升压变体：如何 model JSON？

Question

I'm trying to parse JSON string using Boost Spirit store JSON object into recursive data structures:

Value <== [null, bool, long, double, std::string, Array, Object];
Array <== [Value, Value, Value, ...];
Object <== ["name1": Value, "name2": Value, ...];

And here's my code:

#include <map>
#include <vector>
#include <string>
#include <boost/variant.hpp>
#include <boost/shared_array.hpp>
#include <boost/shared_ptr.hpp>

struct JsonNull {};
struct JsonValue;

typedef std::map<std::string, JsonValue *> JsonObject;
typedef std::vector<JsonValue *> JsonArray;

struct JsonValue : boost::variant<JsonNull, bool, long, double, std::string, JsonArray, JsonObject>
{
};

JsonValue aval = JsonObject();

When compiling I get the error:

Error C2440: 'initializing' : cannot convert from 'std::map<_Kty,_Ty>' to 'JsonValue'

Moreover, how to safely cast JsonValue to JsonObject? When I try doing:

boost::get<JsonObject>(aval) = JsonObject();

This gets run-time exception/fatal failure.

Any help is greatly appreciated.

EDIT:

Following @Nicol's advice, I came out with the following code:

struct JsonNull {};
struct JsonValue;

typedef std::map<std::string, JsonValue *> JsonObject;
typedef std::vector<JsonValue *> JsonArray;
typedef boost::variant<
    JsonNull, bool, long, double, std::string,
    JsonObject, JsonArray,
    boost::recursive_wrapper<JsonValue>
> JsonDataValue;

struct JsonValue
{
    JsonDataValue data;
};

I can work on JsonObject & JsonArray as easy as this:

JsonValue *pJsonVal = new JsonValue();

boost::get<JsonObject>(pCurrVal->data).insert(
    std::pair<std::string, JsonValue *>("key", pJsonVal)
);

boost::get<JsonArray>(pCurrVal->data).push_back(pJsonVal);

Just posting so that everyone could benefit from this.

Answer 1

You have to use a recursive wrapper (and you shouldn't be deriving from boost::variant ):

struct JsonValue;

typedef boost::variant</*types*/, boost::recursive_wrapper<JsonValue> > JsonDataValue;

struct JsonValue
{
    JsonDataValue value;
};

To make Boost.Spirit take a JsonValue, you will need to write one of those Fusion adaptor things to adapt the raw variant type into a struct.

---

Moreover, how to safely cast JsonValue to JsonObject? When I try doing:

That's not how variants work. If you want to set them to a value, just set them like any other value:

JsonValue val;
val.value = JsonValue();