Why is it important for C / C++ Code to be compilable on different compilers?

I'm interested in different aspects of portability (as you can see when browsing my other questions), so I read a lot about it. Quite often, I read/hear that Code should be written in a way that makes it compilable on different compilers.

Without any real life experience with gcc / g++, it seems to me that it supports every major platform one can imagine, so Code that compiles on g++ can run on almost any system. So why would someone bother to have his code run on the MS Compiler, the Intel compiler and others?

I can think of some reasons, too. As the FAQ suggest, I'll try to post them as an answer, opposed to including them into my own question.

Edit: Conclusion

You people got me completely convinced that there are several good reasons to support multiple compilers. There are so many reasons that it was hard to choose an answer to be the accepted one. The most important reasons for me:

• Contributors are much more likely to work an my project or just use it if they can use the compiler of their choice
• Being compilable everywhere, being usable with future compile开发者_运维问答rs and tools, and adhering to the standards are enforcing each other, so it's a good idea

On the other hand, I still believe that there are other things which are more important, and now I know that sometimes it isn't important at all.

And last of all, there was no single answer that could convince me not to choose GCC as the primary or default compiler for my project.

Some reasons from the top of my head:

1) To avoid being locked with a single compiler vendor (open source or not).

2) Compiling code with different compilers is likely to discover more errors: warnings are different and different compilers support the Standard to a different degree.

It is good to be compilable on MSVC, because some people may have projects that they build in MSVC that they want to link your code into, without having to set up an entirely different build system.

It is good to be compilable under the Intel compiler, because it frequently compiles faster code.

It is good to be compilable under Clang, because it can give better error messages and provide a better development experience, and it is an easier project to work on than GCC and so may gain additional benefits in the future.

In general, it is good to keep your options open, because there is no one compiler that fits all needs. GCC is a good compiler, and is great for most purposes, but you sometimes need something else.

And even if you're usually only going to be compiling under GCC, making sure your code compiles under other compilers is also likely to help find problems that could prevent your code from working with past and future versions of GCC, for instance, if there's something that GCC is less strict about now, but later adds checks for, another compiler may catch in advance, helping you keep your code cleaner. I've found this helpful in the reverse case, where GCC caught more potential problems with warnings than MSVC did (MSVC is the only compiler we needed to support, as we were only shipping on Windows, but we did a partial port to the Mac under GCC in our free time), which allowed me to produce cleaner code than I would have otherwise.

Portability. If you want your code to be accessible by the maximum number of people possible, you have to make it work on the widest range of possible compilers. It the same idea as make a web site run on browsers other than IE.

Some of it is political. Companies have standards, people have favorite tools etc. Telling someone that they should use X, really puts some people off, and makes it really inaccessible to others.

Nemanja brings up a good point too, targeting for a certain compiler locks you into to using it. In the Open Source world, this might not be as big of a problem (although people could just stop developing on it and it becomes obsolete), but what if the company you buy it from discontinues the product, or goes out of business?

For most languages I care less about portability and more about conforming to international standards or accepted language definitions, from which properties portability is likely to follow. For C, however, portability is a useful idea, because it is very hard to write a program that is "strictly conforming" to the standard. (Why? Because the standards committees felt it necessary to grandfather some existing practice, including giving compilers some freedom you might not like them to have.)

So why try to conform to a standard or make your code acceptable to multiple compilers as opposed to simply writing whatever gcc (or your other favorite compiler) happens to accept?

• Likely in 2015 gcc will accept a rather different language than it does today. You would prefer not to have to rewrite your old code.

• Perhaps your code might be ported to very small devices, where the GNU toolchain is not as well supported.

• If your code compiles with any ANSI C compiler straight out of the box with no errors and no warnings, your users' lives will be easier and your software may be widely ported and used.

• Perhaps someone will invent a great new tool for analyzing C programs, refactoring C programs, improving performance of C programs, or finding bugs in C programs. We're not sure what version of C that tool will work on or what compiler it might be based on, but almost certainly the tool will accept standard C.

Of all these arguments, it's the tool argument I find most convincing. People forget that there are other things one can do with source code besides just compile it and run it. In another language, Haskell, tools for analysis and refactoring lagged far behind compilers, but people who stuck with the Haskell 98 standard have access to a lot more tools. A similar situation is likely for C: if I am going to go to the effort of building a tool, I'm going to base it on a standard with a lifetime of 10 years or so, not on a gcc version which might change before my tool is finished.

That said, lots of people can afford to ignore portability completely. For example, in 1995 I tried hard to persuade Linus Torvalds to make it possible to compile Linux with any ANSI C compiler, not just gcc. Linus had no interest whatever—I suspect he concluded that there was nothing in it for him or his project. And he was right. Having Linux compile only with gcc was a big loss for compiler researchers, but no loss for Linux. The "tool argument" didn't hold for Linux, because Linux became so wildly popular; people building analysis and bug-finding tools for C programs were willing to work with gcc because operating on Linux would allow their work to have a big impact. So if you can count on your project becoming a wild success like Linux or Mosaic/Netscape, you can afford to ignore standards :-)

If you are building for different platforms, you will end up using different compilers. Moreover, C++ compilers tend to be always slightly behind the C++ standard, which means they usually change their adherence to it as time passes. If you target the common denominator to all major compilers then the code maintenance cost will be lower.

