Ensuring C++ doubles are 64 bits

An improvement on the other answers (which assume a char is 8-bits, the standard does not guarantee this..). Would be like this:

char a[sizeof(double) * CHAR_BIT == 64];

or

BOOST_STATIC_ASSERT(sizeof(double) * CHAR_BIT == 64);

You can find CHAR_BIT defined in <limits.h> or <climits>.

In C99, you can just check if the preprocessor symbol __STDC_IEC_559__ is defined. If it is, then you are guaranteed that a double will be an 8-byte value represented with IEEE 754 (also known as IEC 60559) format. See the C99 standard, Annex F. I'm not sure if this symbol is available in C++, though.

#ifndef __STDC_IEC_559__
#error "Requires IEEE 754 floating point!"
#endif

Alternatively, you can check the predefined constants __DBL_DIG__ (should be 15), __DBL_MANT_DIG__ (should be 53), __DBL_MAX_10_EXP__ (should be 308), __DBL_MAX_EXP__ (should be 1024), __DBL_MIN_10_EXP__ (should be -307), and __DBL_MIN_EXP__ (should be -1021). These should be available in all flavors of C and C++.