I remember fretting about these rules when reading Scott Meyer's Effective C++11, and then later to realize it's better not to use auto at all. Explicit types are good types
const auto start = std::chrono::steady_clock::now();
do_some_work(size);
const auto end = std::chrono::steady_clock::now();
const std::chrono::duration<double> diff = end - start;
std::cout << "diff = " << diff << "; size = " << size << '\n';
TP start = TP::clock::now();
do_some_work(size);
TP end = TP::clock::now();Strong agree here. It's not just because it reduces cognitive load, it's because explicit types allows and requires the compiler to check your work.
Even if this isn't a problem when the code is first written, it's a nice safety belt for when someone does a refactor 6-12 months (or even 5+ years) down the road that changes a type. With auto, in the best case you might end up with 100+ lines of unintelligible error messages. In the worst case the compiler just trudges on and you have some subtle semantic breakage that takes weeks or months to chase down.
The only exceptions I like are iterators (whose types are a pita in C++), and lambda types, where you sometimes don't have any other good options because you can't afford the dynamic dispatch of std::function.
I also prefer not to use auto when getting iterators from STL containers. Often I use a typedef for most STL containers that I use. The one can write MyNiceContainerType::iterator.
auto var = FunctionCall(...);
Then, in the IDE, hover over auto to show what the actual type is, and then replace auto with that type. Useful when the type is complicated, or is in some nested namespace.
There's nothing special about auto here. It deduces the same type as a template parameter, with the same collapsing rules.
decltype(auto) is a different beast and it's much more confusing. It means, more or less, "preserve the type of the expression, unless given a simple identifier, in which case use the type of the identifier."
So for example I'd write:
var x = new List<Foo>();
List<Foo> x = new List<Foo>();
Whereas I'd write:
List<Foo> x = FooBarService.GetMyThings();
Although with newer language features you can also write:
List<Foo> x = new();
With good naming it should be pretty obvious it's a Foo, and then either you know the type by heart, or will need to look up the definition anyway.
With standard containers, you can have the assumption that everyone knows the type, at least high level. So knowing whether it's a list, a vector, a stack, a map or a multimap, ... is pretty useful and avoid a lookup.
List<Foo> x = new();
Nowadays I only use
var x = new List<Foo>();
auto it = some_container.begin();
Not even once have I wished to know the actual type of the iterator.
IDEs are an invention from the late 1970's, early 1980's.
Having syntax highlighting makes me slightly faster, but I want to still be able to understand things, when looking at a diff or working over SSH and using cat.
For example:
auto a;
will always fail to compile not matter what flags.
int a;
is valid.
Also it prevents implicit type conversions, what you get as type on auto is the type you put at the right.
That's good.
What do you mean it is not a source of bugs?
I think what they mean, and what I also think is that the bug does not come from the existence of uninitialized variables. It comes from the USE of uninitialized variables. Making the variables initialized does not make the bug go away, at most it silences it. Making the program invalid instead (which is what UB fundamentally is) is way more helpful for making programs have less bugs. That the compiler still emits a program is a defect, although an unfixable one.
As to my knowledge C (and derivatives like C++) is the only common language where the question "Is this a program?" has false positives. It is certainly an interesting choice.
So, I see uninitialized variables as a good way to find such logic errors. And, therefore, advice to always initialize variable - bad practice.
Of course if you already have a good value to initialize variable - do it. But if you have no - better leave it uninitialized.
Moreover - this will not cause safety issues in production builds because you can use `-ftrivial-auto-var-init` to initialize automatic variables to e.g. zeroes (`-fhardened` will do this too)
Regarding the "auto" in C++, and technically in any language, it seems conceptually wrong. The ONLY use-case I can imagine is when the type name is long, and you don't want to type it manually, or the abstractions went beyond your control, which again I don't think is a scalable approach.
In both Rust and C++ we need this because we have unnameable types, so if their type can't be inferred (in C++ deduced) we can't use these types at all.
