问题
For a language like C++ the existence of a standard is a must. And good compilers try their best (well, most of the good compilers, at least) to comply. Many compilers have language extensions, some of which are allowed by the standard, some of which are not. Of the latter kind 2 examples:
gcc's typeof
microsoft's compilers allow a pure virtual function declaration to have both a pure-specifier(=0) and a definition (which is prohibited by the standard - let's not discuss why, that's another topic:)
(there are many other examples)
Both examples are useful in the following sense: example1 is a very useful feature which will be available in c++0x under a different name. example2 is also useful, and microsoft has decided not to respect the ban that made no sense.
And I am grateful that compilers provide language extensions that help us developers in our routine. But here's a question: shouldn't there be an option which, when set, mandates that the compiler be as standards compliant as it can, no matter whether they agree with the standard or not. For example visual studio has such an option, which is called disable language extensions. But hey, they still allow example2.
I want everyone to understand my question correctly. It is a GREAT thing that MSVC allows example2, and I would very much like that feature to be in the standard. It doesn't break any compliant code, it does nothing bad. It just isn't standard.
Would you like that microsoft disable example2 when disable language extensions is set to true? Note that the words microsoft, example2, etc. are placeholders :) Why?
Again, just to make sure. The crucial point is: Should a compiler bother to provide a compliant version (optionally set in the settings)(in its limits, e.g. I am not talking about export) for a certain feature when they provide a better alternative that is not standard and is perhaps even a superset of the standard, thus not breaking anything.
回答1:
Standards compliance is important for the fundamental reason that it makes your code easier to maintain. This manifests in a number of ways:
Porting from one version of a compiler to another. I once had to post a 1.2 million-LOC app from VC6 to VC9. VC6 was notorious for being horribly non-Compliant, even when it was new. It allowed non-compliant code even on the highest warning levels that the new compiler rejected at the lowest. If the code had been written in a more compliant way in the first place, this project wouldn't (shouldn't)have taken 3 months.
Porting from one platform to another. As you say, the current MS compilers have language extensions. Some of these are shared by compilers on other platforms, some are not. Even if they are shared, the behavior may be subtly different. Writing compliant code, rather that using these extensions, makes your code correct from the word go. "Porting" becomes simply pulling the tree down and doing a rebuild, rather than digging through the bowels of your app trying to figure out why 3 bits are wrong.
C++ is defined by the standard. The extensions used by compilers changes the language. New programmers coming online who know C++ but not the dialect your compiler uses will get up to speed more quickly if you write to Standard C++, rather than the dialect that your compiler supports.
回答2:
First, a reply to several comments. The MS VC extension in question is like this:
struct extension {
virtual void func() = 0 { /* function body here */ }
};
The standard allows you to implement the pure virtual function, but not "in place" like this, so you have to write it something like this instead:
struct standard {
virtual void func() = 0;
};
void standard::func() { ; }
As to the original question, yes, I think it's a good idea for the compiler to have a mode in which it follows (and enforces) the standard as accurately as possible. While most compilers have that, the result isn't necessarily as accurate a representation of the standard as you/I would like.
At least IMO, about the only answer to this is for people who care about portability to have (and use) at least a couple of compilers on a regular basis. For C++, one of those should be based on the EDG front-end; I believe it has substantially better conformance than most of the others. If you're using Intel's compiler on a regular basis anyway, that's fine. Otherwise, I'd recommend getting a copy of Comeau C++; it's only $50, and it's the closest thing to a "reference" available. You can also use Comeau online, but if you use it on a regular basis, it's worth getting a copy of your own.
Not to sound like an EDG or Comeau shill or anything, but even if you don't care much about portability, I'd recommend getting a copy anyway -- it generally produces excellent error messages. Its clean, clear error messages (all by themselves) have saved enough time over the years to pay for the compiler several times over.
Edit: Looking at this again, some of the advice is looking pretty dated, especially the recommendation for EDG/Comeau. In the three years since I originally wrote this, Clang has progressed from purely experimental to being quite reasonable for production use. Likewise, the gcc maintainers have (IMO) made great strides in conformance as well.
