std::map key conversion with to_json #607
Comments
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The problem is already when the unordered map is defined: #include <unordered_map>
enum class EC {
A
};
int main()
{
std::unordered_map<EC, int> imap;
}Error: This can be fixed by defining a hash for type namespace std
{
template<>
struct hash<EC>
{
std::size_t operator()(const EC& e) const
{
return 0;
}
};
}Unfortunately, this does not yet fix the problem: I'm not sure about this yet. :-( |
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@theodelrieu Any idea on this? This may be related to #600. |
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(Not helpful, but worth noting:) The README says:
In this example, there is no conversion from |
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Indeed, only However, you can partially specialize namespace nlohmann {
template <typename T>
adl_serializer<std::unordered_map<EC, T>> {
// define to_json/from_json here
};
} |
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Cool, thanks. I'll give that a try.
Why is this not supported via to/from_json?
…On Tue, Jun 6, 2017, 06:27 Théo DELRIEU ***@***.***> wrote:
Indeed, only map-like types with a key convertible to std::string are
supported by default.
However, you can partially specialize adl_serializer like this:
namespace nlohmann {template <typename T>
adl_serializer<std::unordered_map<EC, T>> {
// define to_json/from_json here
};
}
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We support JSON serialization for enums and enum classes by default, using the underlying numeric value. However, in this case, we are not converting an enum into a JSON value, but as a part of a JSON value (an object here). The library cannot call a The approach I mentioned ahead will allow you to handle that key "stringification" yourself. |
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I agree with @theodelrieu. You need to provide your own enum-to-string conversion. |
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@nlohmann and by that you mean via the I'll try out the |
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The to/from_json functions only describe how a type can be converted to |
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Okay, thanks. What is the proper usage of namespace nlohmann {
template <typename T>
adl_serializer<std::unordered_map<EC, T>> {
// define to_json/from_json here
void to_json(json& j, std::unordered_map<EC, T> const& map) {
// loop on map entries here?
// j = ...
}
// likewise for from_json?
};
} |
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I think @theodelrieu can help with that. |
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First, do not forget to make your methods Something like this will work: for (auto const& elem : map) {
auto const strKey = convertToString(elem.first);
j[strKey] = elem.second;
}Of course there needs to be a Same thing for the for (auto const& elem : json::iterator_wrapper(j)) {
auto const enumKey = enumFromString(elem.key());
map[enumKey] = elem.value();
} |
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@RobotCaleb - Did @theodelrieu s hints work for you? |
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I switched away from this to just being explicit with tinyxml2. I appreciate your prompt responses, though. |
I read that you can use maps as long as a
std::stringcan be constructed from the key.I don't understand why this doesn't work. As I've written it I think it's fulfilling the constraint I mentioned above.
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