In both languages all the lambdas are unnameable and in Rust all the functions are too (C++ doesn't have a type for functions themselves only for function pointers and we can name a function pointer type in either language)
C has this, so I think C++ has as well. You can use a typedef'ed function to declare a function, not just for the function pointer.
typedef void * (type) (void * args);
type foo;
a = foo (b);
A function type describes a function with specified return type. A function type is characterized by its return type and the number and types of its parameters. A function type is said to be derived from its return type, and if its return type is T , the function type is sometimes called ‘‘function returning T’’. The construction of a function type from a return type is called ‘‘function type derivation’’.
> Per ISO/IEC 9899:TC3:
What is it supposed to tell me?
You can read a "draft" of that document here: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3220.pdf
[If you've ever been under the impression that "real" people use the actual ISO text, disabuse yourself of that notion, ISO takes ages to turn the same exact words into an official branded document, then charges $$$ for a PDF, ain't nobody got time or money for that]
I can't tell you what they intended by TC3. It might be a typo or it might be some way to refer to a specific draft or a section within that draft. I doubt this particular section changes frequently so I wouldn't worry about it.
It sounds like this text is essentially the same in C23, maybe moved around a bit.
edit: you literally said this in your original comment. I failed at reading comprehension.
I regularly use that in C, to make sure a function matches an abstract interface. Sure, that often ends up in a function pointer, but not always and when I declare the type signature, it isn't yet a function pointer.
> but not define
I think that is because the type signature only contains the types, but no parameter names, which are required for a definition. This is arbitrary, since for data types, the member names are part of the type. It sounds totally fixable, but then you either have two types of function types, one where all parameter names are qualified and one where they aren't and only could use the former for function definitions. Or you would make names also mandatory for function declarations.
> It sounds totally fixable, but then you either have two types of function types, one where all parameter names are qualified and one where they aren't and only could use the former for function definitions
Making the names part of the type would be a bit weird, although we have seen stranger things. The biggest problem is that it would be a breaking change at least in C++.
Exactly, I believe that to be the case in C as well. In C23 the rules for types to be considered compatible were actually relaxed, so this proposal wouldn't make any sense. It would be a useless breaking change for no gain, other than a bit less typing and maybe feeding some language layers, so there is really no reason to do that. It actually makes the code less clear, since you now need to always lookup the type definition, which would be against C's philosophy.
That said, there are some contexts in which “auto” definitely improves the situation.
My LS can infer it anytime.
On top of that:
* Reduce refactoring overhead (a type can be evolved or substituted, if it duck types the same, the autos don't change)
* If using auto instead of explicit types makes your code unclear, it's not clear enough code to begin with. Improve the names of methods being called or variable names being assigned to.
I can see explicit types when you are shipping a library, as part of the API your library exposes. Then you want types to be explicit and changes to be explicit.
void func(std::vector<double> vec) {
for (auto &v : vec) {
// do stuff
}
}
I've seen much more perf-murdering things being caused by
std::map<std::string, int> my_map;
for(const std::pair<std::string, int>& v: my_map) {
...
}
> warning: loop variable 'v' of type 'const std::pair<std::__cxx11::basic_string<char>, int>&' binds to a temporary constructed from type 'std::pair<const std::__cxx11::basic_string<char>, int>' [-Wrange-loop-construct] 11 | for (const std::pair<std::string, int>& v: m) {
As they say, the power of names...
Is it that iterating over map yields something other than `std::pair`, but which can be converted to `std::pair` (with nontrivial cost) and that result is bound by reference?
std::pair<const std::string, int>
std::pair<std::string, int>Is it really? I rather think that a missing & is easier to spot with "auto" simply because there is less text to parse for the eye.
> If you see "for (auto v : vec)" looks good right?
For me the missing & sticks out like a sore thumb.
> It's easy to forget (or not notice) that auto will not resolve to a reference in this case
Every feature can be misused if the user forgets how it works. I don't think people suddenly forget how "auto" works, given how ubiquitous it is.
If auto deduced reference types transparently, it would actually be more dangerous.
