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84 lines
2.6 KiB
Markdown
84 lines
2.6 KiB
Markdown
# Parsing and Exceptions
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When the input is not valid JSON, an exception of type [`parse_error`](../../home/exceptions.md#parse-errors) is thrown.
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This exception contains the position in the input where the error occurred, together with a diagnostic message and the
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last read input token. The exceptions page contains a
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[list of examples for parse error exceptions](../../home/exceptions.md#parse-errors). In case you process untrusted
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input, always enclose your code with a `#!cpp try`/`#!cpp catch` block, like
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```cpp
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json j;
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try
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{
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j = json::parse(my_input);
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}
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catch (json::parse_error& ex)
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{
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std::cerr << "parse error at byte " << ex.byte << std::endl;
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}
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```
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In case exceptions are undesired or not supported by the environment, there are different ways to proceed:
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## Switch off exceptions
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The [`parse()`](../../api/basic_json/parse.md) function accepts a `#!cpp bool` parameter `allow_exceptions` which
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controls whether an exception is thrown when a parse error occurs (`#!cpp true`, default) or whether a discarded value
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should be returned (`#!cpp false`).
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```cpp
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json j = json::parse(my_input, nullptr, false);
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if (j.is_discarded())
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{
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std::cerr << "parse error" << std::endl;
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}
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```
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Note there is no diagnostic information available in this scenario.
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## Use accept() function
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Alternatively, function [`accept()`](../../api/basic_json/accept.md) can be used which does not return a `json` value,
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but a `#!cpp bool` indicating whether the input is valid JSON.
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```cpp
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if (!json::accept(my_input))
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{
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std::cerr << "parse error" << std::endl;
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}
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```
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Again, there is no diagnostic information available.
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## User-defined SAX interface
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Finally, you can implement the [SAX interface](sax_interface.md) and decide what should happen in case of a parse error.
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This function has the following interface:
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```cpp
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bool parse_error(std::size_t position,
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const std::string& last_token,
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const json::exception& ex);
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```
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The return value indicates whether the parsing should continue, so the function should usually return `#!cpp false`.
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??? example "Example: report parse errors without exceptions"
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The example derives from the library's DOM parser and overrides `parse_error` to print the error instead of
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throwing. Note the DOM parser is an implementation detail (`nlohmann::detail`) and may change between releases;
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see [Do not use the `detail` namespace](../../integration/migration_guide.md#do-not-use-the-detail-namespace).
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```cpp
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--8<-- "examples/sax_no_exception.cpp"
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```
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Output:
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```
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--8<-- "examples/sax_no_exception.output"
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```
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