Support parse std::pair to Json object

[nguoithichkhampha]

Hi, I really interested with this library. But did you support parse a std::pair to Json object ?
Something like this:
std::pair<std::string,std::string>{"test1","value1"}
will have a json object: { "test1":"value1"}

[nlohmann]

No, this is currently not supported. You can, however, create an object with an initializer list

json j = { {"test1", "value1"} };

I'm not sure whether implementing a constructor for a pair would be too helpful. The only scenario where pairs may be of interest would be if they are the value_type of a std::map. Then, you can also assign the map at once:

std::map<std::string, std::string> m;
m["key1"] = "value1";
m["key2"] = "value2";

json j = m;

What do you think?

[nguoithichkhampha]

I think very useful if can support std::pair . Sometimes my json object only one field, so I don't want to use a std::map. Although I can use a std::map with one element but I can't access directly key and value in map in case don't know key. Otherwise , with std::pair, I only call pair.first and pair.second to get info in pair.

[nlohmann]

But you can use the initializer syntax as well.

[nguoithichkhampha]

No, I mean I have a struct
struct A { std::pair<std::string,std::string> value; };
And I want to parse struct A to Json object and reverse

[nlohmann]

You may want to have a look at this if you want to convert arbitrary types to JSON: https://github.com/nlohmann/json#arbitrary-types-conversions

[nguoithichkhampha]

Yes, I know but I need to parse manually , need to get pair.first and pair.second and after that add them in Json object. I did this . But if support parse std::pair I don't need write manual code . And this library should support because pair is basic container of C++

[nlohmann]

I see. But I'm not sure whether a constructor to create a singleton object would be helpful for others.

[TurpentineDistillery]

@nguoithichkhampha

No, I mean I have a struct
struct A { std::pair<std::string,std::string> value; };
And I want to parse struct A to Json object and reverse

Can you provide sample implementation?

[nguoithichkhampha]

yes, this is my case:
I want to create a json aray:
[{"record1":"note1"},{"record2":"note2"}......]
so I use a std::vector<std::pair<std::string,std::string>> I don't use std::map<std::string,std::string> because I don't want to the element auto sorted by key

[nlohmann]

If you use std::vector<std::pair<std::string,std::string>>, where do you use the JSON library?

[nguoithichkhampha]

I'm building a web service. the response of ws is json format. I use JSON library at all when send reply.

[nlohmann]

Again, please provide a concrete example.

[nguoithichkhampha]

I can't give detail code in here. Just my idea to support std::pair. if you feel it not useful. You can discard.

[JohanMabille]

@SylvainCorlay and I are also interested in support for std::pair and std::tuple. The framework that we are building serializes patches of modified attributes in the context of an observer pattern. Some of these attributes may be std::pair. We have implemented specializations of adl_serializer as explained here. Would you be interested in a PR with that stuff ?

[nlohmann]

Meanwhile, this code compiles:

#include "json.hpp"
#include <iostream>

using json =  nlohmann::json;

int main() {
    auto p = std::make_pair(1, 2);
    auto t = std::make_tuple("foo", "bar", "baz");

    json j;
    j["pair"] = p;
    j["tuple"] = t;

    std::cout << std::setw(4) << j << std::endl;
}

Output:

{
    "pair": [
        1,
        2
    ],
    "tuple": [
        "foo",
        "bar",
        "baz"
    ]
}

Also, this is possible:

std::pair<int, int> p2 = j["pair"];
std::tuple<std::string, std::string, std::string> t2 = j["tuple"];

Is this helpful for you, @JohanMabille?

[JohanMabille]

Indeed, this works in develop branch, but not in v2.1.1 (which we are using). We'll wait for the v3.0.0 then.
Thanks for your quick reply.

[nlohmann]

Sorry for the inconvenience!