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Overview of Optional Data Members
As of Ice 3.5, a data member of a Slice class or exception may be declared as optional to indicate that a program can leave its value unset. Data members not declared as optional are known as required members; a program must supply legal values for all required members.
Declaring Optional Data Members
Each optional data member in a type must be assigned a unique, non-negative integer tag:
class C { string name; bool active; optional(2) string alternateName; optional(5) int overrideCode; };
It is legal for a base type's tag to be reused by a derived type:
exception Base { optional(1) int systemCode; }; exception Derived extends Base { optional(1) string diagnostic; // OK };
The scope of a tag is limited to its enclosing type and has no effect on base or derived types.
Language mappings specify an API for setting an optional member and testing whether a member is set. Here is an example in C++:
CPtr c = new C; c->name = "xyz"; // required c->active = true; // required c->alternateName = "abc"; // optional c->overrideCode = 42; // optional if(c->alternateName) cout << "alt name = " << c->alternateName << endl;
As you can see, the C++ language mapping makes setting an optional member as simple as assigning it a value. Refer to the language mapping sections for more details on the optional data member API.
A well-behaved program must test for the presence of an optional member and not assume that it is always set. Dereferencing an unset optional member causes a run-time error.
In all supported language mappings, an optional data member's initial condition is unset if not otherwise assigned during construction. Again using C++ as an example:
CPtr c = new C; // default constructor assert(!c->alternateName); // not set c = new C("xyz", true, "abc", 42); // one-shot constructor assert(c->alternateName); // set by constructor
Optional Data Members with Default Values
You can declare a default value for optional members just as you can for required members:
class C { string name; bool active = true; optional(2) string alternateName; optional(5) int overrideCode = -1; };
An optional data member with a default value is considered to be set by default:
CPtr c = new C; // default constructor assert(!c->alternateName); // not set assert(c->overrideCode); // set to default value
Each language mapping provides an API for resetting an optional data member to its unset condition.
See Also