《C++避坑神器·二十五》简单搞懂json文件的读写之遍历json文件读写-编程知识

`json.hpp库放在文章末尾`

1、遍历json文件读写

（1）插入新键值对到json之情形1
原来json文件如下所示：

{"Connection": {"IpAddress": "192.168.20.1","Rock": 0,"Solt": 1}, "DataBaseNumber":7,"hardWare":{"Axis1_offset_Enable": "0.0","Axis1_offset_Stop": "0.1","Axis1_offset_Speed": "2.0","Axis1_offset_Mov": "4.0","Axis1_offset_Pos": "6.0","Axis1_offset_Busy": "8.1","Axis2_offset_Enable": "10.0","Axis2_offset_Stop": "10.1","Axis2_offset_Speed": "12.0","Axis2_offset_Mov": "14.0","Axis2_offset_Pos": "16.0","Axis2_offset_Busy": "18.1"}
}

现想要在末尾插入新的键值对如下：

{"Connection": {"IpAddress": "192.168.20.1","Rock": 0,"Solt": 1}, "DataBaseNumber":7,"hardWare":{"Axis1_offset_Enable": "0.0","Axis1_offset_Stop": "0.1","Axis1_offset_Speed": "2.0","Axis1_offset_Mov": "4.0","Axis1_offset_Pos": "6.0","Axis1_offset_Busy": "8.1","Axis2_offset_Enable": "10.0","Axis2_offset_Stop": "10.1","Axis2_offset_Speed": "12.0","Axis2_offset_Mov": "14.0","Axis2_offset_Pos": "16.0","Axis2_offset_Busy": "18.1"},"time":{"maxtime":100,"mintime":0}
}

代码实现：

#include <iostream>
#include <fstream>
#include "json.hpp"using json = nlohmann::json;int main() {std::ifstream file("test.json");if (!file.is_open()) {std::cerr << "Failed to open file." << std::endl;return 1;}json jsonData;file >> jsonData;file.close();// 创建新的键值对json timeData = {{"maxtime", 100},{"mintime", 0}};// 将新的键值对插入到现有的 JSON 数据中jsonData["time"] = timeData;// 将更新后的 JSON 数据写入文件std::ofstream outputFile("test.json");if (!outputFile.is_open()) {std::cerr << "Failed to open file for writing." << std::endl;return 1;}outputFile << std::setw(4) << jsonData << std::endl; // 美化输出，每个元素缩进四个空格outputFile.close();return 0;
}

（2）插入新键值对到json之情形2
原来的json文件如下：

{"Connection": {"IpAddress": "192.168.20.1","Rock": 0,"Solt": 1}, "DataBaseNumber":7,"hardWare":{"Axis1_offset_Enable": "0.0","Axis1_offset_Stop": "0.1","Axis1_offset_Speed": "2.0","Axis1_offset_Mov": "4.0","Axis1_offset_Pos": "6.0","Axis1_offset_Busy": "8.1","Axis2_offset_Enable": "10.0","Axis2_offset_Stop": "10.1","Axis2_offset_Speed": "12.0","Axis2_offset_Mov": "14.0","Axis2_offset_Pos": "16.0","Axis2_offset_Busy": "18.1"}
}

现想要在末尾插入一个数组，如下所示：

{"Connection": {"IpAddress": "192.168.20.1","Rock": 0,"Solt": 1}, "DataBaseNumber":7,"hardWare":{"Axis1_offset_Enable": "0.0","Axis1_offset_Stop": "0.1","Axis1_offset_Speed": "2.0","Axis1_offset_Mov": "4.0","Axis1_offset_Pos": "6.0","Axis1_offset_Busy": "8.1","Axis2_offset_Enable": "10.0","Axis2_offset_Stop": "10.1","Axis2_offset_Speed": "12.0","Axis2_offset_Mov": "14.0","Axis2_offset_Pos": "16.0","Axis2_offset_Busy": "18.1"},"ROIS":[{"Name":"ROI1","weight":1},{"Name":"ROI2","weight":0.5}]
}

代码实现：

#include <iostream>
#include <fstream>
#include "json.hpp"using json = nlohmann::json;int main() {std::ifstream file("your_file.json");if (!file.is_open()) {std::cerr << "Failed to open file." << std::endl;return 1;}json jsonData;file >> jsonData;file.close();// 创建新的 ROIS 数据json roisData = {{"Name", "ROI1"},{"weight", 1}};json roisData2 = {{"Name", "ROI2"},{"weight", 0.5}};// 将新的 ROIS 数据插入到 JSON 数据末尾jsonData["ROIS"].push_back(roisData);jsonData["ROIS"].push_back(roisData2);// 将更新后的 JSON 数据写入文件std::ofstream outputFile("updated_file.json");if (!outputFile.is_open()) {std::cerr << "Failed to open file for writing." << std::endl;return 1;}outputFile << std::setw(4) << jsonData << std::endl; // 美化输出，每个元素缩进四个空格outputFile.close();return 0;
}

（3）插入新键值对到json之情形3
原先的json文件如下：

{"test": [{"FixedParameters": {"bit_depth": 255,"dynamic_range": 0.8},"IsUseROIs":{"isUseROIs":false,"ROIS":[{"Name":"ROI1","weight":1}]},"Index": 1},{"FixedParameters": {"bit_depth": 255,"dynamic_range": 0.8},"IsUseROIs":{"isUseROIs":false,"ROIS":[{"Name":"ROI1","weight":1}]},"Index": 2}]
}

