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Using fifo_map #485
Description
The document says "you can specialize the object type with containers like tsl::ordered_map or nlohmann::fifo_map". I had a try with simple code like:
#include "fifo_map.hpp"
#include "json.hpp"
using json = nlohmann::basic_json<nlohmann::fifo_map>;
int main()
{
json j;
}It fails the compilation noisily. Would you please fix the issue, and/or clarify in the document how this should be done?
Activity
nlohmann commentedon Mar 7, 2017
Can you please provide the compiler error messages?
adah1972 commentedon Mar 7, 2017
From GCC:
From Clang:
nlohmann commentedon Mar 7, 2017
Thanks, I shall have a look.
nlohmann commentedon Mar 7, 2017
I can confirm the error, but I have no idea how to fix it. I haven't tried
fifo_mapfor this issue in a long time - I only mentioned it in the REAMDE, because I read somewhere that it was used successfully with this library. There may be issues, becausefifo_mapusesstd::unordered_mapinternally, which is problematic (#164).arvidsson commentedon Apr 4, 2017
Using Visual Studio C++ 2017, I get the following error:
'add_key': is not a member of 'std::less'
Daniel599 commentedon Apr 10, 2017
Hi, I think I found out the problem, I think it's because when you are declaring object_t,
you use std::less, but not all map-like use std::less, i'm guessing it's because you wanted to set the allocator.
I can offer a patch:
Patch (might break people who set the map manually)
If you still want to keep you inner type allocator aware
now if you want to use fifo_map you can by using the follow:
please note that I haven't fully tested it, just basic example, not sure if it's the right way to do it, just an idea.
Also I can offer a workaround without changing the json library itself but it's a bit more ugly and will break if you will fix it using the patch above.
Let me know if you want this patch a pull request.
donho commentedon Sep 10, 2017
Same error under VS2015.
I think I will use nlohmann::json without fifo_map :(
nlohmann commentedon Sep 10, 2017
Related to #660 (comment):
I really need to remove the hint to fifo_map from the documentation... :-(
tomtachi commentedon Sep 10, 2017
tomtachi commentedon Sep 10, 2017
SiebelsTim commentedon Oct 2, 2017
The fifo_map library uses an uncommon comparator type, which needs to hold a key storage.
std::mapfor example usesstd::lessby default, which is a simple functor without state.The json library follows the pattern of
std::mapwhich directly passesstd::less<StringType>to the object type. And of course, there is no constructor forstd::less<std::string>(std::unordered_map<Key, std::size_t>*). That's the problem.It is not possible to solve this without breaking compatibility with one of the libraries.
Daniel599 commentedon Oct 2, 2017
As I said before in my previous comment, there is a way to do it without a change in the libraries, it's a workaround which I don't like that much since it will break if the API will change.
anyway here it's: (be warn - this workaround will break if the template parameters will change)
output:
I don't like this workaround very much since it isn't clean and really patchy, but maybe this will give others a better idea about what to do.
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Daniel599 commentedon Oct 26, 2018
it doesn't compile because they are different types.
if you want same function for both of them, you need to convert
processJsoninto a template function:risa2000 commentedon Jul 10, 2019
I am sorry if this has been already solved, but I am trying to build specialized
jsonwithfifo_map, using following code:and I am hit by the error already reported by @arvidsson above:
I wonder, was it resolved, or is it a know bug? If the latter, is there any other solution?
Daniel599 commentedon Jul 20, 2019
@risa2000 it doesn't seems like you used my workaround.
please define your json type as the example below.
risa2000 commentedon Jul 20, 2019
@Daniel599 thanks for the nudge. Indeed I did not apply the "fix" as I did not understand that it was one! After following your example, it works as expected. Unfortunately, it turned out that I am stuck anyway.
The reason is that I am using
xtensorlibrary in the same project, which implements also a serialization/deserialization throughnlohmann::json, but this library seems to be "hardcoding" nlohmann::json as the type it uses everywhere, with no template for possible specialization with a custom mapping class. So I guess I am going to write another report over there.theresa95 commentedon Apr 1, 2020
hey @nlohmann and @Daniel599 !
I am currently trying to generate JSON with ordered keys and therefore used the workaround method from @Daniel599 . I just copy-pasted your workaround code and it worked as aspected. However, if I try to use it within my base and derived classes, I get an error which I do not really understand. It seems like it fails to call the to_Json methods (because the error appears if I try to map a DerivedClass-instance (test and test2) to my_json.
I have already tried the example without ordered keys (just by using json = nlohmann::json;) and it works completely fine. The keys in the output are sorted alphabetically and looks like this:
What I am trying to achieve through using the nlohmann fifo_map is to keep the insertion order and the final output therefore should look like this:
Executing the following code outputs two errors:
Please have a look at the following code:
In BaseClass.h:
In DerivedClass.h:
In main.cpp:
Daniel599 commentedon Apr 1, 2020
Hi @theresa95, after some investigation I found out the cause of your error is due to multiple "my_json" types: one inside
BaseNamespace::SubNamespace::my_jsonand the other in the global scope.I don't know if it's the same case in your original code, but I mange to fix it by doing the following:
using namespace nlohmann;within your namespace (e.g inside BaseClass.h & DerivedClass.h)my_jsonin single place atBaseClass.hafter doing the steps above, your code example compiles on
g++ 7.5with-std=c++17, I hope it compiles on windows as well (if not let me know)theresa95 commentedon Apr 1, 2020
Hey @Daniel599 , first of all, thank you for the quick answer!
I edited the codesnippet of my question to fit your answer.
I removed the following lines from the DerivedClass.h file:
But I still get the same error messages in the main.cpp file:
Removing the "using namespace nlohmann" in the BaseClass.h file results in other errors so I have left it there:
What should I do with my_json definition and using namespace nlohmann in the main.cpp file?
Thank you so much for your help in advance!
Daniel599 commentedon Apr 1, 2020
@theresa95
in BaseClass.h:
note the addition of
nlohmann::which is needed when you remove theusing namespace nlohmannin main.cpp:
using namespace nlohmannbut I would recommend against it, prefer to usenlohmann::blablawhenever you need (P.S I don't think you need it in main anymore anyway)my_jsonwithBaseNamespace::SubNamespace::my_jsona bit more details:
since
my_jsonis declared inside the namespaceBaseNamespace::SubNamespaceandmainisn't (it's in the global scope), you have two options: add thenamespace prefixor addusing namespace BaseNamespace::SubNamespaceinmain.cpp, I like more the first option since it's more explicit about where things are taken from.theresa95 commentedon Apr 1, 2020
@Daniel599 That did the trick! Thank you so much!
nlohmann commentedon Jul 30, 2021
@ccpearson Thanks for sharing. I am confused - is
fifo_mapbroken with the latest release? We useobject_comparator_tnow.