json/features/parsing/parse_exceptions.md

Parsing and Exceptions

When the input is not valid JSON, an exception of type parse_error is thrown. This exception contains the position in the input where the error occurred, together with a diagnostic message and the last read input token. The exceptions page contains a list of examples for parse error exceptions. In case you process untrusted input, always enclose your code with a #!cpp try/#!cpp catch block, like

json j;
try
{
    j = json::parse(my_input);
}
catch (json::parse_error& ex)
{
    std::cerr << "parse error at byte " << ex.byte << std::endl;
}

In case exceptions are undesired or not supported by the environment, there are different ways to proceed:

Switch off exceptions

The parse() function accepts a #!cpp bool parameter allow_exceptions which controls whether an exception is thrown when a parse error occurs (#!cpp true, default) or whether a discarded value should be returned (#!cpp false).

json j = json::parse(my_input, nullptr, false);
if (j.is_discarded())
{
    std::cerr << "parse error" << std::endl;
}

Note there is no diagnostic information available in this scenario.

Use accept() function

Alternatively, function accept() can be used which does not return a json value, but a #!cpp bool indicating whether the input is valid JSON.

if (!json::accept(my_input))
{
    std::cerr << "parse error" << std::endl;
}

Again, there is no diagnostic information available.

User-defined SAX interface

Finally, you can implement the SAX interface and decide what should happen in case of a parse error.

This function has the following interface:

bool parse_error(std::size_t position,
                 const std::string& last_token,
                 const json::exception& ex);

The return value indicates whether the parsing should continue, so the function should usually return #!cpp false.

??? example "Example: report parse errors without exceptions"

The example derives from the library's DOM parser and overrides `parse_error` to print the error instead of
throwing. Note the DOM parser is an implementation detail (`nlohmann::detail`) and may change between releases;
see [Do not use the `detail` namespace](../../integration/migration_guide.md#do-not-use-the-detail-namespace).

```cpp
--8<-- "examples/sax_no_exception.cpp"
```

Output:

```
--8<-- "examples/sax_no_exception.output"
```