In my C++ library I have a function that is still there, 1) for debugging 2) for small operations.
The function is basically a very slow fallback of more efficient versions.
(think of a loop of individual assigments vs memcpy for example)>
For this reason I would like to emit a warning as soon the function is invoked instantiated directly or indirectly. Without a warning it is not easy to test if the function is being invoked instantiated, because the function might be called instantiated trough several layers of template code.
I found that GCC's __attribute__((warning("slow function!"))) does the job quite well.
template<class T>
__attribute__((warning("careful this fun is very slow, redesign your algorithm")))
void slow_function(T){...}
However it is not standard or compatible with clang.
Is there a better alternative for this kind of compile time warning?
It looks like there is a standard [[deprecated("msg")]] attribute that also does the job, the problem is that it is confusing because there is nothing deprecated about this function, it is there for convenience.
There is also, I found recently a #pragma poison that might be applicable here, but I don't understand how it is used, besides the function is actually a member function of a template class, the examples do not consider this case. https://www.fluentcpp.com/2018/09/04/function-poisoning-in-cpp/
Related
I see this recommended in the dart style guide, and copied in tons of tutorials and flutter source.
final foo = config.foo;
I don't understand it, how is this considered best practice when the readability is so poor? I have no clue what foo is here, surely final String foo = config.foo is preferable if we really want to use final?
This seems the equivalent to using var, which many consider a bad practice because it prevents the compiler from spotting errors and is less readable.
Where am I wrong?
In a lot of cases is does not really matter what type you are using as long the specific type can be statically determined by the compiler. When you are using var/final in Dart it is not that Dart does not know the type, it will just figure it out automatically based on the context. But the type must still be defined when the program are compiled so the types will never be dynamic based on runtime behavior. If you want truly dynamic types, you can use the dynamic keyword where you tell Dart "trust me, I know what I am doing with this types".
So types should still be defined where it matter most. This is e.g. for return and argument types for methods and class variables. The common factor for this types is that they are used to define the interface for using the method or class.
But when you are writing code inside a method, you are often not that interested in the specific types of variables used inside the method. Instead the focus should be the logic itself and you can often make it even more clear by using good describing variable names. As long the Dart analyzer can figure out the type, you will get autocomplete from your IDE and you can even still see the specific type from your IDE by e.g. Ctrl+Q in IntelliJ on the variable if you ends up in a situation where you want to know the type.
This is in particular the case when we are talking about the use of generics where it can be really tiresome to write the full specific type. Especially if you are using multiple generics inside each other like e.g. Map<String, List<String>>.
In my experience, Dart is really good to figure out very specific types and will complain loudly if your code cannot be determined statically. In the coming future, Dart will introduce non-null by default, which will make the Dart compiler and analyzer even more powerful since it will make sure your variable cannot be null unless this is something you specifically want and will make sure you are going to test for null when using methods which are specified to not expecting null.
About "Where am I wrong?". Well, a lot of languages have similar feature of var/final like Dart with the same design principle about the type should still be statically determined by a compiler or runtime. And even Java has introducing this feature. As a experienced Java and Dart programmer I have come to the conclusion for myself that typing inside methods are really not that important in a lot of cases as long I can still easily figure out the specific type by using an IDE when it really matters.
But it does make it more important to name your variables so they are clearly defining the purpose. But I am hoping you already are doing that. :)
Does Swift keep the the method lookup list when compiled or does it call a function in a specific memory location?
Best regards.
Regarding this: http://davedelong.tumblr.com/post/58428190187/an-observation-on-objective-c
I would recommend you have a look at the below links, especially the first one because it explains the concepts with examples from C++ and Objective-C, in order to have a better understanding of the difference between static, late and dynamic dispatch (for methods).
In a nutshell:
Static dispatch
The function and its implementation is determined at compile time and thus can’t fail at runtime (because the compiler will not continue the compilation process unless the binding is successful).