So I guess I depart from you there and thus my issue here is not really about auto
Things are different in Rust because of lifetimes and destructive moves. In this context, copying would be a bad default indeed.
> because who said that's even achievable, let alone cheap?
Nobody said that. The thing is that user-defined types can be anything from tiny and cheap to huge and expensive. A language has to pick one default and be consistent. You can complain one way or the other.
Yes, languages like Rust can automatically move variables if the compiler can prove that they will not be used anymore. Unfortunately, this is not possible in C++, so the user has to move explicitly (with std::move).
``` struct dummy{}; dummy d = ANYTHING_YOU_WANT_GO_GET_THE_TYPE; ```
Compile it with g++, and get the type info from compilation error -:)
For any seriously templated or metaprogrammed code nowadays a concept/requires is going to make it a lot more obvious what your code is actually doing and give you actually useful errors in the event someone is misusing your code.
1. Consistency across the board (places where it's required for metaprogramming, lambdas, etc). And as a nicety it forces function/method names to be aligned instead of having variable character counts for the return type before the names. IMHO it makes skimming code easier.
2. It's required for certain metaprogramming situations and it makes other situations an order of magnitude nicer. Nowadays you can just say `auto foo()` but if you can constrain the type either in that trailing return or in a requires clause, it makes reading code a lot easier.
3. The big one for everyday users is that trailing return type includes a lot of extra name resolution in the scope. So for example if the function is a member function/method, the class scope is automatically included so that you can just write `auto Foo::Bar() -> Baz {}` instead of `Foo::Baz Foo::Bar() {}`.
2. It's incredibly rare for it to be required. It's not like 10% of the time, it's more like < 0.1% of the time. Just look at how many functions are in your code and how many of them actually can't be written without a trailing return type. You don't change habits to fit the tiny minority of your code.
3. This is probably the best reason to use it and the most subjective, but still not a particularly compelling argument for doing this everywhere, given how much it diverges from existing practice. And the downside is the scope also includes function parameters, which means people will refer to parameters in the return type much more than warranted, which is decidedly not always a good thing.
I have been programming in C++ for 25 years, so I'm so used to the original syntax that I don't default to auto ... ->, but I will definitely use it when it helps simplify some complex signatures.
And lifetime specifiers can be jarring. You have to think about how the function will be used at the declaration site. For example usually a function which takes two string views require different lifetimes, but maybe at call site you would only need one. It is just more verbose.
C++ has a host of complexities that come with header/source splits, janky stdlib improvements like lock_guard vs scoped_lock, quirky old syntax like virtual = 0, a lack of build systems and package managers.
Anything in any language can be very verbose and confusing if you one-line it or obfuscate it or otherwise write it in a deliberately confusing manner. That's not a meaningful point imo. What you have to do is compare what idiomatic code looks like between the two languages.
C++ has dozens of pages of dense standardese to specify how to initialise an object, full with such text as
> Only (possibly cv-qualified) non-POD class types (or arrays thereof) with automatic storage duration were considered to be default-initialized when no initializer is used. Each direct non-variant non-static data member M of T has a default member initializer or, if M is of class type X (or array thereof), X is const-default-constructible, if T is a union with at least one non-static data member, exactly one variant member has a default member initializer, if T is not a union, for each anonymous union member with at least one non-static data member (if any), exactly one non-static data member has a default member initializer, and each potentially constructed base class of T is const-default-constructible.
For me, it's all about inherent complexity vs incidental complexity. The having to pay attention to lifetimes is just Rust making explicit the inherent complexity of managing values and pointers thereof while making sure there isn't concurrent mutation, values moving while pointers to them exist, and no data races. This is just tough in itself. The aforementioned C++ example is just the language being byzantine and giving you 10,000 footguns when you just want to initialise a class.
That's just a list of very simple rules for each kind of type. As a C++-phob person, C++ has a lot of footguns, but this isn't one of them.