It's very common for applications (especially open-source application) that other developers would desire to use different compilers. Some would rather be using Visual Studio with MS Compiler for development purposes. Some would rather use Intel compiler for claimed performance benefits and such.

So here are the reasons I can think of

• if speed is the biggest concern and there is special, highly optimized compiler for some platforms
• if you build a library with a C++ interface (classes and templates, instead of just functions). Because of name mangling and other stuff, the library must be compiled with the same compiler as the client code, and if the client wants to use Visual C++, he must be able to compile the lib with it
• if you want to support some very rare platform that does not have gcc support

(For me, those reasons are not significant, since I want to build a library that uses C++ internally, but has a C interface.)

Typically these are the reasons that I've found:

• cross-platform (windows, linux, mac)
• different developers doing development on different OS's (while not optimal, it does happen - testing usually takes place on the target platform only).
• Compiler companies go out of business - or stop development on that language. If you know your program compiles/runs well using another compiler, you've covered your bet.

I'm sure there are other answers as well, but these are the most common reasons I've run into so far.

Several projects use GCC/G++ as a "day-to-day" compiler for normal use, but every so often will check to make sure their code follows the standards with the Comeau C/C++ compiler. Their website looks like a nightmare, and the compiler isn't free, but it's known as possibly the most standards-compliant compiler around, and will warn you about things many compilers will silently accept or explicitly allow as a nonstandard extension (yes, I'm looking at you, Mr. I-don't-mind-and-actually-actively-support-your-efforts-to-do-pointer-arithmetic-on-void-pointers-GCC).

Compiling every so often with a compiler as strict as Comeau (or, even better, compiling with as many compilers as you can get your hands on) will let you know of errors people might experience when trying to compile your code, things your compiler allows you to do that it shouldn't, and potentially things that other compilers don't allow you to do that you should. Writing ANSI C or C++ should be an important goal for code you intend to use on multiple platforms, and using the most standards-compliant compiler around is a good way to do that.

(Disclaimer: I don't have Comeau, and don't plan on getting it, and can't get it because I'm on OS X. I do C, not C++, so I can actually know the whole language, and the average C compiler is much closer to the C standard than the average C++ compiler to the C++ standard, so it's less of an issue for me. Just wanted to put this in here because this started to look like an ad for Comeau. It should be seen more as an ad for compiling with many different compilers.)

This one of those "It depends" questions. For open source code, it's good to be portable to multiple compilers. After all having people in diverse environments build the code is sort of the point.

But for closed source, This is a lot less important. You never want to unnecessarily tie yourself to a specific compiler. But in most of the places I've worked, compiler portability didn't even make into the top 10 of things we cared about. Even if you never use anything other than standerd C/C++, switching a large code base to a new compiler is a dangerous thing to do. Compilers have bugs. Sometimes your code will have bugs that are benign on one compiler, but suddenly a problem on another.

I remember one transition, where one compiler thought this code was just fine:

for (int ii = 0; ii < n; ++ii) { /* some code */ }
for (int ii = 0; ii < y; ++ii) { /* some other code */ }

While the newer compiler complained that ii had been declared twice, so we had to go through all of our code and declare loop variables before the loop in order to switch.

One place I worked was so careful about unintended side effects of compiler switches, that they checked specific compilers into each source tree, and once the code shipped would only use that one compiler to do updates on that code base - forever.

Another place would try out a new compiler for 6 months to a year before they switched over to it.

I find gcc a slow compiler on windows (nothing to compare against under linux). So I (sometimes) want to compile my code under other compilers, just for faster development cycles.

I don't think anyone has mentioned it so far, but another reason may be access to certain platform-specific features: Many operating system vendors have special versions of GCC, or even their own home-grown (or licensed and modified) compilers. So if you want your code to run well on several platforms, you may need to choose the right compiler on each platform. Be that an embedded system, MacOS, Windows etc.

Also, speed may be an issue (both compilation speed and execution speed). Back in the PPC days, GCC produced notoriously slow code on PowerPC CPUs, so Apple put a bunch of engineers on GCC to improve that (GCC was very new for the Mac, and all other PowerPC platforms were small). Platforms that are used less may be optimized less in GCC, so using another compiler that's been written for that platform can be faster.

But as a final summary: While there is ideal value in compiling on several compilers, in practice, this is mainly interesting for cross-platform software (and open-source software, because it often gets made cross-platform fairly quickly, and contributors have it easier if they can use their compiler of choice instead of having to learn a new one). If you need to ship on one platform only, shipping and maintenance are usually much more important than investing in building on several compilers if you're only releasing the builds made with one of them. However, you will want to clearly document any deviations from the standard (GCC-isms, for instance) to make the job of porting easier, should you ever have to do it.

Both Intel compiler and llvm are faster than gcc. The real reasons to use gcc are

• Infinite hardware support (on no other compiler can you compile a lego mindstorm code on your old DEC).

• it's cheap

• best spagety optimizer in the business.