavior

In C++ programming, one of the most frustrating things is that the compiler does too much behind the backs of programmers. This article compiles various situations and actual behaviors of six special member functions: default constructor, copy/move constructor, copy/move assignment operator, and destructor, as outlined in the C++ Standard ([ISO/IEC 14882:2014]). It can serve as supplementary material for “Inside The C++ Object Model,” enhancing the understanding of compiler implementations through standard descriptions.

Additionally, “Inside The C++ Object Model” mainly describes things from the perspective of “compiler implementation,” but from the “C++ standard” perspective, many things in the book are dependent on compiler implementation. For example, the virtual function table; the standard does not specify how the compiler should implement polymorphic behavior, thus it cannot describe details about the virtual function table.

Moreover, many ambiguous interpretations related to the behavior of “compiler-generated” can be addressed here, which is part of the joy of reading the C++ standard—regardless of whether it’s good or bad, the standard specifications cannot be wrong; all implementations that do not adhere to the standard descriptions are unstandard.

In some circumstances, C++ implementations implicitly define the default constructor(12.1), copy constructor(12.8), move constructor(12.8), copy assignment operator(12.8), move assignment operator(12.8), or destructor(12.4) member functions.

#include <string>
struct C {
	std::string s; // std::string is the standard library class (Clause 21)
};
int main() {
	C a;
	C b = a;
	b = a;
}

the implementation will implicitly define functions to make the definition of C equivalent to

struct C {
	std::string s;
	C() : s() { }
	C(const C& x): s(x.s) { }
	C(C&& x): s(static_cast<std::string&&>(x.s)) { }
	// : s(std::move(x.s)) { }
	C& operator=(const C& x) { s = x.s; return *this; }
	C& operator=(C&& x) { s = static_cast<std::string&&>(x.s); return *this; }
	// { s = std::move(x.s); return *this; }
	~C() { }
};

implicitly declare default constructor

A default constructor for a class X is a constructor of class X that can be called without an argument. If there is no user-declared constructor for class X, a constructor having no parameters is implicitly declared as defaulted (8.4). An implicitly-declared default constructor is an inline public member of its class.

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.
Otherwise, the default constructor is non-trivial.

The implicitly-defined default constructor performs the set of initializations of the class that would be performed by a user-written default constructor for that class with no ctor-initializer (12.6.2) and an empty compound-statement. If that user-written default constructor would be ill-formed, the program is ill-formed.

An implicitly-declared default constructor has an exception-specification (15.4). An explicitly-defaulted definition might have an implicit exception-specification, see 8.4.

constructor behavior

For the order of constructor execution, you can refer to my previous article: Order of Object Construction and Destruction.
Any expression in a mem-initializer is evaluated as part of the full-expression that performs the initialization. A mem-initializer where the mem-initializer-id denotes a virtual base class is ignored during the execution of a constructor of any class that is not the most derived class.
In a non-delegating constructor, if a given potentially constructed subobject is not designated by a mem-initializer-id (including the case where there is no mem-initializer-list because the constructor has no ctor-initializer), then

if the entity is a non-static data member that has a brace-or-equal-initializer and either
- the constructor’s class is a union (9.5), and no other variant member of that union is designated by a mem-initializer-id or
- the constructor’s class is not a union, and, if the entity is a member of an anonymous union, no other member of that union is designated by a mem-initializer-id,
  the entity is initialized as specified in 8.5;
otherwise, if the entity is an anonymous union or a variant member (9.5), no initialization is performed;
otherwise, the entity is default-initialized (8.5).

Note: Uninitialized built-type data members within classes will have indeterminate values. The compiler-generated default constructor only does what is necessary.

[ Note: An abstract class (10.4) is never a most derived class, thus its constructors never initialize virtual base classes, therefore the corresponding mem-initializers may be omitted. — end note ] An attempt to initialize more than one non-static data member of a union renders the program ill-formed. [ Note: After the call to a constructor for class X for an object with automatic or dynamic storage duration has completed, if the constructor was not invoked as part of value-initialization and a member of X is neither initialized nor given a value during the execution of the compound-statement of the body of the constructor, the member has an indeterminate value. — end note ]

struct A {
	A();
};
struct B {
	B(int);
};
struct C {
	C() { } // initializes members as follows:
	A a; // OK: calls A::A()
	const B b; // error: B has no default constructor
	int i; // OK: i has indeterminate value
	int j = 5; // OK: j has the value 5
};

