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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 = int64_t, class NumberFloatType = double, template< typename U > class AllocatorType = std::allocator>
Checks if a JSON value has no elements.
- 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 |
object | result of function object_t::empty() |
array | result of function array_t::empty() |
- Complexity
- Constant, as long as array_t and object_t satisfy the Container concept; that is, their empty() functions have constant complexity.
- Requirements
- This function satisfies the Container requirements:
- The complexity is constant.
- Has the semantics of
begin() == end() .
- Example
- The following code uses empty to check if a json object contains any elements.
10 json j_number_integer = 17;
11 json j_number_float = 23.42;
12 json j_object = {{ "one", 1}, { "two", 2}};
14 json j_array = {1, 2, 4, 8, 16};
16 json j_string = "Hello, world";
19 std::cout << std::boolalpha;
20 std::cout << j_null. empty() << '\n';
21 std::cout << j_boolean. empty() << '\n';
22 std::cout << j_number_integer. empty() << '\n';
23 std::cout << j_number_float. empty() << '\n';
24 std::cout << j_object. empty() << '\n';
25 std::cout << j_object_empty.empty() << '\n';
26 std::cout << j_array. empty() << '\n';
27 std::cout << j_array_empty.empty() << '\n';
28 std::cout << j_string. empty() << '\n';
a class to store JSON values Definition: json.hpp:130
bool empty() const noexcept checks whether the container is empty Definition: json.hpp:3380
object (unordered set of name/value pairs)
namespace for Niels Lohmann Definition: json.hpp:55
array (ordered collection of values)
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 -Isrc doc/examples/empty.cpp -o empty
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