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api/operator_ltlt/index.md

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+# nlohmann::operator\<<(basic_json), nlohmann::operator\<<(json_pointer)
+
+```
+std::ostream& operator<<(std::ostream& o, const basic_json& j);      // (1)
+
+std::ostream& operator<<(std::ostream& o, const json_pointer& ptr);  // (2)
+```
+
+1. Serialize the given JSON value `j` to the output stream `o`. The JSON value will be serialized using the [`dump`](https://json.nlohmann.me/api/basic_json/dump/index.md) member function.
+   - The indentation of the output can be controlled with the member variable `width` of the output stream `o`. For instance, using the manipulator `std::setw(4)` on `o` sets the indentation level to `4` and the serialization result is the same as calling `dump(4)`.
+   - The indentation character can be controlled with the member variable `fill` of the output stream `o`. For instance, the manipulator `std::setfill('\\t')` sets indentation to use a tab character rather than the default space character.
+1. Write a string representation of the given JSON pointer `ptr` to the output stream `o`. The string representation is obtained using the [`to_string`](https://json.nlohmann.me/api/json_pointer/to_string/index.md) member function.
+
+## Parameters
+
+`o` (in, out) : stream to write to
+
+`j` (in) : JSON value to serialize
+
+`ptr` (in) : JSON pointer to write
+
+## Return value
+
+the stream `o`
+
+## Exceptions
+
+1. Throws [`type_error.316`](https://json.nlohmann.me/home/exceptions/#jsonexceptiontype_error316) if a string stored inside the JSON value is not UTF-8 encoded. Note that unlike the [`dump`](https://json.nlohmann.me/api/basic_json/dump/index.md) member functions, no `error_handler` can be set.
+1. None.
+
+## Complexity
+
+Linear.
+
+## Notes
+
+Deprecation
+
+Function `std::ostream& operator<<(std::ostream& o, const basic_json& j)` replaces function `std::ostream& operator>>(const basic_json& j, std::ostream& o)` which has been deprecated in version 3.0.0. It will be removed in version 4.0.0. Please replace calls like `j >> o;` with `o << j;`.
+
+## Examples
+
+Example: (1) serialize JSON value to stream
+
+The example below shows the serialization with different parameters to `width` to adjust the indentation level.
+
+```
+#include <iostream>
+#include <iomanip>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // create JSON values
+    json j_object = {{"one", 1}, {"two", 2}};
+    json j_array = {1, 2, 4, 8, 16};
+
+    // serialize without indentation
+    std::cout << j_object << "\n\n";
+    std::cout << j_array << "\n\n";
+
+    // serialize with indentation
+    std::cout << std::setw(4) << j_object << "\n\n";
+    std::cout << std::setw(2) << j_array << "\n\n";
+    std::cout << std::setw(1) << std::setfill('\t') << j_object << "\n\n";
+}
+```
+
+Output:
+
+```
+{"one":1,"two":2}
+
+[1,2,4,8,16]
+
+{
+    "one": 1,
+    "two": 2
+}
+
+[
+  1,
+  2,
+  4,
+  8,
+  16
+]
+
+{
+    "one": 1,
+    "two": 2
+}
+```
+
+Example: (2) write JSON pointer to stream
+
+The example below shows how to write a JSON pointer to a stream.
+
+```
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // create JSON pointer
+    json::json_pointer ptr("/foo/bar/baz");
+
+    // write string representation to stream
+    std::cout << ptr << std::endl;
+}
+```
+
+Output:
+
+```
+/foo/bar/baz
+```
+
+## See also
+
+- [dump](https://json.nlohmann.me/api/basic_json/dump/index.md) - serialize to a JSON-formatted string
+- [Serialization](https://json.nlohmann.me/features/serialization/index.md) - the serialization article
+
+## Version history
+
+1. Added in version 1.0.0. Added support for indentation character and deprecated `std::ostream& operator>>(const basic_json& j, std::ostream& o)` in version 3.0.0.
+1. Added in version 3.11.0.