json/docs/mkdocs/docs/features/no_exceptions.md

# Working Without Exceptions

By default, the library reports errors by throwing exceptions (see [Exceptions](../home/exceptions.md)). This page
collects what to use when exceptions are unwanted or unavailable, for example in embedded systems, game engines, or
code compiled with `-fno-exceptions`.

## Two different questions

1. **Can my code avoid *catching* exceptions?** Yes. For parsing and for accessing values there are functions that
   report failure through a return value. They work in a normal build and need no `#!cpp try`/`#!cpp catch`.
2. **Can the library be *compiled* without exceptions?** Yes, with `-fno-exceptions` or by defining
   [`JSON_NOEXCEPTION`](../api/macros/json_noexception.md). Both have the same effect: every `#!cpp throw` in the
   library becomes a call to `std::abort()`. The functions listed below never reach a `#!cpp throw` for the failures
   they report, so they keep working in this configuration. Anything else that fails aborts the program.

## Parsing

| function                                                                                    | on invalid input                                    |
|---------------------------------------------------------------------------------------------|-----------------------------------------------------|
| `#!cpp json::parse(input, nullptr, false)`                                                  | returns a *discarded* value; test with `is_discarded()` |
| [`accept`](../api/basic_json/accept.md)                                                     | returns `#!cpp false`                               |
| `#!cpp json::from_cbor(input, true, false)` (and the other `from_*` functions)              | returns a *discarded* value                         |
| [`sax_parse`](../api/basic_json/sax_parse.md) with your own handler                         | returns `#!cpp false` after calling `parse_error`   |

??? example "Example: parse and validate without exceptions"

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

    Output:

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

None of the calls in this example throws, so it also compiles and runs with `-fno-exceptions`.

The discarded value carries no message. If you need the position or the reason, use the SAX interface; see
[Parsing and exceptions](parsing/parse_exceptions.md).

## Accessing values

The access functions are not equally safe. Only some of them never throw:

| expression                          | if the key is missing or the type is wrong                                    |
|-------------------------------------|-------------------------------------------------------------------------------|
| `#!cpp j.contains("k")`             | returns `#!cpp false` for a missing key; never throws                         |
| `#!cpp j.find("k")`                 | returns `end()` for a missing key; never throws                               |
| `#!cpp j.is_string()`, ...          | the `is_*` functions test the type; never throw                               |
| `#!cpp j.get_ptr<const std::string*>()` | returns `#!cpp nullptr` for a different type; never throws                |
| `#!cpp j.at("k")`                   | throws `out_of_range.403` for a missing key                                   |
| `#!cpp j.get<int>()`                | throws `type_error.302` for a value that is not a number                      |
| `#!cpp j.value("k", 0)`             | returns the default for a missing key, but **throws `type_error.302` if `k` exists with another type** |
| `#!cpp j["k"]` (non-const)          | inserts `null` for a missing key                                              |
| `#!cpp j["k"]` (const)              | undefined behavior for a missing key                                          |

So reading a value that may be absent or of the wrong type without exceptions takes two steps: look up, then test the
type.

??? example "Example: look up a key and test its type"

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

    Output:

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

The last lines of the example show why `value` is not on the safe list: it returns the default only for a *missing*
key.

For numbers, test with `is_number()` (or `is_number_integer()`, `is_number_unsigned()`, `is_number_float()`) before
calling `get<int>()`, and see [number handling](types/number_handling.md) for range and precision. Whether a value
fits into the target type is not checked by `get`.

## Converting to your own types

`from_json` functions (including the ones generated by the
[`NLOHMANN_DEFINE_TYPE_*` macros](arbitrary_types.md#simplify-your-life-with-macros)) report problems by throwing.
There is no variant that returns an error code. Without exceptions you have two options:

- Validate the JSON value yourself, as shown above, and fill your struct field by field.
- Convert in a function that is compiled **with** exceptions enabled and catches them at the boundary, returning
  `std::optional` or an error code to the rest of your code:

??? example "Example: convert to a struct and report failure as std::optional"

    ```cpp
    --8<-- "examples/no_exceptions_convert.c++17.cpp"
    ```

    Output:

    ```
    --8<-- "examples/no_exceptions_convert.c++17.output"
    ```

## Overriding what happens instead of an exception

With exceptions off, you can replace the `abort()` by your own code, for example to log the message first. See
[Switch off exceptions](../home/exceptions.md#switch-off-exceptions) and
[`JSON_THROW_USER`](../api/macros/json_throw_user.md). The handler must not return normally.

## See also

- [Exceptions](../home/exceptions.md) - all exceptions and their ids
- [Parsing and exceptions](parsing/parse_exceptions.md) - the parsing alternatives in detail
- [Parsing untrusted input](parsing/untrusted_input.md) - harden an application that parses data it does not control
- [`JSON_NOEXCEPTION`](../api/macros/json_noexception.md) - disable exceptions in the library
- [`exception`](../api/basic_json/exception.md) - base class of all exceptions thrown by the library
- [`JSON_THROW_USER`](../api/macros/json_throw_user.md) - replace the throw statements of the library
- [`parse`](../api/basic_json/parse.md) - deserialize from a compatible input
- [`accept`](../api/basic_json/accept.md) - check whether the input is valid JSON
- [Supported Macros](macros.md) - all supported macros