Jonathan Mee - 4 months ago 28

C++ Question

I want to know if there is something in the standard, like a

`#define`

`numeric_limits`

For example, if I have some floating point type the largest value of which is: 1234.567. I'd like something defined in the standard that would tell me 4 for that type.

Is there an option to me doing this?

`template <typename T>`

constexpr auto integral_digits10 = static_cast<int>(log10(numeric_limits<T>::max())) + 1;

Answer

The value that you are looking for is `max_exponent10`

which:

Is the largest positive number n such that 10

^{n}is a representable finite value of the floating-point type

Because of this relationship:

log_{10}*x* = *n*

10^{n} = *x*

Your calculation is doing, is finding *n* the way the first equation works:

```
log10(numeric_limits<T>::max())
```

The definition of `max_exponent10`

is explaining that it is using a 10^{n + 1} would be larger than `numeric_limits<T>::max()`

but 10^{n} is less than or equal to `numeric_limits<T>::max()`

. **So numeric_limits<T>::max_exponent10 is what you're looking for.**

Note that you will still need the `+ 1`

as in your example, to account for the 1's place. (Because log_{10}1 = 0) So your the number of 10-based digits required to represent `numeric_limits<T>::max()`

will be:

```
numeric_limits<T>::max_exponent10 + 1
```

If you feel like validating that by hand you can check here: