Ensuring C++ doubles are 64 bits

问题:

In my C++ program, I need to pull a 64 bit float from an external byte sequence. Is there some way to ensure, at compile-time, that doubles are 64 bits? Is there some other type I should use to store the data instead?

Edit: If you're reading this and actually looking for a way to ensure storage in the IEEE 754 format, have a look at Adam Rosenfield's answer below.

回答1:

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>.

回答2:

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++.

回答3:

Check std::numeric_limits< double >::is_iec559 if you need to know whether your C++ implementation supports standard doubles. This guarantees not only that the total number of bits is 64, but also the size and position of all fields inside the double.

回答4:

I don't think you should focus on the "raw size" of your double (which is generally 80 bit, not 64 bit), but rather on its precision.

Thanks to numeric_limits::digits10 this is fairly easy.

回答5:

You can use the Boost static assertions to do this. Look at the Use at namespace scope example.

回答6:

The solution without boost is to define the array like so

char a[ 8 == sizeof(double) ];

If the double is not 64 bits then the code will looks like

char a[0];

which is an compile time error. Just put the appropriate comment near this instruction.

回答7:

See this post for a similar problem and a non-boost compile time assertion called CCASSERT.