Late dispatch
The function is determined at compile time, but the actual implementation depends on the type of the object at runtime. Important for inheritance. The compiler will check if the the class or any of its parents have the function declared, but its up to the runtime to choose which implementation to use. The late binding can be implemented using virtual tables like in the case of C++.
Dynamic dispatch
The function is determined at runtime, which in the case of Objective-C can be called by name and thus can fail at runtime if the receiver (object) doesn't implement or inherit a method that can respond to a specified message.
References
What is the difference between Dynamic, Static and Late binding?
What is early and late binding?
What is the difference between dynamic dispatch and late binding in C++?
Is it possible to dynamically modify symbol table at runtime in C (in elf format on Linux)?
My eventual goal is the following:
Inside certain function say foo, I want to override malloc function to my custom handler my_malloc. But outside foo, any malloc should still call to malloc as in glibc.
Note: this is different from LD_PRELOAD which would override malloc during the entire program execution.
Is it possible to dynamically modify symbol table at runtime in C (in elf format on Linux)?
In theory this is possible, but in practice it's too hard to do.
Inside certain function say foo, I want to override malloc function to my custom handler my_malloc. But outside foo, any malloc should still call to malloc as in glibc.
Modifying symbol table (even if it were possible) would not get you to your desired goal.
All calls from anywhere inside your ELF binary (let's assume foo is in the main executable), resolve to the same PLT import slot malloc#plt. That slot is resolved to glibc malloc on the first call (from anywhere in your program, assuming you are not using LD_BIND_NOW=1 or similar). After that slot has been resolved, any further modification to the symbol table will have no effect.
You didn't say how much control over foo you have.
If you can recompile it, the problem becomes trivial:
#define malloc my_malloc
int foo() {
// same code as before
}
#undef malloc
If you are handed a precompiled foo.o, you are linking it with my_malloc.o, and you want to redirect all calls from inside foo.o from malloc to my_malloc, that's actually quite simple to do at the object level (i.e. before final link).
All you have to do is go through foo.o relocation records, and change the ones that say "put address of external malloc here" to "put address of external my_malloc here".
If foo.o contains additional functions besides foo, it's quite simple to limit the relocation rewrite to just the relocations inside foo.
Is it possible to dynamically modify symbol table at runtime in C (in elf format on Linux)?
Yes, it is not easy, but the functionality can be packaged into a library, so at the end of the day, it can be made practical.
Typemock Isolator++
(https://www.typemock.com/isolatorpp-product-page/isolate-pp/)
This is free-to-use, but closed source solution. The usage example from documentation should be instructive
TEST_F(IsolatorPPTests, IsExpired_YearIs2018_ReturnTrue) {
Product product;
// Prepare a future time construct
tm* fakeTime = new tm();
fakeTime->tm_year = 2018;
// Fake the localtime method
FAKE_GLOBAL(localtime);
// Replace the returned value when the method is called
// with the fake value.
WHEN_CALLED(localtime(_)).Return(fakeTime);
ASSERT_TRUE(product.IsExpired());
}
Other libraries of this kind
Mimick, from Q: Function mocking in C?
cpp-stub, from Q: Creating stub functionality in C++
Elfspy, for C++, but sometimes it's OK to test C code from C++ unittests, from Q: C++ mock framework capable of mocking non-virtual methods and C functions
HippoMocks, from Q: Mocking C functions in MSVC (Visual Studio)
the subprojects in https://github.com/coolxv/cpp-stub/tree/master/other
... there is still more, feel free to append ...
Alternate approaches
ld's --wrap option and linker scripts, https://gitlab.com/hedayat/powerfake
various approaches described in answers for Q: Advice on Mocking System Calls
and in answers to Q: How to mock library calls?
