Is new C++ backward compatible

I haven't coded in C++ for years. I recently discovered that during those years it has changed quite dramatically. I'm not sure I like the changes, but that's another discussion.

I still have some C++ code knocking around my har开发者_如何转开发d drive. If I got it out and tried to compile it with a nice new C++ compiler, say the latest version of g++, would it compile? Without warnings (assuming it compiled without warnings before)?

I did get to mess around with a little VC++ 2010 recently and found some things I expected to work just not working, and got different messages depending on context when I tried to use NULL. But in one part of that code I used NULL without even a warning.

It depends. Usually newer compilers are more standard-compliant, so many constructs that compiled on earlier compilers don't compile now without fixing. For example:

 for( int i = 0; ... );
 i++;

compiled in Visual C++ 7, but not in Visual C++ 9.

In general, yes it is backwards compatible. However, the devil is in the details. You likely will find things where conventions change, or specific libraries fall into or out of use.

NULL is a macro - prefer to use 0 (or nullptr in C++0x).

Not sure just how old your code is, but Visual C++ v6 suffers from limitations that result in code that simply won't compile on newer compilers. VS2005 and up are a lot better (in the sense of more correct wrt the contemporaneous C++ standard).

I would not expect it to be a huge amount of work to get old code compiled, though. I've done some pretty major ports from VC6 -> VS2005 and for the most part it's a matter of hours, not days. Perhaps once the culture shock has worn off this will not seem so daunting. Really, VC++ 10 is very nice.

This depends on what you're comparing to.

• Visual Studio 2010 partially implements the upcoming C++0x draft (recent versions of GCC also implement a subset of this draft, which is expected to be standardized next year)
• C++98/C++03 was the first standardized version of C++, and is still the official one (as C++0x is still only a draft)
• and of course, there are the dialects from before the language was standardized

C++0x is pretty much backwards compatible with C++03/98. There may be a couple of obscure corner cases that have changed, but you are unlikely to encounter them. However, a lot of changes occurred when the language was first standardized, meaning that C++98 isn't entirely (but nearly) compatible with pre-standard C++.

But more likely, what you're into isn't a question of C++ backwards compatibility, but simply that compilers have gotten stricter. They have become much better at following the standard, and no longer allow many non-standard tricks that were common earlier. Most likely, your old code was never valid C++, but worked because compilers used to deviate more from the standard.

The language itself hasn't changed since it was standardized in 1998. Idioms have changed, and compilers have both improved their support for the standard and become stricter about non-standard code, but any standard-conforming C++ code that compiled in the past should still compile today. Code that relies on non-standard compiler features or quirks might not work, but compilers typically offer command-line options to make them accept non-standard code that used to be accepted by previous versions of the same compiler.

NULL is a macro that's defined in <cstddef>. Other standard headers may include that header as an implementation detail, so it's often possible to use NULL without explicitly including <cstddef>, but relying on that has always been non-portable. It's OK to use NULL as long as you include its header, though just using 0 is preferred in idiomatic C++.

All version of C++ should be backwards compatible.

Although there might be some unusual cases where there could be a problem, e.g. noexcept on destructos in C++0x (although this has not yet been decided).

Newer C++ standards come to clarify things, then take decissions on which usage is "more correct" or accepted, so I would expect warnings. Some other decisions change what can be done or not, so I would also expect errors. I've come across the same situation, and I had to tweak the code for it to compile. It was not a great deal, but this was taking into account my knowledge of C++. In general, you should encounter not great problems.

There are also other things. For example, not all compilers implemented the whole rules of the C++ standard, or they had bugs. When you get used with a compiler, some of those errors or missing features may pass unnoticed to your compiler, but may cause errors in future versions of the same compiler.

That's the major reason why we have standards. You don't have to worry about compatibility, you will just tell the compiler to compile the code as C++98 (or 2003).

MSVC is unfortunately very bad at supporting C++ standards. It is slowly getting better (and that's why old code doesn't compile, since it shouldn't compile in the first place).

Well, I know a lot of our old VS6 code started throwing tons of warnings when we upgraded to VS 2005 (with full warnings on of course, as everyone should work). Most were good things though, warning of potential information loss, warning that some types may be 64-bit instead of 32-bit like old code might expect, etc.

For your specific example of NULL, that wasn't even really standard C back in the day. Every compiler just had a #define for it that set it to 0. These days it is generally considered more correct (and clear) to just use 0.

If you have used old versions of various libraries such as Boost, expect some problems.

Slightly random, but the keyword export is being removed from the standard. So previously standards compliant code the used export would now be illegal. Of course, very few compilers even began implementing that keyword.

Very similar to sharptooth's answer, there are bits of older C and C++ code that will need /ZC:forScope- set (i.e. don't force conformance in for loop scope). e.g.

int myfunc(int x, int y, int z)
{
  int j;

  for (int i=0; i <= 10; i++)
  {
    if  (f(i) == 0)
      break;
  }
  j = i;
  for (i=0; i <= 10; i++)
  {
    if  (g(i) == 0)
      break;
  }
  if (i > j)
    return j;
  return i;
}

This type of thing is quite common in much older code, where bytes cost money and variable re-use was common place.