That prints a full list of all pre-processor symbols defined by default in your g++ compiler. I noticed a bunch of things I didn't know were normally compiler defined. Just some samples from my system (Linux Mint 15, 64-bit):
#define __INT_LEAST32_TYPE__ int
#define __INT32_TYPE__ int
#define __UINT64_TYPE__ long unsigned int
#define __SIZE_TYPE__ long unsigned int
#define __DBL_EPSILON__ double(2.22044604925031308085e-16L)
#define __DBL_MIN_EXP__ (-1021)
#define __FLT_MIN__ 1.17549435082228750797e-38F
#define __VERSION__ "4.7.3"
#define __SIZEOF_SIZE_T__ 8
#define __SIZEOF_INT__ 4
#define __SIZEOF_POINTER__ 8
#define __SIZE_TYPE__ long unsigned int
#define __x86_64__ 1
#define __LP64__ 1
#define __FLT_HAS_INFINITY__ 1