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half 12 is not required as half 12 will compile another means to turn on attribute inference is to store the function inside another function these are called nested functions inference is enabled not only on functions nested directly inside another function but also on most things nested in a type or a template inside the function example safe void parentfun this is auto inferred int half int x return x 2 class nestedtype this is auto inferred final int half1 int x return x 2 this is not auto inferred it s a virtual function and the compiler can t know if it has an unsafe override in a derived class int half2 int x return x 2 int a half 12 works inferred as safe auto cl new nestedtype int b cl half1 18 works inferred as safe int c cl half2 26 error a downside of nested functions is that they can only be used in lexical order the call site must be below the function declaration unless both the nested function and the call are inside the same struct class union or template that is in turn inside the parent function another downside is that they don t work with uniform function call syntax finally attribute inference is always enabled for function literals a k a lambda functions the halving function would be defined as enum half int x x 2 and called exactly as normal however the language does not consider this declaration a function it considers it a function pointer this means that in global scope it s important to use enum or immutable instead of auto otherwise the lambda can be changed to something else from anywhere in the program and cannot be accessed from pure functions in rare cases such mutability can be desirable but most often it is an antipattern like global variables in general limits of inference aiming for minimal manual typing isn t always wise neither is aiming for maximal attribute bloat the primary problem of auto inference is that subtle changes in the code can lead to inferred attributes turning on and off in an uncontrolled manner to see when it matters we need to have an idea of what will be inferred and what will not the compiler in general will go to great lengths to infer safe pure nothrow and nogc attributes if your function can have those it almost always will the specification says that recursion is an exception a function calling itself should not be safe pure or nothrow unless explicitly specified as such but in my testing i found those attributes actually are inferred for recursive functions it turns out there is an ongoing effort to get recursive attribute inference working and it partially works already inference of scope and return on function parameters is less reliable in the most mundane cases it ll work but the compiler gives up pretty quickly the smarter the inference engine is the more time it takes to compile so the current design decision is to infer those attributes in only the simplest of cases where to let the compiler infer a d programmer should get into the habit of asking what will happen if i mistakenly do something that makes this function unsafe impure throwing garbage collecting or escaping if the answer is immediate compiler error auto inference is probably fine on the other hand the answer could be user code will break when updating this library i m maintaining in that case annotate manually in addition to the potential of losing attributes the author intends to apply there is also another risk safe pure nothrow nogc firstnewline string from foreach i 0 from length switch from i case r if from length i 1 from i 1 n return r n else return r case n return n default break return you might think that since the author is manually specifying the attributes there s no problem unfortunately that s wrong suppose the author decides to rewrite the function such that all the return values are slices of the from parameter rather than string literals safe pure nothrow nogc firstnewline string from foreach i 0 from length switch from i case r if from length i 1 from i 1 n return from i i 2 else return from i i 1 case n return from i i 1 default break return surprise the parameter from was previously inferred as scope and a library user was relying on that but now it s inferred as return scope instead breaking client code still for internal functions auto inference is a great way to save both our fingers when writing and our eyes when reading note that it s perfectly fine to rely on auto inference of the safe attribute as long as the function is used in explicitly in safe functions or unit tests if something potentially unsafe is done inside the auto inferred function it gets inferred as system not trusted calling a system function from a safe function results in a compiler error meaning auto inference is safe to rely on in this case it still sometimes makes sense to manually apply attributes to internal functions because the error messages generated when they are violated tend to be better with manual attributes what about templates auto inference is always enabled for templated functions what if a library interface needs to expose one there is a way to block the inference albeit an ugly one private template funcontainer t not auto inferred only eponymous template functions are safe t fun t arg return arg 3 auto inferred per se but since the function it calls is not only safe is inferred auto addthree t t arg return funcontainer t fun arg however which attributes a template should have often depends on its compile time parameters it would be possible to use metaprogramming to designate attributes depending on the template parameters but that would be a lot of work hard to read and easily as error prone as relying on auto inference it s more practical to just test that the function template infers the wanted attributes such testing doesn t have to and probably shouldn t be done manually each time the function is changed instead float multiplyresults alias fun float arr if is typeof fun new float float float result 1 0f foreach ref e arr result fun e return result safe pure nothrow unittest float fun float x return x 1 using a static array makes sure arr argument is inferred as scope or return scope float 5 elements 1 0f 2 0f 3 0f 4 0f 5 0f no need to actually do anything with the result the idea is that since this compiles multiplyresults is proven safe pure nothrow and its argument is scope or return scope multiplyresults fun elements thanks to d s compile time introspection powers testing against unwanted attributes is also covered safe unittest import std traits attr functionattribute functionattributes issafe float fun float x makes the function both throwing and garbage collector dependant if x 5 throw new exception static float impurevar makes the function impure auto result impurevar impurevar 5 makes the argument unscoped impurevar x return result enum attrs functionattributes multiplyresults fun assert attrs attr nothrow_ assert attrs attr nogc checks against accepting scope arguments note that this check does not work with system functions assert issafe float 5 stackfloats multiplyresults fun stackfloats it s a good idea to do positive tests with similar methods to make sure the tests above would fail if the tested function had the wrong attributes assert attrs attr safe assert issafe float heapfloats multiplyresults fun heapfloats if assertion failures are wanted at compile time before the unit tests are run adding the static keyword before each of those assert s will get the job done