现想要在数组的两个内容里面index和weight下面插入新的键值对，如下所示：

{"test": [{"FixedParameters": {"bit_depth": 255,"dynamic_range": 0.8},"IsUseROIs":{"isUseROIs":false,"ROIS":[{"Name":"ROI1","weight":1,"width": 100}]},"Index": 1,"name": "vimba"},{"FixedParameters": {"bit_depth": 255,"dynamic_range": 0.8},"IsUseROIs":{"isUseROIs":false,"ROIS":[{"Name":"ROI1","weight":1,"width": 100}]},"Index": 2,"name": "vimba"}]
}

代码实现：

#include <iostream>
#include <fstream>
#include "json.hpp"using json = nlohmann::json;int main() {std::ifstream file("your_file.json");if (!file.is_open()) {std::cerr << "Failed to open file." << std::endl;return 1;}json jsonData;file >> jsonData;file.close();// 遍历 AutoExposure 数组if (jsonData.find("test") != jsonData.end() && jsonData["test"].is_array()) {for (auto& entry : jsonData["test"]) {// 获取 ROIS 数组if (entry.find("IsUseROIs") != entry.end() && entry["IsUseROIs"].find("ROIS") != entry["IsUseROIs"].end()) {// 在每个 ROIS 数组元素后插入新键值对 "width": 100for (auto& rois_entry : entry["IsUseROIs"]["ROIS"]) {rois_entry["width"] = 100;}}// 在每个对象中添加新键值对 "name": "vimba"entry["name"] = "vimba";}}// 将更新后的 JSON 数据写入文件std::ofstream outputFile("updated_file.json");if (!outputFile.is_open()) {std::cerr << "Failed to open file for writing." << std::endl;return 1;}outputFile << std::setw(4) << jsonData << std::endl; // 美化输出，每个元素缩进四个空格outputFile.close();return 0;
}

（4）修改原先的键值对之情形1
原先的json文件如下：

{"test": [{"FixedParameters": {"bit_depth": 255,"dynamic_range": 0.8},"IsUseROIs":{"isUseROIs":false,"ROIS":[{"Name":"ROI1","weight":1}]},"Index": 1},{"FixedParameters": {"bit_depth": 255,"dynamic_range": 0.8},"IsUseROIs":{"isUseROIs":false,"ROIS":[{"Name":"ROI1","weight":1}]},"Index": 2}]
}

现想修改每个数组元素里面index和weight值，代码如下：

#include <iostream>
#include <fstream>
#include "json.hpp"using json = nlohmann::json;int main() {std::ifstream file("your_file.json");if (!file.is_open()) {std::cerr << "Failed to open file." << std::endl;return 1;}json jsonData;file >> jsonData;file.close();// 遍历 AutoExposure 数组if (jsonData.find("test") != jsonData.end() && jsonData["test"].is_array()) {for (auto& entry : jsonData["test"]) {// 修改每个对象的 "Index" 值为 10 和 20entry["Index"] = (entry["Index"] == 1) ? 10 : 20;  //这里可以通过json其他键值对判断目前遍历的是数组第几个元素// 修改每个对象的 "weight" 值为 0.5 和 0.7if (entry.find("IsUseROIs") != entry.end() && entry["IsUseROIs"].find("ROIS") != entry["IsUseROIs"].end()) {for (auto& rois_entry : entry["IsUseROIs"]["ROIS"]) {rois_entry["weight"] = (rois_entry["weight"] == 1) ? 0.5 : 0.7;  //这里可以通过json其他键值对判断目前遍历的是数组第几个元素}  }}}// 将更新后的 JSON 数据写入文件std::ofstream outputFile("updated_file.json");if (!outputFile.is_open()) {std::cerr << "Failed to open file for writing." << std::endl;return 1;}outputFile << std::setw(4) << jsonData << std::endl; // 美化输出，每个元素缩进四个空格outputFile.close();return 0;
}

（5）修改原先的键值对之情形2
原先的json文件如下：

{"Connection": {"IpAddress": "192.168.20.1","Rock": 0,"Solt": 1}, "DataBaseNumber":7,"hardWare":{"Axis1_offset_Enable": "0.0","Axis1_offset_Stop": "0.1","Axis1_offset_Speed": "2.0","Axis1_offset_Mov": "4.0","Axis1_offset_Pos": "6.0","Axis1_offset_Busy": "8.1","Axis2_offset_Enable": "10.0","Axis2_offset_Stop": "10.1","Axis2_offset_Speed": "12.0","Axis2_offset_Mov": "14.0","Axis2_offset_Pos": "16.0","Axis2_offset_Busy": "18.1"}
}

现想修改修改Axis2_offset_Busy为18.2，代码如下：

#include <iostream>
#include <fstream>
#include "json.hpp"using json = nlohmann::json;int main() {std::ifstream file("your_file.json");if (!file.is_open()) {std::cerr << "Failed to open file." << std::endl;return 1;}json jsonData;file >> jsonData;file.close();// 修改特定键的值if (jsonData.find("hardWare") != jsonData.end()) {jsonData["hardWare"]["Axis2_offset_Busy"] = "18.2";}// 将更新后的 JSON 数据写入文件std::ofstream outputFile("updated_file.json");if (!outputFile.is_open()) {std::cerr << "Failed to open file for writing." << std::endl;return 1;}outputFile << std::setw(4) << jsonData << std::endl; // 美化输出，每个元素缩进四个空格outputFile.close();return 0;
}