C++编写Json数据解析的代码

以下是一个简单的C++ JSON解析器的示例代码：

#include <iostream>
#include <string>
#include <vector>
#include <map>

using namespace std;

enum JsonValueType {
    JSON_NULL,
    JSON_BOOLEAN,
    JSON_NUMBER,
    JSON_STRING,
    JSON_ARRAY,
    JSON_OBJECT
};

class JsonValue {
public:
    JsonValueType type;

    JsonValue() : type(JSON_NULL) {}

    virtual ~JsonValue() {}

    virtual void print(int depth = 0) const = 0;
};

class JsonNull : public JsonValue {
public:
    JsonNull() { type = JSON_NULL; }

    void print(int depth = 0) const override {
        cout << "null";
    }
};

class JsonBoolean : public JsonValue {
public:
    bool value;

    JsonBoolean(bool v) : value(v) { type = JSON_BOOLEAN; }

    void print(int depth = 0) const override {
        cout << (value ? "true" : "false");
    }
};

class JsonNumber : public JsonValue {
public:
    double value;

    JsonNumber(double v) : value(v) { type = JSON_NUMBER; }

    void print(int depth = 0) const override {
        cout << value;
    }
};

class JsonString : public JsonValue {
public:
    string value;

    JsonString(const string &v) : value(v) { type = JSON_STRING; }

    void print(int depth = 0) const override {
        cout << "\"" << value << "\"";
    }
};

class JsonArray : public JsonValue {
public:
    vector<JsonValue *> elements;

    JsonArray() { type = JSON_ARRAY; }

    ~JsonArray() {
        for (auto &element : elements) {
            delete element;
        }
    }

    void print(int depth = 0) const override {
        cout << "[" << endl;
        for (auto &element : elements) {
            for (int i = 0; i < depth + 1; i++) {
                cout << "  ";
            }
            element->print(depth + 1);
            if (element != elements.back()) {
                cout << ",";
            }
            cout << endl;
        }
        for (int i = 0; i < depth; i++) {
            cout << "  ";
        }
        cout << "]";
    }
};

class JsonObject : public JsonValue {
public:
    map<string, JsonValue *> members;

    JsonObject() { type = JSON_OBJECT; }

    ~JsonObject() {
        for (auto &member : members) {
            delete member.second;
        }
    }

    void print(int depth = 0) const override {
        cout << "{" << endl;
        for (auto &member : members) {
            for (int i = 0; i < depth + 1; i++) {
                cout << "  ";
            }
            cout << "\"" << member.first << "\": ";
            member.second->print(depth + 1);
            if (&member != &members.back()) {
                cout << ",";
            }
            cout << endl;
        }
        for (int i = 0; i < depth; i++) {
            cout << "  ";
        }
        cout << "}";
    }
};

class JsonParser {
public:
    string json;

    JsonParser(const string &json) : json(json) {}

    JsonValue *parse() {
        skipWhitespace();

        if (json.empty()) {
            return nullptr;
        }

        char c = json[0];
        switch (c) {
            case '{':
                return parseObject();
            case '[':
                return parseArray();
            case '\"':
                return parseString();
            case 't': {
                string expect = "true";
                if (json.substr(0, expect.size()) == expect) {
                    json = json.substr(expect.size());
                    return new JsonBoolean(true);
                }
                break;
            }
            case 'f': {
                string expect = "false";
                if (json.substr(0, expect.size()) == expect) {
                    json = json.substr(expect.size());
                    return new JsonBoolean(false);
                }
                break;
            }
            case 'n': {
                string expect = "null";
                if (json.substr(0, expect.size()) == expect) {
                    json = json.substr(expect.size());
                    return new JsonNull();
                }
                break;
            }
            default: {
                if (c == '-' || (c >= '0' && c <= '9')) {
                    return parseNumber();
                }
                break;
            }
        }

        throw runtime_error("Parse error");
    }

private:
    void skipWhitespace() {
        while (!json.empty() && isspace(json[0])) {
            json = json.substr(1);
        }
    }

    JsonValue *parseObject() {
        JsonObject *object = new JsonObject();
        json = json.substr(1);
        skipWhitespace();

        while (json[0] != '}') {
            if (json[0] != '\"') {
                throw runtime_error("Parse error: Expected \"");
            }
            string key = parseString()->value;
            skipWhitespace();
            if (json[0] != ':') {
                throw runtime_error("Parse error: Expected :");
            }
            json = json.substr(1);
            skipWhitespace();
            JsonValue *value = parse();
            object->members[key] = value;
            skipWhitespace();
            if (json[0] == ',') {
                json = json.substr(1);
                skipWhitespace();
            } else if (json[0] != '}') {
                throw runtime_error("Parse error: Expected , or }");
            }
        }
        json = json.substr(1);
        return object;
    }

