If an unsigned integer is serialized, deserializing back to the unsigned integer fails

[DUOLabs333]

When reporting a bug please include the following:

Version of Boost

1.84

Steps necessary to reproduce the problem

1. Serialize uint8_t test=8;
2. Parse it back into a value value test_value=serialize(value_from(test));
3. Try to get the number with value_to<uint8_t>(test_value)
4. See the exception Not exact.

[grisumbras]

Step 2 doesn't compile for me at all. Can you please provide an actual minimal, complete and verifiable example (MCVE)?

Although, let's consider this one:

#include <boost/json.hpp>
#include <cassert>

int main()
{
        uint8_t test = 8;
        value test_value = value( serialize(value_from(test)) ); // (1); notice explicit constructor
        auto test2 = value_to<uint8_t>(test_value); // (2)
        assert( test == test2 );
        return 0;
}

Here (1) converts the integer test to json::value, then serializes that into std::string, then assigns that string to a new json::value. This is equivalent to doing value test_value = "8";

(2) tries to convert that JSON string into a number, and predictably fails. But the error is "not a number".

Then, I guess, this is still not something that you are doing. So, please provide an MCVE.

[DUOLabs333]

This replicates it:

#include <boost/json/src.hpp>
using namespace boost::json;
#include <cassert>

int main()
{       
        object test;
        test["value"]=160;
        object test_new = parse( serialize(test) ).as_object(); // (1); notice explicit constructor
        auto test2 = value_to<char>(test_new["value"]);
        printf("%d\n",test2);
        return 0;
}

[grisumbras]

Seems like 160 is outside of the domain of your implmentation's char. This implies, that your char is signed and has the domain [-128;127].

[DUOLabs333]

Oh --- as it turns out, the previous library I used (nlohmann) silently failed, so I didn't notice that this was an issue.

[DUOLabs333]

Is there a way to get the same behavior and ignore the exception (I'm OK with the values wrapping around)?

[grisumbras]

auto c = static_cast<char>( value_to<int>(test_new["value"]) );

[grisumbras]

If you actually have a more complex structure, where the char is deep inside another object, you would have to use contextual conversions. See an example here: https://godbolt.org/z/TK4dhWPEr

[DUOLabs333]

Thanks!