Rewrite code to make it testable
This is easier in other languages than C, but still doable even in C. Structure code into small functions without side-effects that can be unit-tested without resorting to trickery, and so on. I like this blog Modularity. Details. Pick One. about the tradeoffs this brings. Personally, I guturally dislike the "sea of small functions and tons of dependency injection" style of code, but I realize that that's the easiest to work with, all things considered.
Excursion to other languages
What you are asking for is trivial to do in Python, with the unittest.mock.patch, or possibly by just assigning the mock into the original function directly, and undoing that at the end of the test.
In Java, there is Mockito/PowerMock, which can be used to replace static methods for the duration of a test. Static methods in Java approximately correspond to regular functions in C.
In Go, there is Mockey, which works similarly to what needs to be done in C. It has similar limitations in that inlining can break this kind of runtime mocking. I am not sure if in C you can hit the issue that very short methods are unmockable because there is not enough space to inject the redirection code; I think more likely not, if all calls go through the Procedure Linkage Table.
I'd like to get a firmer grasp of how frequently the runtime in any language that requires one is being called. In this case, I'm specifically interested in knowing:
Of all the function calls getting executed on an OS X or iOS system in any given second (approximations are of course necessary) how many of those are Objective-C runtime functions (i.e. functions that are defined by the runtime)?
Of course it depends on your application, but in general the answer is "a whole lot". Like, a whole freaking lot.
If you really want to see numbers, I'd recommend using dtrace to log all runtime functions as they're called. This blog entry talks about how to do such a thing.
A lot. Here are just a few examples.
Every time you send a message, the actual message sending is done by a runtime function (this is in fact the most called runtime function in pretty much any objective C program).
NSObject class and protocol are not part of the standard library but part of the runtime, therefore any method that ends up executing to the default NSObject implementation is in fact executing runtime code.
Every time you execute a default property accessor (either read or write), that's part of the runtime.
If you use ARC, every time you access a weak reference (either for reading or writing it) that's a runtime function.
Objc runtime includes the C runtime, so anything that involves a C runtime function (for example passing a large structure by value or returning it) is in fact calling into the runtime.
and more.
Why are inline closures so rarely used in Actionscript? They are very powerful and I think quite readable. I hardly ever see anyone using them so maybe I'm just looking at the wrong code. Google uses them in their Google Maps API for Flash samples, but I think thats the only place I've seen them.
I favor them because you have access to local variables in the scope that defines them and you keep the logic in one method and dont end up with lots of functions for which you have to come up with a name.
Are there any catches of using them? Do they work pretty much the same way as in C#.
I actually only just discovered that AS3 supports them, and I'm quite annoyed becasue I had thought I read that they were deprecated in AS#. So I'm back to using them!
private function showPanel(index:int):void {
_timer = new Timer(1000, 1);
_timer.addEventListener(TimerEvent.TIMER, function(event:Event):void
{
// show the next panel
showPanel(index++);
});
The biggest gotcha to watch out for is that often 'this' is not defined in the inline closure. Sometimes you can set a 'this', but it's not always the right 'this' that you would have available to set, depending on how you're using them.
But I'd say most of the Flex code I've worked on has had inline closures rampantly throughout the code--since callbacks are the only way to get work done, and often you don't need the bring out a whole separate function.
Sometimes when the function nested is getting to be too much, I'll break it out slightly with Function variables in the function; this helps me organize a bit by giving labels to the functions but keeping some of the characteristics of inline closures (access to the local variables, for example).
Hope this helps.
One additional problem is that garbage collection is broken when it comes to closures (at least in Flash 9). The first instance of a given closure (from a lexical standpoint) will never be garbage collected - along with anything else referenced by the closure in the scope chain.
I found what originally made me NOT want to do this, but I had forgotten the details:
http://livedocs.adobe.com/flex/3/html/16_Event_handling_6.html#119539
(This is what Mitch mentioned - as far as the 'this' keyword being out of scope)
So thats Adobe's answer, however I am much more likely to need to refer to local variables than 'this'.
How do others interpret Adobe's recommendation ?