Non-static data member initializers

Abstract

Acknowledgement

The proposal

As a simple example,

    class A {
    public:
        int a = 7;
    };

    class A {
    public:
        A() : a(7) {}
    };

    class A {
    public:
        A(): a(7), b(5), hash_algorithm("MD5"), s("class A example") {}
        A(int a_val) : a(a_val), b(5), hash_algorithm("MD5"), s("Constructor run") {}
        A(int b_val) : a(7), b(b_val), hash_algorithm("MD5"), s("Constructor run") {}
        A(D d) : a(f(d)), b(g(d)), hash_algorithm("MD5"), s("Constructor run") {}
        int a, b;
    private:
        // Cryptographic hash to be applied to all A instances
        HashingFunction hash_algorithm;
        // String indicating state in object lifecycle
        std::string s;
    };

    class A {
    public:
        A(): a(7), b(5) {}
        A(int a_val) : a(a_val), b(5) {}
        A(int b_val) : a(7), b(b_val) {}
        A(D d) : a(f(d)), b(g(d)) {}
        int a, b;
    private:
        // Cryptographic hash to be applied to all A instances
        HashingFunction hash_algorithm("MD5");
        // String indicating state in object lifecycle
        std::string s("Constructor run");
    };

Now suppose that it is decided that MD5 hashes are not collision resistent enough and that SHA-1 hashes should be used. Without member initializers, all the constructors need to be updated. Unfortunately, if one developer is unaware of this change and creates a constructor that is defined in a different source file and continues to initialize the cryptographic algorithm to MD5, a very hard to detect bug will have been introduced. It seems better to keep the information in one place.

It may happen that a data member will usually have a particular value, but a few specialized constructors will need to be cognizant of that value. If a constructor initializes a particular member explicitly, the constructor initialization overrides the member initializations as shown below:

    class A {
    public:
        A(): a(7), b(5) {}
        A(int a_val) : a(a_val), b(5) {}
        A(int b_val) : a(7), b(b_val) {}
        A(D d) : a(f(d)), b(g(d)) {}
        // Copy constructor
        A(const A& aa) : a(aa.a),
                         b(aa.b),
                         hash_algorithm(aa.hash_algorithm.getName()),
                         s(aa.s) {}
        int a, b;
    private:
        // Cryptographic hash to be applied to all A instances
        HashingFunction hash_algorithm("MD5");
        // String indicating state in object lifecycle
        std::string s("Constructor run");
    };

• By allowing non-static data members of classes to be initialized in the same way as non-static local variables (which should be thought of as non-static data members of the function frame), C++ initialization becomes more consistent. As many of the documents mentioned above point out, this is much needed.

• Because these initializers must be given in the class definition, which may be included in many files, it is possible that the initializers may vary in value depending on context. Although this is troublesome, such ODR problems already exist for default arguments, inline methods, member types, etc.; and member initializers do not appreciably aggravate them.

• There is some overlap between this proposal and constructor delegation and forwarding (N1445, N1581, N1618, and N1898). However, it is easy to see that neither technique obviates the other. For example, overriding member initializers appears difficult to do in generality through constructor forwarding. (And note that Java has both member initialization and constructor forwarding. This is not regarded as confusing, and most experienced Java programmers make regular use of both techniques based on applicability.)

• Member initializers make possible the use of copy-initialization for class data members. It seems likely this flexibility will prove useful just as in the case of local variables.

• It is intended that this proposal also supports initializer lists (N2575) if they are adopted into C++0X.

An issue raised in Kona regarding scope of identifiers:

During discussion in the Core Working Group at the September ’07 meeting in Kona, a question arose about the scope of identifiers in the initializer. Do we want to allow class scope with the possibility of forward lookup; or do we want to require that the initializers be well-defined at the point that they’re parsed?

