c – 读取和写入同一文件fstream

首先是命令json json_data(fs);读到文件末尾设置EOF标志.流将停止工作,直到该标志被清除.

其次,文件指针位于文件的末尾.如果要覆盖文件,则需要再次返回到开头：

if (fs.is_open())
{
    json json_data(fs); // reads to end of file
    fs.clear(); // clear flag
    fs.seekg(0); // move to beginning

遗憾的是,仍然没有解决所有问题,因为如果您回写的文件小于您读回的文件,那么将会有一些旧数据标记到新数据的末尾：

std::cout << "Operation successfully performedn";
    std::cout << json_data.at("Number") << std::endl;
    std::cout << json_data.at("Test") << std::endl;
    std::cout << json_data.at("Foo") << std::endl;

    json_data.at("Foo") = 4.32; // what if new data is smaller?

Json文件：

{
   "Foo": 4.32,// this number is smaller than before
   "Number": 100,"Test": "Test1"
}} // whoops trailing character from previous data!!

在这种情况下,我只需打开一个文件进行读取,然后打开另一个进行写入,它更不容易出错,并表达了覆盖所有内容的意图.

就像是：

#include "json.hpp"
#include <iostream>
#include <fstream>
#include <string>

using json = nlohmann::json;

void readAndWriteDataToFile(std::string fileName) {

    json json_data;

    // restrict scope of file object (auto-closing raii)
    if(auto fs = std::ifstream(fileName))
    {
        json_data = json::parse(fs);

        std::cout << "Operation successfully performedn";
        std::cout << json_data.at("Number") << std::endl;
        std::cout << json_data.at("Test") << std::endl;
        std::cout << json_data.at("Foo") << std::endl;
    }
    else
    {
        throw std::runtime_error(std::strerror(errno));
    }

    json_data.at("Foo") = 4.32;
    std::cout << json_data.at("Foo") << std::endl;
    std::string json_content = json_data.dump(3);

    if(auto fs = std::ofstream(fileName))
    {
        fs.write(json_content.data(),json_content.size());
        std::cout << "Done" << std::endl;
    }
    else
    {
        throw std::runtime_error(std::strerror(errno));
    }

}

int main()
{
    try
    {
        std::string fileName = "C:/new/json1.json";
        readAndWriteDataToFile(fileName);
    }
    catch(std::exception const& e)
    {
        std::cerr << e.what() << 'n';
        return EXIT_FAILURE;
    }
    return EXIT_SUCCESS;
}