Exact decimal datatype for C++?

Question

PHP has a decimal type, which doesn't have the "inaccuracy" of floats and doubles, so that 2.5 + 2.5 = 5 and not 4.999999999978325 or something like that.

So I wonder if there is such a data type implementation for C or C++?

Answer 1

What you're asking is anti-physics.

What phyton (and C++ as well) do is cut off the inaccuracy by rounding the result at the time to print it out, by reducing the number of significant digits:

double x = 2.5;
x += 2.5;
std::cout << x << std::endl;

just makes x to be printed with 6 decimal digit precision (while x itself has more than 12), and will be rounded as 5, cutting away the imprecision.

Alternatives are not using floating point at all, and implement data types that do just integer "scaled" arithmetic: 25/10 + 25/10 = 50/10;

Note, however, that this will reduce the upper limit represented by each integer type. The gain in precision (and exactness) will result in a faster reach to overflow.

Rational arithmetic is also possible (each number is represented by a "numarator" and a "denominator"), with no precision loss against divisions, (that -in fact- are not done unless exact) but again, with increasing values as the number of operation grows (the less "rational" is the number, the bigger are the numerator and denominator) with greater risk of overflow.

In other word the fact a finite number of bits is used (no matter how organized) will always result in a loss you have to pay on the side of small on on the side of big numbers.