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116 lines
3.9 KiB
Markdown
116 lines
3.9 KiB
Markdown
# <small>nlohmann::</small>operator>>(basic_json)
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```cpp
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std::istream& operator>>(std::istream& i, basic_json& j);
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```
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Deserializes an input stream to a JSON value.
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## Parameters
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`i` (in, out)
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: input stream to read a serialized JSON value from
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`j` (in, out)
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: JSON value to write the deserialized input to
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## Return value
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the stream `i`
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## Exceptions
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- Throws [`parse_error.101`](../home/exceptions.md#jsonexceptionparse_error101) in case of an unexpected token.
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## Complexity
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Linear in the length of the input. The parser is a predictive LL(1) parser.
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## Notes
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A UTF-8 byte order mark is silently ignored.
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Invalid Unicode escapes and unpaired surrogates in the input are reported as
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[`parse_error.101`](../home/exceptions.md#jsonexceptionparse_error101) with a detailed message.
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`operator>>` parses exactly one JSON value, so it can be called repeatedly to read a sequence of concatenated JSON
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values from the same stream:
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```cpp
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json j1, j2;
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input >> j1; // parses the first value
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input >> j2; // parses the next value
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```
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!!! warning "A number must be followed by whitespace"
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A number is only terminated by the character that follows it. That character is read from the stream to detect the
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end of the number, and it is **not** put back. When a value that is a number is immediately followed by the next
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value, the first character of that next value is lost:
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```cpp
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std::istringstream input("1true");
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json j1, j2;
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input >> j1; // j1 == 1
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input >> j2; // throws parse_error.101: the stream now starts at "rue"
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```
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Separating the values with whitespace avoids this, because the character that is eaten is then the separator:
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```cpp
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std::istringstream input("1 true");
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json j1, j2;
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input >> j1; // j1 == 1
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input >> j2; // j2 == true
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```
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Only numbers are affected. Values ending in a self-delimiting character do not read past themselves, so
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`truefalse`, `[1][2]`, `{"a":1}{"b":2}`, and `"a""b"` can be read back to back without a separator.
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This is tracked in [#5340](https://github.com/nlohmann/json/issues/5340).
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Note that reading concatenated values does **not** work for [JSON Lines](../features/parsing/json_lines.md)
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(newline-delimited JSON) input -- see that page for why and for the recommended alternative.
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By default, a `'\0'` (NUL) byte encountered while reading a value is treated as end of input, rather than as an
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ordinary (and, outside of a string, invalid) byte; see the [FAQ entry](../home/faq.md#nul-bytes-in-the-input) for
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details and the [`JSON_STRICT_NUL_HANDLING`](macros/json_strict_nul_handling.md) macro to opt into rejecting it
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instead. Because `operator>>` only parses a single value and does not require the rest of the stream to be consumed,
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a NUL byte *after* a complete value has no effect on `operator>>` either way; it only matters while a value is still
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being read.
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!!! warning "Deprecation"
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This function replaces function `#!cpp std::istream& operator<<(basic_json& j, std::istream& i)` which has
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been deprecated in version 3.0.0. It will be removed in version 4.0.0. Please replace calls like `#!cpp j << i;`
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with `#!cpp i >> j;`.
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## Examples
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??? example
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The example below shows how a JSON value is constructed by reading a serialization from a stream.
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```cpp
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--8<-- "examples/operator_deserialize.cpp"
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```
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Output:
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```json
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--8<-- "examples/operator_deserialize.output"
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```
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## See also
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- [accept](basic_json/accept.md) - check if the input is valid JSON
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- [parse](basic_json/parse.md) - deserialize from a compatible input
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- [`JSON_STRICT_NUL_HANDLING`](macros/json_strict_nul_handling.md) - opt in to rejecting a NUL byte in the input
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instead of treating it as end of input
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## Version history
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- Added in version 1.0.0.
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- `JSON_STRICT_NUL_HANDLING` added in version 3.13.0 to optionally reject a NUL byte in the input instead of treating
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it as end of input; planned to become the default in version 4.0.0.
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