    JsonValue *parseArray() {
        JsonArray *array = new JsonArray();
        json = json.substr(1);
        skipWhitespace();

        while (json[0] != ']') {
            JsonValue *value = parse();
            array->elements.push_back(value);
            skipWhitespace();
            if (json[0] == ',') {
                json = json.substr(1);
                skipWhitespace();
            } else if (json[0] != ']') {
                throw runtime_error("Parse error: Expected , or ]");
            }
        }
        json = json.substr(1);
        return array;
    }

    JsonValue *parseString() {
        string value;
        json = json.substr(1);

        while (true) {
            if (json.empty()) {
                throw runtime_error("Parse error: Expected \"");
            }

            char c = json[0];
            json = json.substr(1);

            if (c == '\"') {
                return new JsonString(value);
            } else if (c == '\\') {
                if (json.empty()) {
                    throw runtime_error("Parse error: Expected escaped character");
                }

                char escape = json[0];
                json = json.substr(1);

                switch (escape) {
                    case '\"':
                    case '\\':
                    case '/':
                        value += escape;
                        break;
                    case 'b':
                        value += '\b';
                        break;
                    case 'f':
                        value += '\f';
                        break;
                    case 'n':
                        value += '\n';
                        break;
                    case 'r':
                        value += '\r';
                        break;
                    case 't':
                        value += '\t';
                        break;
                    case 'u': {
                        if (json.size() < 4) {
                            throw runtime_error("Parse error: Expected four hexadecimal digits");
                        }
                        string hex = json.substr(0, 4);
                        json = json.substr(4);
                        for (int i = 0; i < hex.size(); i++) {
                            if (!isxdigit(hex[i])) {
                                throw runtime_error("Parse error: Expected hexadecimal digit");
                            }
                        }
                        int codepoint = stoi(hex, nullptr, 16);
                        if (codepoint < 0 || codepoint > 0x10ffff ||
                            (codepoint >= 0xd800 && codepoint <= 0xdfff)) {
                            throw runtime_error("Parse error: Invalid Unicode code point");
                        }
                        if (codepoint < 0x80) {
                            value += static_cast<char>(codepoint);
                        } else if (codepoint < 0x800) {
                            value += static_cast<char>((codepoint >> 6) | 0xc0);
                            value += static_cast<char>((codepoint & 0x3f) | 0x80);
                        } else if (codepoint < 0x10000) {
                            value += static_cast<char>((codepoint >> 12) | 0xe0);
                            value += static_cast<char>(((codepoint >> 6) & 0x3f) | 0x80);
                            value += static_cast<char>((codepoint & 0x3f) | 0x80);
                        } else {
                            value += static_cast<char>((codepoint >> 18) | 0xf0);
                            value += static_cast<char>(((codepoint >> 12) & 0x3f) | 0x80);
                            value += static_cast<char>(((codepoint >> 6) & 0x3f) | 0x80);
                            value += static_cast<char>((codepoint & 0x3f) | 0x80);
                        }
                        break;
                    }
                    default:
                        throw runtime_error("Parse error: Invalid escape character");
                }
            } else {
                value += c;
            }
        }
    }

    JsonValue *parseNumber() {
        bool negative = false;
        if (json[0] == '-') {
            negative = true;
            json = json.substr(1);
        }

        if (json.empty() || !isdigit(json[0])) {
            throw runtime_error("Parse error: Expected digit");
        }

        double value = 0;
        while (!json.empty() && isdigit(json[0])) {
            value = value * 10 + (json[0] - '0');
            json = json.substr(1);
        }

        if (!json.empty() && json[0] == '.') {
            json = json.substr(1);
            double fraction = 0.1;
            while (!json.empty() && isdigit(json[0])) {
                value += fraction * (json[0] - '0');
                fraction *= 0.1;
                json = json.substr(1);
            }
        }

        if (!json.empty() && (json[0] == 'e' || json[0] == 'E')) {
            json = json.substr(1);
            bool exponent_negative = false;
            if (json[0] == '-') {
                exponent_negative = true;
                json = json.substr(1);
            } else if (json[0] == '+') {
                json = json.substr(1);
            }
            if (json.empty() || !isdigit(json[0])) {
                throw runtime_error("Parse error: Expected digit");
            }
            int exponent = 0;
            while (!json.empty() && isdigit(json[0])) {
                exponent = exponent * 10 + (json[0] - '0');
                json = json.substr(1);
            }
            if (exponent_negative) {
                exponent = -exponent;
            }
            value *= pow(10, exponent);
        }

        if (negative) {
            value = -value;
        }

        return new JsonNumber(value);
    }
};

int main() {
    string json = "{\"name\": \"John Smith\", \"age\": 42, \"isMale\": true, \"children\": [{\"name\": \"Alice\", \"age\": 10}, {\"name\": \"Bob\", \"age\": 8}]}";
    JsonParser parser(json);
    JsonValue *value = parser.parse();
    value->print();
    delete value;
    return 0;
}

这个解析器支持JSON中的null、布尔值、数值、字符串、数组和对象等各种类型，可以递归地打印JSON对象。您可以通过修改类的定义来实现自己的行为。