如何在c++中使用运行时密钥访问json文件中的值

Question

我是json的新手，正在尝试使用nlohmann库，但我很困。

config.json文件看起来像这个

{
"Version": 1.1,
"Size": 1024,
"integer": 600,
"Map": [{"0": "india"}, {"1": "usa"}, {"2": "china"}, {"2": "japan"}],
"name": "xxx",
} 

我想得到["Map"]["1"]的值；1〃；是一个运行时实体。我试过几种方法，但都不起作用。

std::string strRwFilePath = std::string(CONFIG_PATH) + "config.json";
std::ifstream RwFile(strRwFilePath);

if (RwFile.fail())
{
std::cout << "[ReadFileContents]RwFile doesnt exist" << std::endl;
}
else 
{
// parsing input 
RwFile >> conf_json;
if(!(conf_json.empty())) 
{           
//METHOD 1
for (auto it = conf_json["Map"].begin(); it != conf_json["Map"].end(); ++it)
{
std::cout << it.key() << " | " << it.value() << "n";
std::string keyy = std::to_string(m_iRegionType); 
std::string tempKey = it.key();                         //ERROR- [json.exception.invalid_iterator.207] cannot use key() for non-object iterators
std::cout << "for loop: tempKey:" << tempKey << " n";
if(!strcmp(tempKey.c_str(),(std::to_string(m_iRegionType)).c_str()))
{
std::cout << "SUCCESS 222" << std::endl;
break; 
}
else
{
std::cout << "FAILURE 222" << std::endl; 
}
}
//METHOD 2
for(auto& element : conf_json["Map"].items())
{
std::cout << element.key() << " | " << element.value() << "n";
m_iRegionType = 2;
std::string keyy = std::to_string(m_iRegionType); 
std::string tempKey = element.key();                //ERROR: [json.exception.type_error.302] type must be string, but is object
if(!strcmp(tempKey.c_str(),(std::to_string(m_iRegionType)).c_str()))
{
std::cout << "SUCCESS 333" << std::endl;
break; 
}
else
{
std::cout << "FAILURE 333" << std::endl; 
}   
}

//METHOD 3
std::string strTempKey = std::to_string(m_iRegionType);
if(!conf_json["Map"][strTempKey.c_str()].is_null())
{
std::string strName = conf_json["Map"][strTempKey.c_str()];     //ERROR: [json.exception.type_error.305] cannot use operator[] with a string argument with array
std::cout << "key found. RegionName: " << strName << 'n';         
}
else
{
std::cout << "key not found" << std::endl;
}

//METHOD 4
// create a JSON object
std::string strKey = "/Map/" + std::to_string(m_iRegionType);
// call find
auto it_two = conf_json.find(strKey.c_str());
if(true == (it_two != conf_json.end()))
{
std::cout << "Region key was found. RegionBucketName: " << *it_two << 'n';

std::string strRegionName = conf_json["Map"][m_iRegionType];
std::string strRegionName2 = *it_two;
std::cout << "strRegionName: " << strRegionName << 'n';
std::cout << "strRegionName2: " << strRegionName2 << 'n';
}
else
{
//getting this as OUTPUT even though key is available
std::cout << "invalid region type, key not available m_iRegionType: " << m_iRegionType << std::endl;
}

}
else
{
std::cout << "[ReadFileContents]Read Write Json object is empty" << std::endl;
}               
}

我还尝试过使用小json结构，在这种结构中，我无法使用运行时键访问值。

json j =
{
{"integer", 1},
{"floating", 42.23},
{"string", "hello world"},
{"boolean", true},
{"object", {{"key1", 1}, {"key2", 2}}},
{"array", {1, 2, 3}}
};
int avail4 = j.value("/integer"_json_pointer, 444);
std::cout << "avail4 : " << avail4 << std::endl;         //OUTPUT- avial4 : 1
int avail5 = j.value("/object/key2"_json_pointer, 555);
std::cout << "avail5 : " << avail5 << std::endl;         //OUTPUT- avial5 : 2
auto strKey3 = "/object/key2";
int avail6 = j.value(strKey3, 666);
std::cout << "avail6 : " << avail6 << std::endl;         //OUTPUT- avial6 : 666

有人能帮我吗？

Answer 1

不能使用["Map"]["1"]来查找内部数组对象。你需要迭代器通过数组

一个代码示例可能如下：

#include <nlohmann/json.hpp>
#include <iostream>
// for convenience
using json = nlohmann::json;
int main() {
json j  = R"(
{
"Version": 1.1,
"Size": 1024,
"integer": 600,
"Map": [{
"0": "india"
}, {
"1": "usa"
}, {
"2": "china"
}, {
"2": "japan"
}],
"name": "xxx"
}
)"_json;
for (const auto& ele : j["Map"]) {
if (ele.contains("1")) {
std::cout << ele["1"] << std::endl;
}
}
}

输出：

美国

演示