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nlohmann
65 lines
2.9 KiB
Markdown
65 lines
2.9 KiB
Markdown
# Access with default value: value
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## Overview
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In many situations, such as configuration files, missing values are not exceptional, but may be treated as if a default
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value was present. For this case, use [`value(key, default_value)`](../../api/basic_json/value.md) which takes the key
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you want to access and a default value in case there is no value stored with that key.
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## Example
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??? example
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Consider the following JSON value:
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```json
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{
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"logOutput": "result.log",
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"append": true
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}
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```
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Assume the value is parsed to a `json` variable `j`.
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| expression | value |
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|---------------------------------------------|------------------------------------------------------|
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| `#!cpp j` | `#!json {"logOutput": "result.log", "append": true}` |
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| `#!cpp j.value("logOutput", "logfile.log")` | `#!json "result.log"` |
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| `#!cpp j.value("append", true)` | `#!json true` |
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| `#!cpp j.value("append", false)` | `#!json true` |
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| `#!cpp j.value("logLevel", "verbose")` | `#!json "verbose"` |
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## Notes
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!!! failure "Exceptions"
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- With string keys, `value` can only be used with objects. For other types, a [`basic_json::type_error`](../../home/exceptions.md#jsonexceptiontype_error306) is thrown.
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- With JSON Pointers, `value` can be used with both objects and arrays. For other types (null, boolean, number, string), a [`basic_json::type_error`](../../home/exceptions.md#jsonexceptiontype_error306) is thrown.
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!!! warning "Return type"
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The value function is a template, and the return type of the function is determined by the type of the provided
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default value unless otherwise specified. This can have unexpected effects. In the example below, we store a 64-bit
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unsigned integer. We get exactly that value when using [`operator[]`](../../api/basic_json/operator[].md). However,
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when we call `value` and provide `#!c 0` as default value, then `#!c -1` is returned. This occurs, because `#!c 0`
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has type `#!c int` which overflows when handling the value `#!c 18446744073709551615`.
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To address this issue, either provide a correctly typed default value or use the template parameter to specify the
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desired return type. Note that this issue occurs even when a value is stored at the provided key, and the default
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value is not used as the return value.
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```cpp
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--8<-- "examples/value__return_type.cpp"
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```
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Output:
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```json
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--8<-- "examples/value__return_type.output"
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```
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## See also
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- [`value`](../../api/basic_json/value.md) for access with default value
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- documentation on [checked access](checked_access.md)
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