If a given non-static data member has both a brace-or-equal-initializer and a mem-initializer, the initialization specified by the mem-initializer is performed, and the non-static data member’s brace-or-equal-initializer is ignored.

struct A {
	int i = /* some integer expression with side effects */ ;
	A(int arg) : i(arg) { }
	// ...
};

the A(int) constructor will simply initialize i to the value of arg, and the side effects in i’s brace-or-equal-initializer will not take place.
In a non-delegating constructor, the destructor for each potentially constructed subobject of class type is potentially invoked (12.4). [ Note: This provision ensures that destructors can be called for fully-constructed sub-objects in case an exception is thrown (15.2). — end note ]

In a non-delegating constructor, initialization proceeds in the following order:

First, and only for the constructor of the most derived class (1.8), virtual base classes are initialized in the order they appear on a depth-first left-to-right traversal of the directed acyclic graph of base classes, where “left-to-right” is the order of appearance of the base classes in the derived class base-specifier-list.
Then, direct base classes are initialized in declaration order as they appear in the base-specifier-list (regardless of the order of the mem-initializers).
Then, non-static data members are initialized in the order they were declared in the class definition (again regardless of the order of the mem-initializers).
Finally, the compound-statement of the constructor body is executed. [ Note: The declaration order is mandated to ensure that base and member subobjects are destroyed in the reverse order of initialization.– end note ]

struct V {
	V();
	V(int);
};
struct A : virtual V {
	A();
	A(int);
};
struct B : virtual V {
	B();
	B(int);
};
struct C : A, B, virtual V {
	C();
	C(int);
};
A::A(int i) : V(i) { /* ... */ }
B::B(int i) { /* ... */ }
C::C(int i) { /* ... */ }
V v(1); // use V(int)
A a(2); // use V(int)
B b(3); // use V()
C c(4); // use V()

implicitly declare destructor

default destructor

If a class has no user-declared destructor, a destructor is implicitly declared as defaulted (8.4). An implicitly-declared destructor is an inline public member of its class.

delete destructor

A defaulted destructor for a class X is defined as deleted if:

X is a union-like class that has a variant member with a non-trivial destructor,
any potentially constructed subobject has class type M (or array thereof) and M has a deleted destructor or a destructor that is inaccessible from the defaulted destructor,
or, for a virtual destructor, lookup of the non-array deallocation function results in an ambiguity or in a function that is deleted or inaccessible from the defaulted destructor.

trivial destructor

A destructor is trivial if it is not user-provided and if:

the destructor is not virtual,
all of the direct base classes of its class have trivial destructors, and
for all of the non-static data members of its class that are of class type (or array thereof), each such class has a trivial destructor.
Otherwise, the destructor is non-trivial.

A destructor that is defaulted and not defined as deleted is implicitly defined when it is odr-used (3.2) to destroy an object of its class type (3.7) or when it is explicitly defaulted after its first declaration.
Before the defaulted destructor for a class is implicitly defined, all the non-user-provided destructors for its base classes and its non-static data members shall have been implicitly defined.

executing the destructor

After executing the body of the destructor and destroying any automatic objects allocated within the body, a destructor for class X calls the destructors for X’s direct non-variant non-static data members, the destructors for X’s direct base classes, and, if X is the type of the most derived class (12.6.2), its destructor calls the destructors for X’s virtual base classes. All destructors are called as if they were referenced with a qualified name, that is, ignoring any possible virtual overriding destructors in more derived classes. Bases and members are destroyed in the reverse order of the completion of their constructor (see 12.6.2). A return statement (6.6.3) in a destructor might not directly return to the caller; before transferring control to the caller, the destructors for the members and bases are called. Destructors for elements of an array are called in reverse order of their construction (see 12.6).

A destructor can be declared virtual (10.3) or pure virtual (10.4); if any objects of that class or any derived class are created in the program, the destructor shall be defined. If a class has a base class with a virtual destructor, its destructor (whether user- or implicitly-declared) is virtual.
[ Note: some language constructs have special semantics when used during destruction; see 12.7. - end note ]
A destructor is invoked implicitly

for a constructed object with static storage duration (3.7.1) at program termination (3.6.3),
for a constructed object with thread storage duration (3.7.2) at thread exit,
for a constructed object with automatic storage duration (3.7.3) when the block in which an object is created exits (6.7),
for a constructed temporary object when its lifetime ends (12.2).

