Are C++ applications cross-platform?
Source code compatible. If I compile the source code, will it run everywhere?
API/ABI compatibility. Does the OS provide the interface to its components in a way that the code will understand?
Binary compatibility. Is the code capable of running on the target host?
Source code compatible
C++ is a standard which defines how structures, memory, files can be read and written.
#include <iostream>
int main( int argc, char ** argv )
{
std::cout << "Hello World" << std::endl;
}
Code written to process data (e.g. grep, awk, sed) is generally cross-platform.
When you want to interact with the user, modern operating systems have a GUI, these are not cross-platform, and cause code to be written for a specific platform.
Libraries such as qt or wxWidgets have implementations for multiple platforms and allow you to program for qt instead of Windows or iOS, with the result being compatible with both.
The problem with these anonymizing libraries, is they take some of the specific benefits of platform X away in the interest of uniformity across platforms.
Examples of this would be on Windows using the WaitForMultipleObjects function, which allows you to wait for different types of events to occur, or the fork function on UNIX, which allows two copies of your process to be running with significant shared state. In the UI, the forms look and behave slightly different (e.g. color-picker, maximize, minimize, the ability to track mouse outside of your window, the behaviour of gestures).
When the work you need to be done is important to you, then you may end up wanting to write platform specific code to leverage the advantages of the specific application.
The C library sqlite is broadly cross-platform code, but its low-level IO is platform specific, so it can make guarantees for database integrity (that the data is really written to disk).
So libraries such as Qt do work, they may produce results which are unsatisfactory, and you end up having to write native code.
API/ABI compatibility
Different releases of UNIX and Windows have some form of compatibility with each other. These allow a binary built for one version of the OS to run on other versions of the OS.
In UNIX the choice of your build machine defines the compatibility. The lowest OS revision you wish to support should be your build machine, and it will produce binaries compatible with subsequent minor versions until they make a breaking change (deprecate a library).
On Windows and Mac OS X, you choose an SDK which allows you to target a set of OS's with the same issues with breaking changes.
On Linux, each kernel revision is ABI incompatible with any other, and kernel modules need to be re-compiled for each kernel revision.
Binary compatibility
This is the ability of the CPU to understand the code. This is more complex than you might think, as the x64 chips, can be capable (depending on OS support) of running x86 code.
Typically a C++ program is packaged inside a container (PE executable, ELF format) which is used by the operating system to unpack the sections of code and data and to load libraries. This makes the final program have both binary (type of code) and API (format of the container) forms of incompatibilities.
Also today if you compile a x86 Windows Application (targeting Windows 7 on Visual Studio 2015), then the code may fail to execute if the processor does not have SSE2 instructions (about 10 years old CPU).
Finally when Apple changed from PowerPC to x86, they provided an emulation layer which allowed the old PowerPC code to run in an emulator on the x86 platform.
So in general binary incompatibility is a murky area. It would be possible to produce an OS which identified invalid instructions (e.g. SSE2) and in the fault, emulated the behaviour, this could be updated as new features come out, and keeps your code running, even though it is binary incompatible.
Even if your platform is incapable of running a form of instruction set, it could be emulated and behave compatibly.
Yes. No. Maybe. What is cross-platform C++ code? Cross-platform C++ code is such a code that can be compiled under different operation systems without the need to be modified.
That means, if you explicitly use any platform-dependant headers, your code is no longer cross-platform. Qt solves this problem in the following way: they provide wrappers for everything that is platform-specific. For example, imagine that you are using QFile to open/read/write a file. Your code looks like
QFile file(filename);
file.open(QFile::ReadOnly);
//other stuff
You can compile this code under any OS as long as you have a suitable compiler and Qt libraries for that OS. The code hidden under QFile will use the OS-appropriate file-handling functions, but that shouldn't concern you.
Also, if you only use the standard library, your code can be compiled anywhere where a C++ compiler is present.
The already-compiled applications, however, are not cross-platform in a way that, say, Java applications are - for instance, you can't compile an app for Windows and then run in in Linux, you will have to recompile your code under Linux instead.
You can achieve that your source compiles on various platforms, giving you various binaries from the same source base.
This is not "compile once, run anywhere with an appropriate VM" as Java or C# do it, but "write once, compile anywhere with an appropriate environment" the way C has done it all the time.
Since the standard library does not provide everything you might need, you have to look for third-party libraries to provide that functionality. Certain frameworks -- like Boost, Qt, GTK+, wxWidgets etc. -- can provide that. Since these frameworks are written in a way that they compile on different platforms, you can achieve cross-platform functionality in the aforementioned sense.
There are various things to be aware of if you want your C++ code to be cross-platform.
The obvious thing is source that makes assumption on data types. Your long might be 32bit here and 64bit there. Data type alignment and struct padding might differ. There are ways to "play it safe" here, like size_t / size_type / uint16_t typedefs etc., and ways to get it wrong, like wchar_t and std::wstring. It takes discipline and some experience to "get it right".
Not all compilers are created equal. You cannot use all the latest C++ language features, or use libraries that rely on those features, if you require your source to compile on other C++ compilers. Check the compatibility chart first.
Another thing is endianess. Just one example, when you're writing a stream of integers to file on one platform (say, x86 or x86_64), and then read it back again on a different platform (say, POWER), you can run into problems. Why would you write integers to file? Well, UTF-16 is integers... again, discipline and some experience go a long way toward making this rather painless.
Once you've checked all those boxes, you need to make sure of the availability of the libraries you base your code on. While std:: is safe (but see "not all compilers are created equal" above), something as innocent as boost:: can become a problem if you're looking beyond the mainstream. (I helped the Boost guys to fix one or two showstoppers regarding AIX / Visual Age in past years simply because they didn't have access to that platform for testing new releases...)
Oh, and watch out for the various licensing schemes out there. Some frameworks that improve your cross-platform capabilities -- like Qt or Cygwin -- have their strings attached. That is not to say they are not a big help in the right circumstances, just that you need to be aware of copyleft / proprietary licensing requirements.
All that being said, there is Wine ("Wine is not emulation"), which makes executables compiled for Windows run on a variety of Unix-alike systems (Linux, OS X, *BSD, Solaris). There are certain limits to its capabilities, but it's getting better all the time.
Standard C++ is cross platform in the "write once, compile anywhere" sense, but not in the "compile once, run anywhere" sense.
That means that if you write a program in standard C++, you can compile and then run it on any target environment that has a standard conforming implementation of C++.
You can however not compile your program on your machine, ship the binary and then expect it to work on other targets. (At least not in general. One can of course distribute binaries from C++ code under certain conditions, but those depend on the actual target. This is a broad field.)
Of course, if you use extra, non-standard features like gcc's variable length arrays or third party libraries, you can only compile on systems that provide those extensions and libraries.
Some libraries like Qt and Boost are available on many systems (those two on Linux, Mac and Windows at least I believe), so your code will stay cross platform if you use those.