c++程序中使用nlohmann解析JSON文件

Question

I have a very basic function below that I am trying to use in hopes of parsing a simple JSON object from a file. I have looked at similar questions that get the same [json.exception.parse_error.101 which all point to the fact that the file is not being opened, however from my understanding that does not seem to be the issue here. I am able to print out the buffer stream which matches my JSON file exactly, so I know it is finding the file and extracting its contents. I am new to using the nlohmann library and am truly at a loss for why the program keeps throwing this error. I would greatly appreciate any pointers as to why this might be happening.

.cpp file

#include <nlohmann/json.hpp>
#include <fstream>
#include <iostream>
 
int main(int argc, char *argv[]) {
    std::ifstream myfile("test.json");
    if (myfile.is_open()) {
 
        std::cout << myfile.rdbuf() << std::endl;
        nlohmann::json data = nlohmann::json::parse(myfile);
    }
    return 0;
}

.json file

{
    "firstName" : "Jane",
    "lastName" : "Doe",
    "age" : 100
}

print statement/ error message

{
    "firstName" : "Jane",
    "lastName" : "Doe",
    "age" : 100
}
terminate called after throwing an instance of 'nlohmann::detail::parse_error'
  what():  [json.exception.parse_error.101] parse error at line 1, column 1: syntax error while parsing value - unexpected end of input; expected '[', '{', or a literal

other solutions

I have also tried using an older method to store the JSON object in my data variable using this operator myfile >> data , but the same error is thrown.

Answer 1

std::cout << myfile.rdbuf() reads the entire file, and sends it to std::cout . Once that's done, there's no more data left to read, so when you pass myfile to json::parse , that function immediately gets an end of file indication.

If you want to both print the file and parse it, you'll need to rewind the file (seek to 0) or reopen it after you print it.

I don't know if this has anything to do with your original problem. If you haven't yet done this, try calling json::parse before you print the file (which will mean that the EOF will be received by std::cout << myfile.rdbuf() instead of json::parse ; it still won't both print and parse the file).