In each case, the context of the invocation is the context of the construction of the object. A destructor is also invoked implicitly through use of a delete-expression (5.3.5) for a constructed object allocated by a new-expression (5.3.4); the context of the invocation is the delete-expression. [ Note: An array of class type contains several subobjects for each of which the destructor is invoked. — end note ] A destructor can also be invoked explicitly. A destructor is potentially invoked if it is invoked as specified in 5.3.4 and 12.6.2. A program is ill-formed if a destructor that is potentially invoked is deleted or not accessible from the context of the invocation.

implicitly declare copy/move constructor

copy constructor

If the class definition does not explicitly declare a copy constructor, one is declared implicitly. If the class definition declares a move constructor or move assignment operator, the implicitly declared copy constructor is defined as deleted; otherwise, it is defined as defaulted (8.4). The latter case is deprecated if the class has a user-declared copy assignment operator or a user-declared destructor.

move constructor

If the definition of a class X does not explicitly declare a move constructor, one will be implicitly declared as defaulted if and only if

X does not have a user-declared copy constructor,
X does not have a user-declared copy assignment operator,
X does not have a user-declared move assignment operator, and
X does not have a user-declared destructor.

When the move constructor is not implicitly declared or explicitly supplied, expressions that otherwise would have invoked the move constructor may instead invoke a copy constructor.

An implicitly-declared copy/move constructor is an inline public member of its class. A defaulted copy/move constructor for a class X is defined as deleted (8.4.3) if X has:

a variant member with a non-trivial corresponding constructor and X is a union-like class,
a potentially constructed subobject type M (or array thereof) that cannot be copied/moved because overload resolution (13.3), as applied to M’s corresponding constructor, results in an ambiguity or a function that is deleted or inaccessible from the defaulted constructor,
any potentially constructed subobject of a type with a destructor that is deleted or inaccessible from the defaulted constructor, or,
for the copy constructor, a non-static data member of rvalue reference type.

A defaulted move constructor that is defined as deleted is ignored by overload resolution (13.3, 13.4). [ Note: A deleted move constructor would otherwise interfere with initialization from an rvalue which can use the copy constructor instead. - end note ]

A copy/move constructor that is defaulted and not defined as deleted is implicitly defined if it is odr-used (3.2) or when it is explicitly defaulted after its first declaration. [ Note: The copy/move constructor is implicitly defined even if the implementation elided its odr-use (3.2, 12.2). - end note ] If the implicitly-defined constructor would satisfy the requirements of a constexpr constructor (7.1.5), the implicitly-defined constructor is constexpr.

Before the defaulted copy/move constructor for a class is implicitly defined, all non-user-provided copy/move constructors for its potentially constructed subobjects shall have been implicitly defined. [ Note: An implicitly-declared copy/move constructor has an exception-specification (15.4). - end note ]

implicitly-defined copy/move constructor behavior

The implicitly-defined copy/move constructor for a non-union class X performs a memberwise copy/move of its bases and members. [ Note: brace-or-equal-initializers of non-static data members are ignored. See also the example in 12.6.2. - end note ] The order of initialization is the same as the order of initialization of bases and members in a user-defined constructor (see 12.6.2). Let x be either the parameter of the constructor or, for the move constructor, an xvalue referring to the parameter. Each base or non-static data member is copied/moved in the manner appropriate to its type:

if the member is an array, each element is direct-initialized with the corresponding subobject of x;
if a member m has rvalue reference type T&&, it is direct-initialized with static_cast<T&&>(x.m);
otherwise, the base or member is direct-initialized with the corresponding base or member of x.

Virtual base class subobjects shall be initialized only once by the implicitly-defined copy/move constructor(see 12.6.2).
The implicitly-defined copy/move constructor for a union X copies the object representation (3.9) of X.

trivial copy/move constructor

A copy/move constructor for class X is trivial if it is not user-provided, its parameter-type-list is equivalent to the parameter-type-list of an implicit declaration, and if

class X has no virtual functions (10.3) and no virtual base classes (10.1), and
class X has no non-static data members of volatile-qualified type, and
the constructor selected to copy/move each direct base class subobject is trivial, and
for each non-static data member of X that is of class type (or array thereof), the constructor selected to copy/move that member is trivial;
otherwise the copy/move constructor is non-trivial.

implicit declare copy/move assignment operator

implicit declare copy assignment operator

If the class definition does not explicitly declare a copy assignment operator, one is declared implicitly. If the class definition declares a move constructor or move assignment operator, the implicitly declared copy assignment operator is defined as deleted; otherwise, it is defined as defaulted (8.4). The latter case is deprecated if the class has a user-declared copy constructor or a user-declared destructor. The implicitly-declared copy assignment operator for a class X will have the form

