Difference in digits10 between GCC and MSVC

问题

I have the following code:

#include <iostream>
#include <limits>

int main()
{
   std::cout << std::numeric_limits<unsigned long long>::digits10 << std::endl;
   return 0;
}

• GCC 4.4 returns 19
• MS VS 9.0 returns 18

Can someone please explain Why is there a difference between the two? I would have expected such a constant would be the same regardless of the compiler.

回答1:

If Visual C++ 2008 returns 18 for std::numeric_limits<unsigned long long>::digits10, it is a bug (I don't have Visual C++ 2008 installed to verify the described behavior).

In Visual C++ (at least for 32-bit and 64-bit Windows), unsigned long long is a 64-bit unsigned integer type and is capable of representing all of the integers between zero and 18,446,744,073,709,551,615 (2⁶⁴ - 1).

Therefore, the correct value for digits10 here is 19 because an unsigned long long can represent 9,999,999,999,999,999,999 (19 digits) but cannot represent 99,999,999,999,999,999,999 (20 digits). That is, it can represent every 19 digit number but not every 20 digit number.

When compiled with Visual C++ 2010, your program prints the expected 19.

回答2:

numeric_limits::digits10 specifies the number of decimal digits to the left of the decimal point that can be represented without a loss of precision. So, I guess it will differ from compiler to compiler depending on their implementation detail.

回答3:

I would have expected such a constant would be the same regardless of the compiler.

This is not correct. A compiler can implement any value as long as it comforms to the standards. For example some weird compilers on a 32 or 64-bit computer may have CHAR_BIT = 9 and unsigned long long wouldn't be 64 bit any more, or it may use different 1's complement or some other number encodings, so the result may vary between compilers.

I've just check and VS 2008 still returns 19. Maybe because of one of the hotfixes when updating. VS2008 result http://imageshack.com/a/img826/6228/6qo4.png

来源：https://stackoverflow.com/questions/5925951/difference-in-digits10-between-gcc-and-msvc