Can't throw exception when starting file is a number

[h3x4n1um]

• What is the issue you have?

When parsing binary file, if several first bytes of file is number, the json parser not throw an exception

• Please describe the steps to reproduce the issue. Can you provide a small but working code example?

The below example show that it recognize fake_undetected.json file is a json file

#include <filesystem>
#include <fstream>
#include <iomanip>
#include <iostream>

#include "json.hpp"

int main(){
    std::ifstream input;
    std::ofstream output;
    nlohmann::json js;
    int test = 2019;

    //create real json
    output.open("real.json");
    js["Hello"] = "Hi";
    js["Year"] = test;
    output << std::setw(4) << js;
    output.close();

    //create detected fake json file
    output.open("fake_detected.json", std::ofstream::binary);
    output.write(reinterpret_cast <const char*> (&test), sizeof test);
    output.close();

    //create undetected fake json file
    output.open("fake_undetected.json", std::ofstream::binary);
    output << test;
    output.write(reinterpret_cast <const char*> (&test), sizeof test);
    output.close();

    //verify json
    std::filesystem::path p = std::filesystem::current_path();
    for (auto &file : std::filesystem::recursive_directory_iterator(p)){
        if (std::filesystem::is_regular_file(file.path())){
            input.open(file.path());
            try{
                input >> js;
                std::cout << file.path() << " is a json" << std::endl;
            }
            catch(nlohmann::json::exception &e){
                std::cerr << file.path() << " is not a json" << std::endl;
            }
            input.close();
        }
    }
    return 0;
}

• What is the expected behavior?

It should throw nlohmann::json::exception when parse fake_undetected.json

• And what is the actual behavior instead?

Opposite above

• Which compiler and operating system are you using? Is it a supported compiler?

I use GCC 9.0 (MSYS2 with MinGW-w64 package) and compile using

g++ -lstdc++fs -std=c++17 -c test.cpp -o test.o

• Did you use a released version of the library or the version from the develop branch?

I use released version 3.6.1 of the library

• If you experience a compilation error: can you compile and run the unit tests?

Hello, world!

[nlohmann]

The issue is that parsing via input >> json is not required to read the stream until the last character: if the prefix is a JSON value, it is returned and the stream is left at the first character after the parsed value. This allows for incremental parsing from a stream.

For your case, try using the static function json::parse. It takes the input as parameter and will throw if the input is not completely read.

Here is an example:

#include "json.hpp"
#include <iostream>
#include <sstream>

using json = nlohmann::json;

int main()
{
    // works and prints "123"
    {
        std::stringstream ss;
        ss << "123ab" << std::endl;
        
        json j;
        ss >> j;
        
        std::cout << j << std::endl;
    }
    
    // throws [json.exception.parse_error.101] parse error at line 1, column 4: syntax error while parsing value - invalid literal; last read: '123a'; expected end of input
    {
        std::stringstream ss;
        ss << "123ab" << std::endl;

        json j = json::parse(ss);
    }
}

[nlohmann]

@h3x4n1um Do you need further assistance?

[h3x4n1um]

@h3x4n1um Do you need further assistance?

Thankyou, that's very kind of you but I think using json::parse as a workaround is fine for me until the problem is fixed

[nlohmann]

For operator>> I cannot pass a parameter to control whether trailing characters should be treated like an error. For parse I could add another parameter, but this is not yet planned. So at least for the near future, there will be no fix for this.