simdjson 库未捕获 C++ 异常

Question

我正在尝试使用 simdjson 库:https://github.com/simdjson/simdjson#documentation

但是，我需要解析的 json 通过 websocket 连接传输，并且并不总是包含相同的键。因此，有时尝试从解析的 json 对象中按键提取值会引发异常:

terminate called after throwing an instance of 'simdjson::simdjson_error'
  what():  The JSON field referenced does not exist in this object.
Aborted (core dumped)

下面尝试使异常处理起作用的示例代码:

#include <iostream>
#include <string>
#include <chrono>

#include "simdjson.h"

using namespace std;
using namespace simdjson;

int main() {

    auto cars_json = R"( [
  { "make": "Toyota", "model": "Camry",  "year": 2018, "tire_pressure": [ 40.1, 39.9, 37.7, 40.4 ] },
  { "make": "Kia",    "model": "Soul",   "year": 2012, "tire_pressure": [ 30.1, 31.0, 28.6, 28.7 ] },
  { "make": "Toyota", "model": "Tercel", "year": 1999, "tire_pressure": [ 29.8, 30.0, 30.2, 30.5 ] }
] )"_padded;

    simdjson::error_code error_c;

    dom::parser  parser;
    dom::object  doc;
    dom::element elem;

    const char* value;

    doc = parser.parse(cars_json);

    try
    {
        doc["clOrdID"].get<const char*>().tie(value, error_c);
    } 
    catch (simdjson_error e)
    {
        cout << 1 << endl;
    }

    return 0;
}

我在这里阅读了有关库错误处理的文档: https://github.com/simdjson/simdjson/blob/master/doc/basics.md#error-handling

但是，使用 try-catch block 和上面文档中描述的错误处理方法仍然会导致程序退出。我对 C++ 编程非常陌生，因此对该语言的异常处理也很陌生 - 任何有关如何从该库捕获任何异常的指导将不胜感激!

Answer 1

首先，您应该将 parser.parse(cars_json); 移动到 try/catch block 中，以便捕获解析器异常

try {
    doc = parser.parse(cars_json);
} catch (simdjson::simdjson_error e) {
    cout << e.what() << endl;
}

那么您可能会意识到解析器不会返回simdjson::dom::object而是simdjson::dom::array相反。

这里是转换为 const char* 的更新示例

const char* value;

try {
    auto root = parser.parse(cars_json);
    cout << "root: " << root.type() << endl;

    auto car = root.at(0);
    if(car["make"].is_string()){
        cout << "first car: " << car["make"] << endl;
    }

    //check non-existing element
    if(car["foo"].is_string()){
        // exception won't be thrown
    }

    std::string_view sv = root.at(1)["model"];
    cout << "string view: " << sv << endl;
    //conversion to const char*
    value = std::string(sv).c_str();
    cout << "const char: " << value << endl;


} catch (const simdjson::simdjson_error &e) {
    cout << e.what() << endl;
}

输出应该是:

root: array
first car: "Toyota"
string view: Soul
const char: Soul