What’s desired:

    int x();

    struct S {
        int i;
        S() : i(x()) {} // currently well-formed, uses S::x()
        // ...
        static int x();
    };

    struct T {
        int i = x(); // should use T::x(), ::x() would be a surprise
        // ...
        static int x();
    };

Problem 1:

    struct S {
        int i(x); // data member with initializer
        // ...
        static int x;
    };

    struct T {
        int i(x); // member function declaration
        // ...
        typedef int x;
    };

    struct S {
        int i(j); // ill-formed...parsed as a member function,
                  // type j looked up but not found
        // ...
        static int j;
    };

    struct S {
        int i(x); // unabmiguously a data member
        int j(typename y); // unabmiguously a member function
    };

The solution proposed in this paper is to allow only initializers of the “= initializer-clause” and “{ initializer-list }” forms. That solves the ambiguity problem in most cases, for example:

    HashingFunction hash_algorithm{"MD5"};

In especially tricky cases, a type might have to be mentioned twice. Consider:

    vector<int> x = 3; // error:  the constructor taking an int is explicit
    vector<int> x(3);  // three elements default-initialized
    vector<int> x{3};  // one element with the value 3

    vector<int> x = vector<int>(3); // rather than vector<int> x(3);
    vector<int> x{3}; // one element with the value 3

Problem 2:

    struct S {
               const int i = f(); // well-formed with forward lookup
        static const int j = f(); // always ill-formed for statics
        // ...
        constexpr static int f() { return 0; }
    };

Problem 3:

    struct S {
        int i = j; // ill-formed without forward lookup, undefined behavior with
        int j = 3;
    };

The proposal:

We believe:

Problem 1: This problem does not occur as we don’t propose the () notation. The = and {} initializer notations do not suffer from this problem.

Problem 2: adding the static keyword makes a number of differences, this being the least of them.

Problem 3: this is not a new problem, but is the same order-of-initialization problem that already exists with constructor initializers.

A recent issue regarding auto:

template< class T >
struct MyType : T {
  auto data = func();
  static const size_t erm = sizeof(data);
};

In order to determine the layout of X, we now have 2-phase name lookup and ADL. Note that func could be either a type or a function; it may be found in T, the namespace of MyType, the associated namespace(s) of T when instantiated, the global namespace, an anonymous namespace, or any namespaces subject to a using directive. With care we could probably throw some concept_map lookup in for luck.

Depending on the order of header inclusion I might even get different results for ADL, and break the One Definition Rule - which is not required to be diagnosed.

Suggested changes to the Working Paper, N2588

5.1 Primary expressions [expr.prim]

The keyword this names a pointer to the object for which a non-static member function (9.3.2) is invoked or a non-static data member’s initializer (9.2) is evaluated. The keyword this shall be used only inside a non-static class member function body (9.3) or in an assignment-initializer for a non-static data member (9.2). The type of the expression is a pointer to the function’s or non-static data member’s class (9.3.2), possibly with cv-qualifiers on the class type. The expression is an rvalue.

An id-expression that denotes a non-static data member or non-static member function of a class can only be used:

— as part of a class member access (5.2.5) in which the object-expression refers to the member’s class or a class derived from that class, or

— to form a pointer to member (5.3.1), or

— in the body of a non-static member function of that class or of a class derived from that class (9.3.1), or

— in a mem-initializer for a constructor for that class or for a class derived from that class (12.6.2), or

— in an assignment-initializer for a non-static data member of that class or of a class derived from that class (12.6.2), or

— if that id-expression denotes a non-static data member and it is the sole constituent of an unevaluated operand, except for optional enclosing parentheses. [ Example:
...

7.1.6.4 auto specifier [dcl.spec.auto]

Change paragraph 4:

The auto type-specifier can also be used in declaring an object in the condition of a selection statement (6.4) or an iteration statement (6.5), in the type-specifier-seq in a new-type-id (5.3.4), and in declaring a static data member with a constant-initializer an assignment-initializer that appears within the member-specification of a class definition (9.4.2).

8.5 Initializers [dcl.init]

initializer:
    =  initializer-clause  assignment-initializer
    (  expression-list  )

assignment-initializer:
    =  initializer-clause

The initialization that occurs in argument passing, function return, throwing an exception (15.1), handling an exception (15.3), initializing non-static data members with assignment-initializers, and brace-enclosed initializer lists (8.5.1) is called copy-initialization and is equivalent to the form

T x = a;

The initialization that occurs in new expressions (5.3.4), static_cast expressions (5.2.9), functional notation type conversions (5.2.3), and base and member initializers mem-initializers (12.6.2) is called direct-initialization and is equivalent to the form

T x(a);

9.2 Class members [class.mem]

member-declarator:
    declarator  pure-specifier_opt
    declarator  constantassignment-initializer_opt
    identifier_opt  :  constant-expression

constant-initializer:
    = constant-expression

A class is considered a completely-defined object type (3.9) (or complete type) at the closing } of the class-specifier. Within the class member-specification, the class is regarded as complete within function bodies, default arguments, exception-specifications, and non-static data member assignment-initializers (including such things in nested classes). Otherwise it is regarded as incomplete within its own class member-specification.

