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Document when value_from is needed #845
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First of all, yes, this code is correct. Second, I honestly can't see the source of confusion: you need |
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. |
As, I've mentioned previosuly, the behaviour was never intended. If it was,
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. |
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. |
This could be related. In 1.80 the following used to work but in 1.81 it doesn't compile:
In 1.81 I need to use value_from otherwise it doesn't compile:
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? |
Yes, this behaviour was never intended and wasn't documented and so we removed the code that accidentally allowed it to work. |
Since 1.81 this no longer compiles:
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 followingIs this correct?
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