parse vs accept

[bajaj689]

Hi All, I am writing an API where the request will contain a JSON string.
After reading this request, I want to check if JSON is valid or not. For that I came across the function named "accept". I want to know what is the use of the function "parse", because this function can also tell us whether JSON string was valid or not, ofcourse it has ability to throw expections a/c to parameter passed.
My question is that usage wise, when should parse be used and where should accept be used ? Do they provide same functionality?

[nlohmann]

accept is to check validity, so you only get a boolean as result.

parse is to get a basic_json object from the input.

[bajaj689]

In the first image, the functional signature shows 4 arguments. In image 2, it rightly says that 3rd argument is for specifying exception flag. But If I give 4 arguments, it gives compilation error."no matching function for call to ‘nlohmann::basic_json<>::parse(char [1024], std::nullptr_t, bool, bool, "

Does the signature need to be updated in document? Or I am getting this wrong ?

[falbrechtskirchinger]

What version are you using?

[bajaj689]

2.1.1

[falbrechtskirchinger]

That is ancient. 3.11.0 will be released soon. The latest available version is 3.10.5, I believe.

[bajaj689]

Ok, Thanks

[lefebvresam]

If you use a file, for an unknown reason, the file variabele can not be used twice after each other in parse or accept. The second time an empty file is reading.

FILE *file = fopen(fullpath.c_str(), "rb");
if (!json::accept(file)) {
    unmountSD();
    ERROR("invalid json in %s", fullpath.c_str());
    return 1;
}
try{
   json app = json::parse(file); // this throws error 101 unless you comment the accept line
} catch (json::parse_error& ex) {
    unmountSD();
    ERROR("%s", ex.what());
    return 1;
}

[json.exception.parse_error.101] parse error at line 1, column 1: attempting to parse an empty input; check that your input string or stream contains the expected JSON

[nlohmann]

That is expected behavior.

Reading from a C FILE* or a C++ std::ifstream twice without proper handling doesn't work as expected because file streams maintain an internal position marker that advances with each read operation.

Before reading a second time, you need to rewind the file stream.

[lefebvresam]

Any example?

[natri0]

If you're reading from a FILE *:

FILE *fp = fopen(/* ... */);
if (json::accept(file)) {
  fseek(fp, 0, SEEK_SET);
  auto j = json::parse(fp);
  printf("read json: %s", j.dump().c_str());
} else {
  fprintf(stderr, "not valid json\n");
}

[nlohmann]

Why don't you just parse the JSON immediately? This will take twice as long as you need to go through the file twice.

[lefebvresam]

Ok now I use the method with discarded and this works.