A member-declarator can contain a constant-initializer only if it declares a static member (9.4) of const literal type, see 9.4.2.

A member can be initialized using an assignment-initializer. (For static data members, see 9.4.2 [class.static.data]; for non-static data members, see 12.6.2 [class.base.init]).

9.4.2 Static data members [class.static.data]

If a static data member is of const literal type, its declaration in the class definition can specify a constant-initializer. A declaration of a static data member in a class definition may only specify an assignment-initializer if that data member is of const literal type, in which case the initializer-clause shall be an integral constant expression (5.19). A static data member of literal type can be declared in the class definition with the constexpr specifier; if so, its declaration shall specify a constant-initializer an assignment-initializer with an initializer-clause that is an integral constant expression (5.19). In both these cases, the member A static data member that is of const literal type, or that is declared with the constexpr specifier, may appear in integral constant expressions. The member shall still be defined in a namespace scope if it is used in the program; and if the member-declarator contains an assignment-initializer, the namespace scope definition shall not contain an initializer.

12.1 Constructors [class.ctor]

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 default constructor is implicitly declared. An implicitly-declared default constructor is an inline public member of its class. A default constructor is trivial if it is not user-provided (8.4) 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 an assignment-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.

12.6.2 Initializing bases and members [class.base.init]

Add a new first bullet to 12.6.2p4:

If a given non-static data member or base class is not named 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 an assignment-initializer, the entity is copy-initialized with the value of the initializer-clause.

— If Otherwise, if the entity is a non-static ...

After the call to a constructor for class X has completed, if a member of X is neither specified in the constructor’s mem-initializers, nor copy-initialized, nor default-initialized, nor value-initialized, nor given a value during execution of the compound-statement of the body of the constructor, the member has indeterminate value.

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

[ Example: given

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 assigmnent-initializer will not take place. — end example ]

12.7 Construction and destruction [class.cdtor]

Member functions, including virtual functions (10.3), can be called during construction or destruction (12.6.2). When a virtual function is called directly or indirectly from a constructor (including from the assignment-initializer or mem-initializer for a data member) or from a destructor, and the object to which the call applies is the object under construction or destruction, the function called is the one defined in the constructor or destructor’s own class or in one of its bases, but not a function overriding it in a class derived from the constructor or destructor’s class, or overriding it in one of the other base classes of the most derived object (1.8). If the virtual function call uses an explicit class member access (5.2.5) and the object-expression refers to the object under construction or destruction but its type is neither the constructor or destructor’s own class or one of its bases, the result of the call is undefined. [ Example:
...

Same for 12.7p5:

The typeid operator (5.2.8) can be used during construction or destruction (12.6.2). When typeid is used in a constructor (including from the assignment-initializer or mem-initializer for a data member) or in a destructor, or used in a function called (directly or indirectly) from a constructor or destructor, if the operand of typeid refers to the object under construction or destruction, typeid yields the std::type_info object representing the constructor or destructor’s class. If the operand of typeid refers to the object under construction or destruction and the static type of the operand is neither the constructor or destructor’s class nor one of its bases, the result of typeid is undefined.

And for 12.7p6:

Dynamic_casts (5.2.7) can be used during construction or destruction (12.6.2). When a dynamic_cast is used in a constructor (including from the assignment-initializer or mem-initializer for a data member) or in a destructor, or used in a function called (directly or indirectly) from a constructor or destructor, if the operand of the dynamic_cast refers to the object under construction or destruction, this object is considered to be a most derived object that has the type of the constructor or destructor’s class. If the operand of the dynamic_cast refers to the object under construction or destruction and the static type of the operand is not a pointer to or object of the constructor or destructor’s own class or one of its bases, the dynamic_cast results in undefined behavior.

12.8 Copying class objects [class.copy]

The implicitly-defined or explicitly-defaulted copy constructor for class X performs a memberwise copy of its subobjects. [ Note: assignment-initializers of non-static data members are ignored. See also the example in 12.6.2p5. — end note ] The order of copying is the same as the order of initialization of bases and members in a user-defined constructor (see 12.6.2). Each subobject is copied in the manner appropriate to its type: