# nlohmann::basic_json_view::source_offset

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

Returns the byte offset of this value in the document's [`source()`](https://json.nlohmann.me/api/basic_json_document/source/index.md) text, without materializing anything.

## Return value

- For a string with no escapes, a number, `true`/`false`/`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()`](https://json.nlohmann.me/api/basic_json_document/source/index.md).
- `static_cast<std::size_t>(-1)` for a [discarded](https://json.nlohmann.me/api/basic_json_view/is_discarded/index.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()`](https://json.nlohmann.me/api/basic_json_document/source/index.md) for a string, whose token length is not the same as its decoded [`size()`](https://json.nlohmann.me/api/basic_json_view/size/index.md).

## Examples

Example

```
#include <iostream>
#include <string>
#include <nlohmann/json_view.hpp>

using json_document = 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:

```
2 123
a
true
true
```

## See also

- [source](https://json.nlohmann.me/api/basic_json_document/source/index.md) - the parsed text
- [is_string](https://json.nlohmann.me/api/basic_json_view/is_string/index.md) - return whether the value is a string

## Version history

- Added in version 3.13.0 unreleased.