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nlohmann
101 lines
2.6 KiB
Markdown
101 lines
2.6 KiB
Markdown
# nlohmann::basic_json::empty
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```
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bool empty() const noexcept;
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```
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Checks if a JSON value has no elements (i.e., whether its [`size()`](https://json.nlohmann.me/api/basic_json/size/index.md) is `0`).
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## Return value
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The return value depends on the different types and is defined as follows:
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| Value type | return value |
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| ---------- | -------------------------------------- |
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| null | `true` |
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| boolean | `false` |
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| string | `false` |
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| number | `false` |
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| binary | `false` |
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| object | result of function `object_t::empty()` |
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| array | result of function `array_t::empty()` |
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## Exception safety
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No-throw guarantee: this function never throws exceptions.
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## Complexity
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Constant, as long as [`array_t`](https://json.nlohmann.me/api/basic_json/array_t/index.md) and [`object_t`](https://json.nlohmann.me/api/basic_json/object_t/index.md) satisfy the [Container](https://en.cppreference.com/w/cpp/named_req/Container) concept; that is, their `empty()` functions have constant complexity.
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## Possible implementation
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```
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bool empty() const noexcept
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{
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return size() == 0;
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}
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```
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## Notes
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This function does not return whether a string stored as JSON value is empty -- it returns whether the JSON container itself is empty which is `false` in the case of a string.
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## Examples
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Example
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The following code uses `empty()` to check if a JSON object contains any elements.
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```
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#include <iostream>
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#include <nlohmann/json.hpp>
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using json = nlohmann::json;
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int main()
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{
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// create JSON values
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json j_null;
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json j_boolean = true;
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json j_number_integer = 17;
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json j_number_float = 23.42;
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json j_object = {{"one", 1}, {"two", 2}};
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json j_object_empty(json::value_t::object);
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json j_array = {1, 2, 4, 8, 16};
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json j_array_empty(json::value_t::array);
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json j_string = "Hello, world";
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// call empty()
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std::cout << std::boolalpha;
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std::cout << j_null.empty() << '\n';
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std::cout << j_boolean.empty() << '\n';
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std::cout << j_number_integer.empty() << '\n';
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std::cout << j_number_float.empty() << '\n';
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std::cout << j_object.empty() << '\n';
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std::cout << j_object_empty.empty() << '\n';
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std::cout << j_array.empty() << '\n';
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std::cout << j_array_empty.empty() << '\n';
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std::cout << j_string.empty() << '\n';
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}
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```
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Output:
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```
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true
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false
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false
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false
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false
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true
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false
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true
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false
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```
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## Version history
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- Added in version 1.0.0.
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- Extended to return `false` for binary types in version 3.8.0.
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