Brandon Kohn wrote:
Dimensionally qualified quantities can be calculated for geometries for whose dimensions the quantity makes no sense (e.g. area( point ), length( point ).) These functions return default constructed instances of their corresponding return type as shown here:
template<typename ReturnType, typename Geometry, typename Strategy> struct calculate_null { static inline ReturnType apply(Geometry const& , Strategy const&) { return ReturnType(); } };
In my test ReturnType was double. This would seem to silently compile and return garbage on these inputs. In practice on vs2008( VC9 ) these return values were 0 (with optimizations on) even though the return resolves to double();. I'm guessing other platforms won't be so lucky.
This expression is correct and well defined. If I may, here is reference from the C++ standard document: """ 5.2.3 Explicit type conversion (functional notation) 2 The expression T(), where T is a simple-type-specifier (7.1.5.2) for a non-array complete object type or the (possibly cv-qualified) void type, creates an rvalue of the specified type, which is value-initialized (8.5; no initialization is done for the void() case). """ and """ 5 To zero-initialize an object of type T means: — if T is a scalar type (3.9), the object is set to the value of 0 (zero) converted to T; — if T is a non-union class type, each nonstatic data member and each base-class subobject is zero- initialized; — if T is a union type, the object’s first named data member89) is zero-initialized; — if T is an array type, each element is zero-initialized; — if T is a reference type, no initialization is performed. To default-initialize an object of type T means: — if T is a non-POD class type (clause 9), the default constructor for T is called (and the initialization is ill-formed if T has no accessible default constructor); — if T is an array type, each element is default-initialized; — otherwise, the object is zero-initialized. To value-initialize an object of type T means: — if T is a class type (clause 9) with a user-declared constructor (12.1), then the default constructor for T is called (and the initialization is ill-formed if T has no accessible default constructor); — if T is a non-union class type without a user-declared constructor, then every non-static data member and base-class component of T is value-initialized; — if T is an array type, then each element is value-initialized; — otherwise, the object is zero-initialized A program that calls for default-initialization or value-initialization of an entity of reference type is ill-formed. If T is a cv-qualified type, the cv-unqualified version of T is used for these definitions of zero-initialization, default-initialization, and value-initialization. """ Best regards, -- Mateusz Loskot, http://mateusz.loskot.net Charter Member of OSGeo, http://osgeo.org