nlohmann::basic_json::start_pos

#if JSON_DIAGNOSTIC_POSITIONS
constexpr std::size_t start_pos() const noexcept;
#endif

Returns the position of the first character in the JSON string from which the value was parsed from.

JSON type	return value
object	position of the opening `{`
array	position of the opening `[`
string	position of the opening `"`
number	position of the first character
boolean	position of `t` for `true` and `f` for `false`
null	position of `n`

Return value

the position of the first character of the value in the parsed JSON string, if the value was created by the parse function, or std::string::npos if the value was constructed otherwise

Exception safety

No-throw guarantee: this member function never throws exceptions.

Complexity

Constant.

Notes

!!! note "Note"

The function is only available if macro [`JSON_DIAGNOSTIC_POSITIONS`](../macros/json_diagnostic_positions.md) has
been defined to `#!cpp 1` before including the library header.

!!! warning "Invalidation"

The returned positions are only valid as long as the JSON value is not changed. The positions are *not* updated
when the JSON value is changed.

Examples

??? example "Example"

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

Output:

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

The output shows the start/end positions of all the objects and fields in the JSON string.

Version history

Added in version 3.12.0.

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