Type inference is usually reserved for more general algorithms that can inspect not only how a variable is initialized, but how the variable used, such as what functions it's passed into, etc...
In a modern context, both would be called "type inference" because unidirectional type inference is quite a bit more common now than the bidirectional kind, given that many major languages adopted it.
If you want to specify constraint-based type inference then you can say global HM (e.g. Haskell), local HM (e.g. Rust), or just bidirectional type inference.
95%[1] of the time, deduction is enough, but occasionally you really wish you had proper inference.
[1] percentage made up on the spot.
Most of the "ugly" of these examples only really matters for library authors and even then most of the time you'd be hard pressed to put yourself in these situations. Otherwise it "just works".
Basically any adherence to a modicum of best practices avoids the bulk of the warts that come with type deduction or at worst reduces them to a compile error.
Those errors are essentially the compiler telling you in order:
1. I tried to do this thing and I could not make it work.
2. Here's everything I tried in order and how each attempt failed.
If you read error messages from the top they make way more sense and if reading just the top line error doesn't tell you what's wrong, then reading through the list of resolution/type substitution failures will be insightful. In most cases the first few things it attempted will give you a pretty good idea of what the compiler was trying to do and why it failed.
If the resolution failures are a particularly long list, just ctrl-f/grep to the thing you expected to resolve/type-substitute and the compiler will tell you exactly why the thing you wanted it to use didn't work.
They aren't perfect error messages and the debugging experience of C++ metaprogramming leaves a lot to be desired but it is an order of magnitude better than it ever has been in the past and I'd still take C++ wall-o-error over the extremely over-reductive and limited errors that a lot of compilers in other languages emit (looking at you Java).
But "I tried to do this thing" error is completely useless in helping to find the reason why the compiler didn't do the thing it was expected to do, but instead chose to ignore.
Say, you hit ambiguous overload resolution, and have no idea what actually caused it. Or, conversely, implicit conversion gets hidden, and it helpfully prints all 999 operator << overloads. Or there is a bug in consteval bool type predicate, requires clause fails, and compiler helpfully dumps list of functions that have differet arguments.
How do you debug consteval, if you cannot put printf in it?
Not everyone can use clang or even latest gcc in their project, or work in a familiar codebase.
Modern C++ in general is so hostile to debugging I think it's astounding people actually use it.
Citation needed. This is common for embedded application, since why would anyone program a STL for that?
And with each std release the particularly nasty parts of std get decoupled from the rest of the library. So it's at the point nowadays where you can use all the commonly used parts of std in an embedded environment. So that means you get all your containers, iterators, ranges, views, smart/RAII pointers, smart/RAII concurrency primitives. And on the bleeding edge you can even get coroutines, generators, green threads, etc in an embedded environment with "pay for what you use" overhead. Intel has been pushing embedded stdlib really hard over the past few years and both they and Nvidia have been spearheading the senders and receivers concurrency effort. Intel uses S&R internally for handling concurrency in their embedded environments internal to the CPU and elsewhere in their hardware.
(Also fun side note but STL doesn't "really" stand for "standard template library". Some projects have retroactively decided to refer to it as that but that's not where the term STL comes from. STL stands for the Adobe "Software Technology Lab" where Stepanov's STL project was formed and the project prior to being proposed to committee was named after the lab.)
The other apocryphal derivation of STL I have heard is "STepanov and Lee".
freestanding requires almost all std library. Please note that -fno-rtti and -fno-exceptions are non-conformant, c++ standard does not permit either.
Also, such std:: members as initializer_list, type_info etc are directly baked into compiler and stuff in header must exactly match internals — making std library a part of compiler implementation
have you actually read the page you linked to? None of the standard containers is there, nor <iostream> or <algorithm>. <string> is there but marked as partial.
If anything, I would expect more headers like <algorithm>, <span>, <array> etc to be there as they mostly do not require any heap allocation nor exceptions for most of their functionality. And in fact they are available with GCC.
The only bit I'm surprised is that coroutine is there, as they normally allocate, but I guess it has full support for custom allocators, so it can be made to work on freestanding.
