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◆ empty()
template<template< typename U, typename V, typename... Args > class ObjectType = std::map, template< typename U, typename... Args > class ArrayType = std::vector, class StringType = std::string, class BooleanType = bool, class NumberIntegerType = std::int64_t, class NumberUnsignedType = std::uint64_t, class NumberFloatType = double, template< typename U > class AllocatorType = std::allocator, template< typename T, typename SFINAE=void > class JSONSerializer = adl_serializer, class BinaryType = std::vector<std::uint8_t>>
bool nlohmann::basic_json< ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType >::empty |
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Checks if a JSON value has no elements (i.e. whether its size is 0 ).
- Returns
- The return value depends on the different types and is defined as follows:
Value type | return value |
null | true |
boolean | false |
string | false |
number | false |
binary | false |
object | result of function object_t::empty() |
array | result of function array_t::empty() |
- Example
- The following code uses
empty() to check if a JSON object contains any elements.
2 #include <nlohmann/json.hpp>
10 json j_boolean = true;
11 json j_number_integer = 17;
12 json j_number_float = 23.42;
13 json j_object = {{ "one", 1}, { "two", 2}};
14 json j_object_empty(json::value_t::object);
15 json j_array = {1, 2, 4, 8, 16};
16 json j_array_empty(json::value_t::array);
17 json j_string = "Hello, world";
20 std::cout << std::boolalpha;
21 std::cout << j_null.empty() << '\n';
22 std::cout << j_boolean.empty() << '\n';
23 std::cout << j_number_integer.empty() << '\n';
24 std::cout << j_number_float.empty() << '\n';
25 std::cout << j_object.empty() << '\n';
26 std::cout << j_object_empty.empty() << '\n';
27 std::cout << j_array.empty() << '\n';
28 std::cout << j_array_empty.empty() << '\n';
29 std::cout << j_string.empty() << '\n';
basic_json<> json default JSON class
Output (play with this example online): true
false
false
false
false
true
false
true
false
The example code above can be translated withg++ -std=c++11 -Isingle_include doc/examples/empty.cpp -o empty
- Complexity
- Constant, as long as array_t and object_t satisfy the Container concept; that is, their
empty() functions have constant complexity.
- Iterator validity
- No changes.
- Exception safety
- No-throw guarantee: this function never throws exceptions.
- Note
- 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.
- Requirements
- This function helps
basic_json satisfying the Container requirements:
- The complexity is constant.
- Has the semantics of
begin() == end() .
- See also
- see size() – returns the number of elements
- Since
- version 1.0.0
Definition at line 22503 of file json.hpp.
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