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api/basic_json_view/source_offset.md

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+# <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 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:
+
+    ```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
+
+## Version history
+
+- Added in version 3.13.0.