LLVM Compiler 3.0 (Xcode 4.2) or later

Integers up to 64 bits: 0
Floating-point up to long double: 0.0
Pointers: nil
Structs: {0}
Any _Complex type: {0, 0}

Notes

C++ objects returned by value are initialized to {0}, even if the type has a default constructor that does something else. This may be fixed in the future.
Struct return is undefined if you call objc_msgSend_stret() directly.
Struct return is undefined if you use an older compiler.
Floating-point return is undefined on Mac OS X 10.4 and earlier on Power PC.
_Complex long double return is undefined if you use an older compiler.