Document when value_from is needed

[gummif]

Since 1.81 this no longer compiles:

struct customer
{
    int id;
    std::string name;
    std::vector<int> vec
};
void tag_invoke( value_from_tag, value& jv, customer const& c )
{
    jv = {
        { "id" , c.id },
        { "name", c.name },
        { "vec", c.vec }
    };
}

I see from #823 that this is intended. But it is not clear when calling value_from is needed and maybe the idiomatic way of doing it should be documented e.g. in the example here https://www.boost.org/doc/libs/1_81_0/libs/json/doc/html/json/quick_look.html. I assume the optimal way to do this now is the following

void tag_invoke( value_from_tag, value& jv, customer const& c )
{
    jv = {
        { "id" , c.id },
        { "name", c.name },
        { "vec", value_from(c.vec, jv.storage()) } // note pass storage to use the correct allocator
    };
}

Is this correct?

[grisumbras]

First of all, yes, this code is correct.

Second, I honestly can't see the source of confusion: you need value_from when your type is not implicitly convertible to value. This is just like with any other type in C++. Is the problem with seemingly magical behaviour of std::initializer_list<value_ref> constructor/assignment?

[Triasmus]

For me, I feel that it would make sense for boost::json to implicitly know to convert std::vector to a json array in this case. I just ran into this issue myself, where the implicit conversion was working fine in 1.80, and now I have build errors in 1.81 (I'm going back to 1.80 with the hope that I won't find the desire to use std::variant or std::optional).

So that's my source of confusion.
The quick look example page shows that value jv = {{"list", {1, 0, 2}}}; works. Replacing {1, 0, 2} with a pre-initialized vector<int> just makes sense.

[grisumbras]

As, I've mentioned previosuly, the behaviour was never intended. If it was, json::value jv = std::vector{1,2,3} would also have been working, but it wasn't. The behaviour was a bug.

{{"list", {1, 0, 2}}}; does not mean any_map{ {"list", any_sequence{1, 0, 2}} };, it means json::object{ {"list, json::array{1, 0, 2} }.

I am not aware of any container type that does implicit conversions from other container types. In particular, standard container types definitely do not do that. There is a reason for that: implicit conversions cannot be locally disabled, they are pervasive, they create potential for lots of unintended conversions. They are a particularly terrible idea when memory is likely to be allocated.

[gummif]

I agree that this is a better design than implicit conversion. My heuristic is if an allocation is probably required then I need value_from and usually pass the storage ptr.

[rjahanbakhshi]

This could be related. In 1.80 the following used to work but in 1.81 it doesn't compile:

#include <boost/describe/enum.hpp>

enum class states
{
    failed,
    completed,
};
BOOST_DESCRIBE_ENUM(states, failed, completed);

#include <boost/json.hpp>

int main()
{
    boost::json::value value = {
        {"id", 42},
        {"state", states::failed},
    };
}

In 1.81 I need to use value_from otherwise it doesn't compile:

#include <boost/describe/enum.hpp>

enum class states
{
    failed,
    completed,
};
BOOST_DESCRIBE_ENUM(states, failed, completed);

#include <boost/json.hpp>

int main()
{
    boost::json::value value = {
        {"id", 42},
        {"state", boost::json::value_from(states::failed)},
    };
}

Of course, in 1.80 I had my own tag_invoke overload for described enum but it is added in 1.81.

It's a pity because the pretty initializer format is not working with 1.81+ for custom types or am I missing something?

[grisumbras]

Yes, this behaviour was never intended and wasn't documented and so we removed the code that accidentally allowed it to work.