api/basic_json_view/source_offset.md

# <small>nlohmann::basic_json_view::</small>source_offset

```cpp
std::size_t source_offset() const noexcept;
```

Returns the byte offset of this value in the document's [`source()`](../basic_json_document/source.md) text, without
materializing anything.

## Return value

- For a string with no escapes, a number, `#!json true`/`#!json false`/`#!json null`, an array, or an object: the
  byte offset of the first byte of the value's token (for a string: the first byte after the opening quote) in
  [`source()`](../basic_json_document/source.md).
- `#!cpp static_cast<std::size_t>(-1)` for a [discarded](is_discarded.md) view, and for a string that contains
  escapes — such a string was decoded once into the document's own buffer, so there is no single byte range in
  `source()` left to point at.

## Exception safety

No-throw guarantee: this function never throws exceptions.

## Complexity

Constant.

## Notes

This is a raw offset, not a length: the API does not (yet) expose how many bytes the token occupies in the source
text, so `source_offset()` alone is enough to report *where* a value came from (for an error message, for syntax
highlighting, ...) but not to slice its exact text back out of [`source()`](../basic_json_document/source.md) for a
string, whose token length is not the same as its decoded [`size()`](size.md).

## Examples

??? example

    ```cpp
    #include <iostream>
    #include <string>
    #include <nlohmann/json_view.hpp>
    
    using nlohmann::json_document;
    
    int main()
    {
        std::cout << std::boolalpha;
    
        // source_offset() points into source(): useful to report *where* in the
        // original text a value came from (error messages, syntax highlighting,
        // forwarding a sub-range verbatim, ...) without materializing it
        json_document doc = json_document::parse(R"(  123)");
        auto v = doc.root();
        std::cout << v.source_offset() << ' '
                  << std::string(doc.source().data() + v.source_offset(), 3) << '\n';
    
        // a string without escapes also stays in the source text
        json_document plain = json_document::parse(R"("ab")");
        std::cout << plain.source()[plain.root().source_offset()] << '\n';
    
        // a string with escapes is decoded once into the document's own buffer, so
        // there is no single byte range in source() to point at: source_offset()
        // returns the "not applicable" sentinel
        json_document escaped = json_document::parse(R"("a\nb")");
        std::cout << (escaped.root().source_offset() == static_cast<std::size_t>(-1)) << '\n';
    
        // a discarded view -- default-constructed, or the root of a failed parse
        // with allow_exceptions == false -- has no offset either
        json_document failed = json_document::parse("not json", false);
        std::cout << (failed.root().source_offset() == static_cast<std::size_t>(-1)) << '\n';
    }
    
    ```

    Output:

    ```json
    2 123
    a
    true
    true
    
    ```

## See also

- [`source`](../basic_json_document/source.md) - the parsed text
- [`is_string`](is_string.md) - return whether the value is a string
- [Zero-copy JSON views](../../features/json_view.md#images) - images and the source text

## Version history

- Added in version 3.13.0.