Saturday, February 11, 2012

c++ CONST methods treat *this as bitwise-constant

Item 29 of [[effC++]] shows that a const method in class C1 can return a non-const pointer field of class C1. If the pointee lives at a offsite address, then whoever using the pointer can /edit/ the pointee.

The "const" on the method tells compiler to treat "this" as a pointer-to-const. More precisely, this particular method can't modify the bitwise state of the host object. Still more precisely, it means all the bits in the real estate of the host object must remain intact during this function call.

Therefore, all fields are implicitly converted to const variables when used in this function. See also P215 [[eff STL]]

If the host object has sizeof() of 55 bytes, then that's the size of its real estate. All the bits in the real estate must remain intact. However, the pointee lives outside the real estate and lives unprotected by const.

In java/c#, every reference field is a pointer. In contrast, java primitives and c# structs are the primary examples of "embedded" fields in a class.

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