When passing SGVector (or any non-primitive type) as an argument, I think it is a good idea to use either const reference (for input or read-only parameters) or pointer (for input and output parameters). In this way it is clear from the calling site whether a method/function will modify its arguments.
A bit more on this: https://google.github.io/styleguide/cppguide.html#Reference_Arguments On 12 May 2018 at 11:47, Heiko Strathmann <heiko.strathm...@gmail.com> wrote: > BTW if you see pointers to SGVectors being passed around, that should > probably be changed. Can you share the locations of it? > H > > 2018-05-12 10:46 GMT+01:00 Heiko Strathmann <heiko.strathm...@gmail.com>: > >> Hi >> >> I cc the list as others might have the same question. >> >> SGVector (same for SGMarix) is a memory wrapper for c/c++ arrays, that >> implements an automatic reference counter and therefore shared ownership. >> If you assign the vector to another one using operator=, (or similarly, >> pass it by value), then what happens is that a new SGVector structure is >> generated, but it points to the same memory. Once all SGVector instances >> that point to the memory block are destroyed, the memory block is freed. >> This is why passing it by value is relatively efficient (it doesnt copy >> the actual memory) and in particular it allows shared ownership, such as >> SGVector<float64_t> get_vector() { return m_vector; } >> which returns returns a copy of the SGVector instance that points to the >> same memory that m_vector does. >> This would allow me to modify a member variable as >> auto vec = obj->get_vector() >> vec[0] = 5; // now the memory block where Object::m_vector points to is >> changed >> >> Now sometimes, helper methods that accept vectors are called many times >> (in a loop). Copy- assigning the SGVector doesnt copy the memory block but >> creates a new instance everytime, which can be slow. This is why we >> sometimes pass around references, in particular const references make a >> speed difference (see linalg). >> There should never be the need to pass a pointer to an SGVector to a >> helper method. >> >> Summary answer to your questions: >> Passing SGVector by value creates a shared ownership of a fixed memory >> block. >> With respect to the memory block, passing SGVector by value allows for >> what you called inplace updates (of the memory block). >> Whenever you pass vectors to a helper method, you dont need to share >> ownership, and therefore you can pass a by reference (if you want to >> modify) or even const reference (if you just want to read). Obviously, >> shared ownership comes at a (small) cost, which you might want to avoid in >> low-level methods (say linalg). >> >> Hope that helps! >> H >> >> >> 2018-05-12 2:54 GMT+01:00 Elfarouk Harb <eyfmh...@gmail.com>: >> >>> Hi Heiko, >>> >>> Just a quick question about something that is confusing me a bit. Some >>> times, if there is a need to do an inplace update of a variable, I see that >>> SGVector<T> is passed by a pointer, meaning: >>> SGVector<T>* ref_vars. However, sometimes, they are passed to functions >>> which do in place updates but they are passed by value: SGVector<T> >>> ref_vars (example: http://shogun-toolbox.org/api/ >>> latest/DescendUpdaterWithCorrection_8cpp_source.html#l00054 >>> variable_reference >>> is passed by value but the function acts on it as if it is passed by >>> reference). Is SGVector somehow being passed by reference in both ways >>> or am I missing something? >>> >>> Thanks a lot, >>> Elfarouk >>> >> >> >