1	X& X::operator=(const X&)

each direct base class B of X has a copy assignment operator whose parameter is of type const B&,
const volatile B& or B, and
for all the non-static data members of X that are of a class type M (or array thereof), each such class type has a copy assignment operator whose parameter is of type const M&, const volatile M& or M.
Otherwise, the implicitly-declared copy assignment operator will have the form

1	X& X::operator=(X&)

implicit declare move assignment operator

If the definition of a class X does not explicitly declare a move assignment operator, one will be implicitly declared as defaulted if and only if

X does not have a user-declared copy constructor,
X does not have a user-declared move constructor,
X does not have a user-declared copy assignment operator, and
X does not have a user-declared destructor.

The class definition

struct S {
	int a;
	S& operator=(const S&) = default;
};

will not have a default move assignment operator implicitly declared because the copy assignment operator has been user-declared. The move assignment operator may be explicitly defaulted.

struct S {
	int a;
	S& operator=(const S&) = default;
	S& operator=(S&&) = default;
};

The implicitly-declared move assignment operator for a class X will have the form

1	X& X::operator=(X&&);

The implicitly-declared copy/move assignment operator for class X has the return type X&; it returns the object for which the assignment operator is invoked, that is, the object assigned to. An implicitly-declared copy/move assignment operator is an inline public member of its class.

A defaulted copy/move assignment operator for class X is defined as deleted if X has:

a variant member with a non-trivial corresponding assignment operator and X is a union-like class, or
a non-static data member of const non-class type (or array thereof), or
a non-static data member of reference type, or
a potentially constructed subobject of class type M (or array thereof) that cannot be copied/moved because overload resolution (13.3), as applied to M’s corresponding assignment operator, results in an ambiguity or a function that is deleted or inaccessible from the defaulted assignment operator.
A defaulted move assignment operator that is defined as deleted is ignored by overload resolution (13.3,13.4).

Because a copy/move assignment operator is implicitly declared for a class if not declared by the user, a base class copy/move assignment operator is always hidden by the corresponding assignment operator of a derived class (13.5.3). A using-declaration (7.3.3) that brings in from a base class an assignment operator with a parameter type that could be that of a copy/move assignment operator for the derived class is not considered an explicit declaration of such an operator and does not suppress the implicit declaration of the derived class operator; the operator introduced by the using-declaration is hidden by the implicitly-declared operator in the derived class.

A copy/move assignment operator for a class X that is defaulted and not defined as deleted is implicitly defined when it is odr-used (3.2) (e.g., when it is selected by overload resolution to assign to an object of its class type) or when it is explicitly defaulted after its first declaration. The implicitly-defined copy/move assignment operator is constexpr if

X is a literal type, and
the assignment operator selected to copy/move each direct base class subobject is a constexpr function, and
for each non-static data member of X that is of class type (or array thereof), the assignment operator selected to copy/move that member is a constexpr function.

Before the defaulted copy/move assignment operator for a class is implicitly defined, all non-user-provided copy/move assignment operators for its direct base classes and its non-static data members shall have been implicitly defined. [ Note: An implicitly-declared copy/move assignment operator has an exception-specification (15.4). — end note ]### implicitly copy/move assignment operator behavior

The implicitly-defined copy/move assignment operator for a non-union class X performs memberwise copy/move assignment of its subobjects. The direct base classes of X are assigned first, in the order of their declaration in the base-specifier-list, and then the immediate non-static data members of X are assigned, in the order in which they were declared in the class definition. Let x be either the parameter of the function or, for the move operator, an xvalue referring to the parameter. Each subobject is assigned in the manner appropriate to its type:

if the subobject is of class type, as if by a call to operator= with the subobject as the object expression and the corresponding subobject of x as a single function argument (as if by explicit qualiﬁcation; that is, ignoring any possible virtual overriding functions in more derived classes);

if the subobject is an array, each element is assigned, in the manner appropriate to the element type;

if the subobject is of scalar type, the built-in assignment operator is used.