During the same time, Comeau hasn't released a single new version of their compiler, and there's been a new release of the C++ standard. As a result, Comeau is now fairly out of date with respect to the current standard (and the situation seems to be getting worse, not better -- the committee has already approved a committee draft of a new standard that is likely to become C++14).
As such, although I recommended Comeau at that time, I'd have difficulty (at best) doing so today. Fortunately, most of the advantages it provided are now available in more mainstream compilers -- both Clang and gcc have improved compliance (substantially) as outlined above, and their error messages have improved considerably as well (Clang has placed a strong emphasis on better error messages, almost from its inception).
Bottom line: I'd still recommend having at least two compilers installed and available, but today I'd probably choose different compilers than I did when I originally wrote this answer.
回答3:
"Not breaking anything" is such a slippery slope in the long run, that it's better to avoid it altogether. My company's main product outlived several generations of compilers (first written in 1991, with RW), and combing through compiler extensions and quiet standards violations whenever it was the time to migrate to a newer dev system took a lot of effort.
But as long as there's an option to turn off or at least warn about 'non-standard extension', I'm good with it. 34, 70, 6.
回答4:
I would certainly want an option that disables language extensions to disable all language extensions. Why?
- All options should do what they say they do.
- Some people need to develop portable code, requiring a compiler that only accepts the standard form of the language.
"Better" is a subjective word. Language extensions are useful for some developers, but make things more difficult for others.
回答5:
I think that it's critical that a compiler provide a standards-only mode if it wants to be the primary one used while developing. All compilers should, of course, compile standards compliant code, but it's not critical they they don't extend if they don't think of themselves as the primary compiler -- for example, a cross-compiler, or a compiler for a less popular platform that is nearly always ported to, rather than targeted.
Extensions are fine for any compiler, but it would be nice if I had to turn them on if I want them. By default, I'd prefer a standards-only compiler.
So, given that, I expect MSVC to be standards-only by default. The same with gcc++.
Stats: 40, 90, 15
回答6:
I think standards compliance is very important.
I always consider source code is more for the human readers than for the machine(s). So, to communicate programmer's intention to the reader, abiding the standard is like speaking a language of lowest common denominator.
Both at home and work, I use g++, and I have aliased it with the following flags for strict standard compliance.
-Wall -Wextra -ansi -pedantic -std=c++98
Check out this page on Strict ANSI/ISO
I am not a standards expert, but this has served me well. I have written STL-style container libraries which run as-is on different platforms, e.g. 32-bit linux, 64-bit linux, 32-bit solaris, and 32-bit embedded OSE.
回答7:
Consider indicators on cars (known as "turn signals" in some jurisdictions); they are a reliable way to determine which direction someone's going to turn off a roundabout... until just one person doesn't use them at all. Then the whole system breaks down.
It didn't "hurt anyone" or obviously "break anything" in IE when they allowed document.someId to be used as a shortcut for document.getElementById('someId').... however, it did spawn an entire generation of coders and even books that consequently thought it was okay and right, because "it works". Then, suddenly, the ten million resulting websites were entirely non-portable.
Standards are important for interoperability, and if you don't follow them then there's little point in having them at all.
Standards-compliance hounds may get hated for "pedanticism" but, really, until everybody follows suit you're going to have portability and compatibility problems for ever.
回答8:
How important standards-compliance is depends on what you are trying to achieve.
If you are writing a program that will never be ported outside of its current environment (especially a program that you're not planning to develop/support for a long time) then it's not very important. Whatever works, works.
If you need your program to remain relevant for a long time, and be easily portable to different environments, than you will want it to be standards compliant, since that's the only way to (more or less) guarantee that it will work everywhere.
The trick, of course, is figuring out which situation you are actually in. It's very common to start a program thinking it is a short-term hack, and later on find that it's so useful that you're still developing/maintaining it years later. In that situation your life will be much less unpleasant if you didn't make any short-sighted design decisions at the beginning of the program's lifetime.
来源：https://stackoverflow.com/questions/3916608/how-important-is-standards-compliance