C++如何实现一个简单的JSON解析器_从零开始编写C++递归下降JSON解析器

class JsonParser {
private:
    const std::string& input;
    size_t pos;
<pre class="brush:php;toolbar:false;"><pre class="brush:php;toolbar:false;">// 跳过空白字符
void skip_whitespace() {
    while (pos < input.size() && (input[pos] == ' ' || input[pos] == '	' || input[pos] == '
' || input[pos] == '
'))
        ++pos;
}

// 匹配并消耗指定字符
bool match(char c) {
    skip_whitespace();
    if (pos < input.size() && input[pos] == c) {
        ++pos;
        return true;
    }
    return false;
}

// 解析 "null"
JsonValue parse_null() {
    if (input.substr(pos, 4) == "null") {
        pos += 4;
        return JsonValue(nullptr);
    }
    throw std::runtime_error("Expected 'null'");
}

// 解析 "true" / "false"
JsonValue parse_boolean() {
    if (input.substr(pos, 4) == "true") {
        pos += 4;
        return JsonValue(true);
    }
    if (input.substr(pos, 5) == "false") {
        pos += 5;
        return JsonValue(false);
    }
    throw std::runtime_error("Expected 'true' or 'false'");
}

// 解析数字（简单版本，支持整数和小数）
JsonValue parse_number() {
    size_t start = pos;
    if (pos < input.size() && input[pos] == '-') ++pos;

    if (pos == input.size() || !isdigit(input[pos]))
        throw std::runtime_error("Invalid number");

    while (pos < input.size() && isdigit(input[pos])) ++pos;

    if (pos < input.size() && input[pos] == '.') {
        ++pos;
        if (pos == input.size() || !isdigit(input[pos]))
            throw std::runtime_error("Invalid number");
        while (pos < input.size() && isdigit(input[pos])) ++pos;
    }

    std::string numStr = input.substr(start, pos - start);
    try {
        return JsonValue(std::stod(numStr));
    } catch (...) {
        throw std::runtime_error("Invalid number format");
    }
}

// 解析带引号的字符串（未处理转义字符）
JsonValue parse_string() {
    if (!match('"')) throw std::runtime_error("Expected '"'");

    size_t start = pos;
    while (pos < input.size() && input[pos] != '"') {
        ++pos;
    }

    if (pos == input.size()) throw std::runtime_error("Unterminated string");

    std::string str = input.substr(start, pos - start);
    ++pos; // 跳过结尾的 "
    return JsonValue(str);
}

// 解析数组
JsonValue parse_array() {
    if (!match('[')) throw std::runtime_error("Expected '['");

    JsonArray arr;
    skip_whitespace();

    if (match(']')) return JsonValue(arr); // 空数组

    while (true) {
        arr.push_back(parse_value());
        skip_whitespace();

        if (match(']')) break;
        if (!match(',')) throw std::runtime_error("Expected ',' or ']'");
    }

    return JsonValue(arr);
}

// 解析对象
JsonValue parse_object() {
    if (!match('{')) throw std::runtime_error("Expected '{'");

    JsonObject obj;
    skip_whitespace();

    if (match('}')) return JsonValue(obj); // 空对象

    while (true) {
        JsonValue keyVal = parse_string();
        std::string key = keyVal.as_string();

        skip_whitespace();
        if (!match(':')) throw std::runtime_error("Expected ':'");

        JsonValue value = parse_value();
        obj[key] = value;

        skip_whitespace();
        if (match('}')) break;
        if (!match(',')) throw std::runtime_error("Expected ',' or '}'");
    }

    return JsonValue(obj);
}

// 主解析入口
JsonValue parse_value() {
    skip_whitespace();
    if (pos >= input.size()) throw std::runtime_error("Unexpected end of input");

    char c = input[pos];
    if (c == 'n') return parse_null();
    if (c == 't' || c == 'f') return parse_boolean();
    if (c == '-' || isdigit(c)) return parse_number();
    if (c == '"') return parse_string();
    if (c == '[') return parse_array();
    if (c == '{') return parse_object();

    throw std::runtime_error("Unexpected character: " + std::string(1, c));
}

JsonValue parse() {
    JsonValue result = parse_value();
    skip_whitespace();
    if (pos != input.size())
        throw std::runtime_error("Extra characters after JSON");
    return result;
}

#include <iostream>
#include <stdexcept>
<p>void print_json(const JsonValue& val, int indent = 0) {
std::string space(indent, ' ');
if (val.is_null()) {
std::cout << "null";
} else if (val.is_bool()) {
std::cout << (val.as_bool() ? "true" : "false");
} else if (val.is_number()) {
std::cout << val.as_number();
} else if (val.is_string()) {
std::cout << """ << val.as_string() << """;
} else if (val.is_array()) {
std::cout << "[
";
const auto& arr = val.as_array();
for (size_t i = 0; i < arr.size(); ++i) {
std::cout << std::string(indent + 2, ' ');
print_json(arr[i], indent + 2);
if (i != arr.size() - 1) std::cout << ",";
std::cout << "
";
}
std::cout << std::string(indent, ' ') << "]";
} else if (val.is_object()) {
std::cout << "{
";
const auto& obj = val.as_object();
auto it = obj.begin();
while (it != obj.end()) {
std::cout << std::string(indent + 2, ' ') << """ << it->first << "": ";
print_json(it->second, indent + 2);
++it;
if (it != obj.end()) std::cout << ",";
std::cout << "
";
}
std::cout << std::string(indent, ' ') << "}";
}
}</p><p>int main() {
std::string json_str = R"({
"name": "Alice",
"age": 30,
"is_student": false,
"grades": [85.5, 92.0, 78.5],
"address": {
"city": "Beijing",
"zipcode": "100000"
},
"spouse": null
})";</p><pre class="brush:php;toolbar:false;"><pre class="brush:php;toolbar:false;">try {
    JsonParser parser(json_str);
    JsonValue root = parser.parse();
    print_json(root);
    std::cout << std::endl;
} catch (const std::exception& e) {
    std::cerr << "Parse error: " << e.what() << std::endl;
    return 1;
}

return 0;