It is unspecified whether subobjects representing virtual base classes are assigned more than once by the implicitly-defined copy assignment operator.

struct V { };
struct A : virtual V { };
struct B : virtual V { };
struct C : B, A { };

It is unspecified whether the virtual base class subobject V is assigned twice by the implicitly-defined copy assignment operator for C. — end example ] [ Note: This does not apply to move assignment, as a defaulted move assignment operator is deleted if the class has virtual bases. — end note ]
A program is ill-formed if the copy/move constructor or the copy/move assignment operator for an object is implicitly odr-used and the special member function is not accessible (Clause 11). [ Note: Copying/moving one object into another using the copy/move constructor or the copy/move assignment operator does not change the layout or size of either object. — end note ]

trivial copy/move assignment operator

A copy/move assignment operator for class X is trivial if it is not user-provided, its parameter-type-list is
equivalent to the parameter-type-list of an implicit declaration, and if

class X has no virtual functions (10.3) and no virtual base classes (10.1), and
class X has no non-static data members of volatile-qualified type, and
the assignment operator selected to copy/move each direct base class subobject is trivial, and
for each non-static data member of X that is of class type (or array thereof), the assignment operator selected to copy/move that member is trivial;
otherwise the copy/move assignment operator is non-trivial.

elision of copy/move

When certain criteria are met, an implementation is allowed to omit the copy/move construction of a class object, even if the constructor selected for the copy/move operation and/or the destructor for the object have side effects. In such cases, the implementation treats the source and target of the omitted copy/move operation as simply two different ways of referring to the same object, and the destruction of that object occurs at the later of the times when the two objects would have been destroyed without the optimization (Because only one object is destroyed instead of two, and one copy/move constructor is not executed, there is still one object destroyed for each one constructed.). This elision of copy/move operations, called copy elision, is permitted in the following circumstances (which may be combined to eliminate multiple copies):

in a return statement in a function with a class return type, when the expression is the name of a non-volatile automatic object (other than a function or catch-clause parameter) with the same cv-unqualified type as the function return type, the copy/move operation can be omitted by constructing the automatic object directly into the function’s return value
in a throw-expression, when the operand is the name of a non-volatile automatic object (other than a function or catch-clause parameter) whose scope does not extend beyond the end of the innermost enclosing try-block (if there is one), the copy/move operation from the operand to the exception object (15.1) can be omitted by constructing the automatic object directly into the exception object
when a temporary class object that has not been bound to a reference (12.2) would be copied/moved to a class object with the same cv-unqualified type, the copy/move operation can be omitted by constructing the temporary object directly into the target of the omitted copy/move
when the exception-declaration of an exception handler (Clause 15) declares an object of the same type (except for cv-qualification) as the exception object (15.1), the copy operation can be omitted by treating the exception-declaration as an alias for the exception object if the meaning of the program will be unchanged except for the execution of constructors and destructors for the object declared by the exception-declaration. [ Note: There cannot be a move from the exception object because it is always an lvalue. — end note ]

class Thing {
	public:
	Thing();
	~Thing();
	Thing(const Thing&);
};
Thing f() {
	Thing t;
	return t;
}
Thing t2 = f();

Here the criteria for elision can be combined to eliminate two calls to the copy constructor of class Thing: the copying of the local automatic object t into the temporary object for the return value of function f() and the copying of that temporary object into object t2. Effectively, the construction of the local object t can be viewed as directly initializing the global object t2, and that object’s destruction will occur at program exit. Adding a move constructor to Thing has the same effect, but it is the move construction from the temporary object to t2 that is elided. — end example ]

When the criteria for elision of a copy/move operation are met, but not for an exception-declaration, and the object to be copied is designated by an lvalue, or when the expression in a return statement is a (possibly parenthesized) id-expression that names an object with automatic storage duration declared in the body or parameter-declaration-clause of the innermost enclosing function or lambda-expression, overload resolution to select the constructor for the copy is first performed as if the object were designated by an rvalue. If the first overload resolution fails or was not performed, or if the type of the first parameter of the selected constructor is not an rvalue reference to the object’s type (possibly cv-qualified), overload resolution is performed again, considering the object as an lvalue. [ Note: This two-stage overload resolution must be performed regardless of whether copy elision will occur. It determines the constructor to be called if elision is not performed, and the selected constructor must be accessible even if the call is elided. — end note ]

class Thing {
public:
	Thing();
	~Thing();
	Thing(Thing&&);
private:
	Thing(const Thing&);
};
Thing f(bool b) {
	Thing t;
	if(b)
		throw t; // OK: Thing(Thing&&) used (or elided) to throw t
	return t; // OK: Thing(Thing&&) used (or elided) to return t
}
Thing t2 = f(false); // OK: Thing(Thing&&) used (or elided) to construct t2