those compiler errors can even be had in non unittest builds by converting that unit test to a regular function e g by replacing safe unittest with say private safe testattrs the live fire exercise system let s not forget that d is a systems programming language as this series has shown in most d code the programmer is well protected from memory errors but d would not be d if it didn t allow going low level and bypassing the type system in the same manner as c or c bit arithmetic on pointers writing and reading directly to hardware ports executing a struct destructor on a raw byte blob d is designed to do all of that the difference is that in c and c it takes only one mistake to break the type system and cause undefined behavior anywhere in the code a d programmer is only at risk when not in a safe function or when using dangerous compiler switches such as release or check assert off failing a disabled assertion is undefined behavior and even then the semantics tend to be less ub prone for example float cube float arg float result result arg result arg return result this is a language agnostic function that compiles in c c and d someone intended to calculate the cube of arg but forgot to initialize result with arg in d nothing dangerous happens despite this being a system function no initialization value means result is default initialized to nan not a number which leads to the result also being nan which is a glaringly obvious error value when using this function the first time however in c and c not initializing a local variable means reading it is sans a few narrow exceptions undefined behavior this function does not even handle pointers yet according to the standard calling this function could just as well have int rand 0xdeadbeef in it all due to a trivial mistake while many compilers with enabled warnings will catch this one not all do and these languages are full of similar examples where even warnings don t help in d even if you explicitly requested no default initialization with float result void it d just mean the return value of the function is undefined not anything and everything that happens if the function is called consequently that function could be annotated safe even with such an initializer still for anyone who cares about memory safety as they probably should for anything intended for a wide audience it s a bad idea to assume that d system code is safe enough to be the default mode two examples will demonstrate what can happen what undefined behavior can do some people assume that undefined behavior simply means erroneous behavior or crashing at runtime while that is often what ultimately happens undefined behavior is far more dangerous than say an uncaught exception or an infinite loop the difference is that with undefined behavior you have no guarantees at all about what happens this might not sound any worse than an infinite loop but an accidental infinite loop is discovered the first time it s entered code with undefined behavior on the other hand might do what was intended when it s tested but then do something completely different in production even if the code is tested with the same flags it s compiled with in production the behavior may change from one compiler version to another or when making completely unrelated changes to the code time for an example return whether the exception itself is in the array bool replaceexceptions object arr ref exception e bool result foreach ref o arr if o is e result true if cast exception o o e return result the idea here is that the function replaces all exceptions in the array with e if e itself is in the array it returns true otherwise false and indeed testing confirms it works the function is used like this auto arr new exception a null null new exception c auto result replaceexceptions cast object arr arr 3 this cast is not a problem right object references are always of the same size regardless of their type and we re casting the exceptions to the parent type object it s not like the array contains anything other than object references unfortunately that s not how the d specification views it having two class references or any references for that matter in the same memory location but with different types and then assigning one of them to the other is undefined behavior that s exactly what happens in if cast exception o o e if the array does contain the e argument since true can only be returned when undefined behavior is triggered it means that any compiler would be free to optimize replaceexceptions to always return false this is a dormant bug no amount of testing will find but that might years later completely mess up the application when compiled with the powerful optimizations of an advanced compiler it may seem that requiring a cast to use a function is an obvious warning sign that a good d programmer would not ignore i wouldn t be so sure casts aren t that rare even in fine high level code even if you disagree other cases are provably bad enough to bite anyone last summer this case appeared in the d forums string foo in string s return s void main import std stdio string result foreach c hello result foo c writeln result this problem was encountered by steven schveighoffer a long time d veteran who has himself lectured about safe and system on more than one occasion anything that can burn him can burn any of us normally this works just as one would think and is fine according to the spec however if one enables another soon to be default language feature with the preview in compiler switch along with dip1000 the program starts malfunctioning the old semantics for in are the same as const but the new semantics make it const scope since the argument of foo is scope the compiler assumes that foo will copy c before returning it or return something else and therefore it allocates c on the same stack position for each of the hello letters the result is that the program prints o o o o o at least for me it s already somewhat hard to understand what s happening in this simple example hunting down this sort of bug in a complex codebase could be a nightmare with my nightly dmd version somewhere between 2 100 and 2 101 a compile time error is printed instead with 2 100 2 the example runs as described above the fundamental problem in both of these examples is the same safe is not used had it been both of these undefined behaviors would have resulted in compilation errors the replaceexceptions function itself can be safe but the cast at the usage site cannot by now it should be clear that system code should be used sparingly when to proceed anyway sooner or later though the time comes when the guard rail has to be temporarily lowered here s an example of a good use case undefined behavior passing a non null pointer to a standalone character other than 0 or to an array without 0 at or after the pointed character as utf8stringz extern c system pure bool phobosvalidateutf8 const char utf8stringz import std string std utf try utf8stringz fromstringz validate catch utfexception return false return true this function lets code written in another language validate a utf 8 string using phobos c being c it tends to use zero terminated strings so the function accepts a pointer to one as the argument instead of a d array this is why the function has to be unsafe there is no way to safely check that utf8stringz is pointing to either null or a valid c string if the character being pointed to is not 0 meaning the next character has to ...