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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>
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() |
- 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.
- Complexity\n Constant, as long as @ref array_t and @ref object_t satisfy
- the Container concept; that is, their
empty() functions have constant complexity.
- Requirements\n This function helps
basic_json satisfying the - Container requirements:
- The complexity is constant.
- Has the semantics of
begin() == end() .
- Example\n The following code uses
empty() to check if a JSON - object contains any elements.
19 std::cout << std::boolalpha;
bool empty() const noexcept checks whether the container is empty
static basic_json array(std::initializer_list< basic_json > init=std::initializer_list< basic_json >()) explicitly create an array from an initializer list
static basic_json object(std::initializer_list< basic_json > init=std::initializer_list< basic_json >()) explicitly create an object from an initializer list
a class to store JSON values
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 with g++ -std=c++11 -Isrc doc/examples/empty.cpp -o empty
- See also
- size() – returns the number of elements
- Since
- version 1.0.0
Definition at line 4995 of file json.hpp.
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