I did not know that.
My understanding was that C does not require standard library functions to be present in freestanding. The Linux kernel famously does not build in freestanding mode, since then GCC can't reason about the standard library functions which they want. This means that they need to implement stuff like memcpy and pass -fno-builtin.
Does that mean that freestanding C++ requires the C++ standard library, but not the C standard library? How does that work?
The "abstract machine" C++ assumes in the standard is itself a deeply puzzling construct. Luckily, compiler authors seem much more pragmatic and reasonable, I do not fear -fno-exceptions dissapearing suddenly, or code that accesses mmapped data becoming invalid because it didn't use start_lifetime_as
> freestanding C++ requires the C++ standard library, but not the C standard library
is true?
> The "abstract machine" C++ assumes in the standard is itself a deeply puzzling construct.
I find the abstract machine to be quite a neat abstraction, but I am also more of a C guy.
https://eel.is/c++draft/compliance
One of required headers in freestanding, <cstdlib>, is labelled "C standard library", but it is not <stdlib.h> Something similar with other <csomething> headers.
This kinda implies C library is required, if I read it correctly, but maybe someone else can correct me: https://eel.is/c++draft/library.c
> The ISO C standard defines (in clause 4) two classes of conforming implementation. A "conforming hosted implementation" supports the whole standard including all the library facilities; a "conforming freestanding implementation" is only required to provide certain library facilities: those in '<float.h>', '<limits.h>', '<stdarg.h>', and '<stddef.h>'; since AMD1, also those in '<iso646.h>'; since C99, also those in '<stdbool.h>' and '<stdint.h>'; and since C11, also those in '<stdalign.h>' and '<stdnoreturn.h>'. In addition, complex types, added in C99, are not required for freestanding implementations.
> The standard also defines two environments for programs, a "freestanding environment", required of all implementations and which may not have library facilities beyond those required of freestanding implementations, where the handling of program startup and termination are implementation-defined; and a "hosted environment", which is not required, in which all the library facilities are provided and startup is through a function 'int main (void)' or 'int main (int, char *[])'. An OS kernel is an example of a program running in a freestanding environment; a program using the facilities of an operating system is an example of a program running in a hosted environment.
> GCC aims towards being usable as a conforming freestanding implementation, or as the compiler for a conforming hosted implementation. By default, it acts as the compiler for a hosted implementation, defining '__STDC_HOSTED__' as '1' and presuming that when the names of ISO C functions are used, they have the semantics defined in the standard. To make it act as a conforming freestanding implementation for a freestanding environment, use the option '-ffreestanding'; it then defines '__STDC_HOSTED__' to '0' and does not make assumptions about the meanings of function names from the standard library, with exceptions noted below. To build an OS kernel, you may well still need to make your own arrangements for linking and startup. *Note Options Controlling C Dialect: C Dialect Options.
> GCC does not provide the library facilities required only of hosted implementations, nor yet all the facilities required by C99 of freestanding implementations on all platforms. To use the facilities of a hosted environment, you need to find them elsewhere (for example, in the GNU C library). *Note Standard Libraries: Standard Libraries.
> Most of the compiler support routines used by GCC are present in 'libgcc', but there are a few exceptions. GCC requires the freestanding environment provide 'memcpy', 'memmove', 'memset' and 'memcmp'. Finally, if '__builtin_trap' is used, and the target does not implement the 'trap' pattern, then GCC emits a call to 'abort'.
So the last paragraph means that my remark about the Linux kernel might be wrong.
So the required headers are all about basic constants for types, the types themselves (bool), and basic language features like stdarg, iso646 or stdalign. Sounds sensible to me. Not sure what C++ does with that.
This matches with https://en.cppreference.com/w/c/language/conformance.html . Since C23, stdlib.h is also required for dynamic allocation, string conversions and some other things.
This also actually matches the links provided by you. In https://eel.is/c++draft/cstdlib.syn you see that not all declarations are actually marked for freestanding implementations.