C++17 in-place JSON parser

InPlaceJSON.cpp

// In-place JSON parsing concept by gatopeich 2020
// Taking advantage of C++17 features like std::string_view
// No rights reserved, use at your own risk!

#include <cstdlib>
#include <iostream>
#include <stdexcept>
#include <string_view>

namespace inplacejson
{
    using JSONError = std::runtime_error;
    struct InPlaceJSON
    {
        const char *data;
        InPlaceJSON(const char* data) : data{data} {
            while (*data && *data <= ' ') ++data;
        }

        int asInt() { return atol(data); }
        float asFloat() { return atof(data); }

        std::string_view asString() {
            if (*data != '"')
                throw JSONError("Not a JSON String");
            auto str = data + 1;
            for (int len = 0; str[len]; len++) {
                if (str[len] == '"')
                    return std::string_view(str, len);
                if (str[len] == '\\' && !str[++len])
                    break;
            }
            throw JSONError("Badly terminated JSON String");
        }

        InPlaceJSON operator[](const std::string_view& key) {
            if (*data != '{')
                throw JSONError("Not a JSON Object");
            InPlaceJSON next {data + 1};
            while (*next.data != '}') {
                auto ikey = next.asString();
                InPlaceJSON colon { ikey.end() + 1 };
                if (*colon.data != ':')
                    throw JSONError("Missing colon after JSON key");
                InPlaceJSON value {colon.data + 1};
                if (ikey == key)
                    return value;
                next = value.skip();
                if (*next.data == ',')
                    next = InPlaceJSON {next.data + 1};
            }
            throw JSONError("Key not found");
        }
        
        InPlaceJSON skip() {
            char* after_number;
            if (strtof(data, &after_number) || after_number != data)
                return InPlaceJSON(after_number);
            if (*data == '"')
                return InPlaceJSON {asString().end()+1};
            if (*data == '{') {
                InPlaceJSON next {data + 1};
                while (*next.data != '}') {
                    auto ikey = next.asString();
                    InPlaceJSON colon { ikey.end() + 1 };
                    if (*colon.data != ':')
                        throw JSONError("Missing colon after JSON key");
                    InPlaceJSON value {colon.data + 1};
                    next = value.skip();
                    if (*next.data == ',')
                        next = InPlaceJSON {next.data + 1};
                }
                return InPlaceJSON {next.data + 1};
            }
            throw JSONError(std::string("Cannot skip: ") + data);
        }
        // Parser operator[](const int n) {}
    };
}

using inplacejson::InPlaceJSON;

int main(int argc, char* argv[])
{
    auto input = "{\"string\":\"my-string\",\"number\":123.5,\"boolean\":true,\"list\":[1,2,3]}";
    std::clog << InPlaceJSON{input}["string"].asString() << std::endl;
    std::clog << InPlaceJSON{input}["number"].asFloat() << std::endl;
    std::clog << InPlaceJSON{input}["list"].asFloat() << std::endl;

    return 0;
}