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nlohmann
69 lines
1.9 KiB
Markdown
69 lines
1.9 KiB
Markdown
# <small>nlohmann::basic_json::</small>start_pos
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```cpp
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#if JSON_DIAGNOSTIC_POSITIONS
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constexpr std::size_t start_pos() const noexcept;
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#endif
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```
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Returns the position of the first character in the JSON string from which the value was parsed from.
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| JSON type | return value |
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|-----------|------------------------------------------------|
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| object | position of the opening `{` |
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| array | position of the opening `[` |
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| string | position of the opening `"` |
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| number | position of the first character |
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| boolean | position of `t` for `true` and `f` for `false` |
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| null | position of `n` |
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## Return value
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the position of the first character of the value in the parsed JSON string, if the value was created by the
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[`parse`](parse.md) function, or `std::string::npos` if the value was constructed otherwise
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## Exception safety
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No-throw guarantee: this member function never throws exceptions.
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## Complexity
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Constant.
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## Notes
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!!! note "Note"
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The function is only available if macro [`JSON_DIAGNOSTIC_POSITIONS`](../macros/json_diagnostic_positions.md) has
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been defined to `#!cpp 1` before including the library header.
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!!! warning "Invalidation"
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The returned positions are only valid as long as the JSON value is not changed. The positions are *not* updated
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when the JSON value is changed.
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## Examples
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??? example "Example"
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```cpp
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--8<-- "examples/diagnostic_positions.cpp"
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```
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Output:
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```
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--8<-- "examples/diagnostic_positions.output"
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```
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The output shows the start/end positions of all the objects and fields in the JSON string.
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## See also
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- [end_pos](end_pos.md) to access the end position
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- [JSON_DIAGNOSTIC_POSITIONS](../macros/json_diagnostic_positions.md) for an overview of the diagnostic positions
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## Version history
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- Added in version 3.12.0.
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