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json Conversions to/from C++ objects #223

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JoeStroupe opened this issue Aug 28, 2020 · 33 comments
Closed

json Conversions to/from C++ objects #223

JoeStroupe opened this issue Aug 28, 2020 · 33 comments

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@JoeStroupe
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Does Boost.json support conversions from C++ objects to json, and from json to C++ objects? Use case is in the interface between C++ application and calls to the ArangoDB HTTP REST API. I'm using libcurl to generate REST calls (with json bodies) into ArangoDB, and I need to convert C++ object members and their values to json for those calls. Then, on the return route, I need to populate C++ object members with the json data returned from ArangoDB calls. I also need to know if such conversions can be done recursively, or only singly.

What of the case where my C++ objects exist in a vector or other std container? I'm looking for functionality akin to that noted here: https://stackoverflow.com/questions/17549906/c-json-serialization

Scroll down to the solution with 85 votes, posted by Guillaume Racicot.
@sdkrystian
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The library supports conversions to and from user-defined types via value_to<T>(expr) and value_from(expr). Generic conversions for container types can be done by the library out of the box so long as they fit a certain "container-like" criteria, and such conversions are performed recursively. You can find some more details of what is supported here: http://vinniefalco.github.io/doc/json/json/usage/conversion.html

If the generic conversions that we provide aren't enough, you can implement the conversion yourself by providing overloads for tag_invoke. These will be found by ADL and value_to/from will use them for the conversion.

@vinniefalco
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vinniefalco commented Aug 28, 2020 •
edited

Short answer: Yes (http://vinniefalco.github.io/doc/json/json/usage/conversion.html)

Long answer:

Given a user defined type T:

    T t;
    ...
    // Convert from T to json::value
    json::value jv = json::value_from( t );

    // Convert json::value to T
    T t2 = json::value_to( jv );

This is accomplished by providing overloads of tag_invoke for T in the namespace of T:

    // Called to convert json::value to T
    T tag_invoke( json::value_to_tag<T>, json::value const& jv );

    // Called to convert T to json::value
    void tag_invoke( json::value_from_tag, json::value& jv, T const& );

Furthermore as @sdkrystian noted, the library provides generic conversions for ranges, which handles std::vector<T> (and maps with key/value pairs).

@JoeStroupe
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JoeStroupe commented Aug 28, 2020 via email

@JoeStroupe
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JoeStroupe commented Aug 28, 2020 via email

@JoeStroupe
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JoeStroupe commented Aug 28, 2020 via email

@vinniefalco
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I want to build the json lib as MD/d

The easiest thing to do is ignore my CMakeLists.txt and just add src.cpp to your project:
https://github.com/CPPAlliance/json/blob/397a31f1667150a94c1b079de83bec85065729f6/src/src.cpp

Alternatively, just add the following line to any one of your translation units:

#include <boost/json/src.hpp>

@JoeStroupe
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JoeStroupe commented Aug 28, 2020 via email

@vinniefalco
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Looks like this is resolved, thanks!

@JoeStroupe
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JoeStroupe commented Sep 1, 2020 via email

@vinniefalco
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If you would be willing to critique it, what is your preferred way for me to get you the code and explanation of what I’m trying to do?

I am more than happy to comment on the usage of my libraries in other programs. The best way to do that is to open a pull request against your own GitHub repository, and add me as a reviewer so I can leave comments.

@JoeStroupe
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JoeStroupe commented Sep 1, 2020 via email

@vinniefalco
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But when I’m ready I could send you a zip file with the code and a good explanation of what I’m trying/needing to accomplish.

I encourage you to create a repository. It is easy, and free. I can't really comment on a zip file. GitHub has wonderful tools for collaboration - we should use it!

@JoeStroupe
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JoeStroupe commented Sep 1, 2020 via email

@JoeStroupe
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JoeStroupe commented Sep 8, 2020 via email

@vinniefalco
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vinniefalco commented Sep 8, 2020 •
edited

I just recently changed to_string to serialize. Have you picked up the latest develop branch?

When I use to_string() I get a string with ‘[‘ ‘]’ brkts and it’s not suitable for a json body in an HTTP request.

Yeah...that's the problem. The standard library and Boost already provide many overloads of to_string so even though I took out the function, another overload is still being found! So it is being converted incorrectly...

@JoeStroupe
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JoeStroupe commented Sep 8, 2020 via email

@vinniefalco
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Any suggestions on how to get the new serialize() to work correctly?

Can you please provide a small sample program that demonstrates the problem?

@JoeStroupe
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JoeStroupe commented Sep 8, 2020 via email

@vinniefalco
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I don't see any zip file, can you paste the minimal sample program here?

@JoeStroupe
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JoeStroupe commented Sep 8, 2020 via email

@vinniefalco
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can you please reduce that down to a small program, with main, of just a few lines, that reproduces the problem? In other words a minimal reproducible example.

@JoeStroupe
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JoeStroupe commented Sep 8, 2020 via email

@JoeStroupe
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JoeStroupe commented Sep 8, 2020 via email

@vinniefalco
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Your program works fine for me, I get this output
image

@JoeStroupe
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JoeStroupe commented Sep 8, 2020 via email

@vinniefalco
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jval = { t->name + CURL_PAIR_SEPARATOR + t->value };

Well, what you are doing here is creating an array of one-element arrays each containing a string. If you want an object with key/value pairs then you need something resembling a map. This is one way to do it:

int main()
{
    std::map<std::string, std::string> ReqV;
    ReqV.emplace( "Hello", "World" );
    ReqV.emplace( "Goodbye", "World" );
    value requestBodyjs = boost::json::value_from(ReqV);
    std::string str = boost::json::serialize(requestBodyjs);
    std::cout << str << std::endl;
    return 0;
}

@JoeStroupe
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JoeStroupe commented Sep 8, 2020 via email

@vinniefalco
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there’s a stinking bug in VS 2015

Try boost::container::map

@JoeStroupe
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JoeStroupe commented Sep 9, 2020 via email

@vinniefalco
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See: https://stackoverflow.com/help/minimal-reproducible-example

@JoeStroupe
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JoeStroupe commented Sep 9, 2020 via email

@vinniefalco
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You are including src.hpp which is required for linking, but you are not including the headers which give you all the declarations:

#Include <boost/json.hpp>

Alternatively you can include just what you need:

#Include <boost/json/value_to.hpp>
#Include <boost/json/value_from.hpp>

@JoeStroupe
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JoeStroupe commented Sep 9, 2020 via email

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@vinniefalco @JoeStroupe @sdkrystian