The FDIS has a section for incompatibilities, at appendix C.2 "C++ and ISO C++ 2003".

Summary, paraphrasing the FDIS here, to make it (better) suitable as a SO answer. I added some examples of my own to illustrate the differences.

There are a few library-related incompatibilities where I don't exactly know the implications of, so I leave those for others to elaborate on.

Core language

#define u8 "abc" const char *s = u8"def"; // Previously "abcdef", now "def"

#define _x "there" "hello"_x // now a user-defined-string-literal. Previously, expanded _x .

New keywords: alignas, alignof, char16_t, char32_t, constexpr, decltype, noexcept, nullptr, static_assert, and thread_local

Certain integer literals larger than can be represented by long could change from an unsigned integer type to signed long long.

Valid C++ 2003 code that uses integer division rounds the result toward 0 or toward negative infinity, whereas C++0x always rounds the result toward 0.

(admittedly not really a compatibility problem for most people).

Valid C++ 2003 code that uses the keyword auto as a storage class specifier may be invalid in C++0x.

Narrowing conversions cause incompatibilities with C++03. For example, the following code is valid in C++ 2003 but invalid in this International Standard because double to int is a narrowing conversion:

int x[] = { 2.0 };

Implicitly-declared special member functions are deﬁned as deleted when the implicit definition would have been ill-formed. A valid C++ 2003 program that uses one of these special member functions in a context where the definition is not required (e.g., in an expresion that is not potentially evaluated) becomes ill-formed.

Example by me:

struct A { private: A(); }; struct B : A { }; int main() { sizeof B(); /* valid in C++03, invalid in C++0x */ }

Such sizeof tricks have been used by some SFINAE, and needs to be changed now :)

User-declared destructors have an implicit exception specification.

Example by me:

struct A { ~A() { throw "foo"; } }; int main() { try { A a; } catch(...) { } }

This code calls terminate in C++0x, but does not in C++03. Because the implicit exception specification of A::~A in C++0x is noexcept(true) .

A valid C++ 2003 declaration containing export is ill-formed in C++0x.

A valid C++ 2003 expression containing > followed immediately by another > may now be treated as closing two templates.

In C++03, >> would always be the shift-operator token.

Allow dependent calls of functions with internal linkage.

Example by me:

static void f(int) { } void f(long) { } template<typename T> void g(T t) { f(t); } int main() { g(0); }

In C++03, this calls f(long) , but in C++0x, this calls f(int) . It should be noted that in both C++03 and C++0x, the following calls f(B) (the instantiation context still only considers extern linkage declarations).

struct B { }; struct A : B { }; template<typename T> void g(T t) { f(t); } static void f(A) { } void f(B) { } int main() { A a; g(a); }

The better matching f(A) is not taken, because it does not have external linkage.

Library changes

Valid C++ 2003 code that uses any identifiers added to the C++ standard library of C++0x may fail to compile or produce different results in This International Standard.

Valid C++ 2003 code that #includes headers with names of new C++0x standard library headers may be invalid in this International Standard.

Valid C++ 2003 code that has been compiled expecting swap to be in <algorithm> may have to instead include <utility>

The global namespace posix is now reserved for standardization.