在 nlohmann json 中，如何将嵌套对象数组转换为嵌套结构向量？

Question

I asked this question, and the answer worked perfectly for regular (un-nested) objects:

[
  {
    "Name": "test",
    "Val": "test_val"
  },
  {
    "Name": "test2",
    "Val": "test_val2"
  }
]

with the struct:

struct Test {
  string Name;
  string Val;
};

However when I tried to use nested structs, like so:

struct Inner {
  string Name;
  string Value;
};

struct Outer {
  string Display;
  int    ID;
  Inner  Nested
};

//with json

"
[
  {
    "Display": "abcd",
    "ID": 100,
    "Nested": {
      "Name": "Test Name",
      "Value": "Test Value"
    }
  }
]
"

It gave me this error:

In function 'void from_json(const json&, Outer&)':
parser/run.cc:16:41: error: no matching function for call to 'nlohmann::basic_json<>::get_to(std::vector<Inner>&) const'
     j.at("Inner").get_to(p.Inner);

Answer 1

The error message sounds like you wrote a helper function for Outer , but not Inner . As long as you write a helper function for each user-defined type, the library can handled nested structs:

void from_json(const nlohmann::json& j, Inner& i) {
    j.at("Name").get_to(i.Name);
    j.at("Value").get_to(i.Value);
}

void from_json(const nlohmann::json& j, Outer& o) {
    j.at("Display").get_to(o.Display);
    j.at("ID").get_to(o.ID);
    j.at("Nested").get_to(o.Nested);
}

Then it works like you want it to:

auto parsed = json.get<std::vector<Outer>>();

Demo: https://godbolt.org/z/pGsxxn