On Monday, September 14, 2015 01:12:02 Ola Fosheim Grostad via Digitalmars-d-learn wrote: > On Monday, 14 September 2015 at 00:41:28 UTC, Jonathan M Davis > wrote: > > Regardless, idiomatic D involves a lot more stack allocations > > than you often get even in C++, so GC usage tends to be low in > > Really? I use VLAs in my C++ (a C extension) and use very few > mallocs after init. In C++ even exceptions can be put outside the > heap. Just avoid STL after init and you're good.
>From what I've seen of C++ and understand of typical use cases from other folks, that's not at all typical of C++ usage (though there's enough people using C++ across a wide enough spectrum of environments and situations that there's obviously going to be quite a wide spread of what folks do with it). A lot of C++ folks use classes heavily, frequently allocating them on the heap. Major C++ libraries such as Qt certainly are designed with the idea that you're going to be allocating as the program runs. And C++ historically has been tooted fairly heavily by many folks as an OO language, in which case, inheritance (and therefore heap allocation) are used heavily by many programs. And with C++11/14, more mechanisms for safely handling memory have been added, thereby further encouraging the use of certain types of heap allocations in your typical C++ program - e.g. make_shared has become the recommended way to allocate memory in most cases. And while folks who are trying to get the bare metal performance that some stuff like games require, most folks are going to use the STL quite a bit. And if they aren't, they're probably using similar classes from a 3rd party library such as Qt. It's the folks who are in embedded environments or who have much more restrictive performance requirements who are more likely to avoid the STL or do stuff like avoid heap allocations after the program has been initialized. So, a _lot_ of C++ code uses the heap quite heavily, and I expect that very little of it tries to allocate everything up front. I, for one, have never worked on an application where that even made sense aside from something very small. I know that applications like that definitely exist, but from everything I've seen, I'd expect them to be the exception to the rule rather than the norm. Regardless, idiomatic D promotes ranges, which naturally help reduce heap allocation. It also means using structs heavily and classes sparingly (though there are plenty of cases where inheritance is required and thus classes get used). And while arrays/strings get allocated on the heap, slicing seriously reduces how often they need to be copied in memory, which reduces heap allocations. So, idiomatic D encourages programs to be written in a way that keeps heap allocation to a minimum. The big place that it happens in most D programs is probably strings, but slicing helps consderably with that, and even some of the string stuff can be made to live on the stack rather than the heap (which is what a lot of Walter's recent work in Phobos has been for - making string-based stuff work as lazy ranges), reducing heap allocations for strings ever further. C++ on the other hand does not have such idioms as the norm or as promoted in any serious way. So, programmers are much more likely to use idioms that involve a lot of heap allocations, and the language and standard library don't really have much to promote idioms that avoid heap allocations (and std::string definitely isn't designed to avoid copying). You can certainly do it - and many do - but since it's not really what's promoted by the language or standard library, it's less likely to happen in your average program. It's much more likely to be done by folks who avoid the STL. So, you _can_ have low heap allocation in a C++ program, and many people do, but from what I've seen, that really isn't the norm across the C++ community in general. - Jonathan M Davis
