nlohmann::basic_json::empty¶

boolempty()constnoexcept;

Checks if a JSON value has no elements (i.e., whether its size() is 0).

Return value¶

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()`

Exception safety¶

No-throw guarantee: this function never throws exceptions.

Complexity¶

Constant, as long as array_t and object_t satisfy the Container concept; that is, their empty() functions have constant complexity.

Possible implementation¶

boolempty()constnoexcept
{
returnsize()==0;
}

Notes¶

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.

Examples¶

Example

The following code uses empty() to check if a JSON object contains any elements.

#include<iostream>
#include<nlohmann/json.hpp>
usingjson=nlohmann::json;
intmain()
{
// create JSON values
jsonj_null;
jsonj_boolean=true;
jsonj_number_integer=17;
jsonj_number_float=23.42;
jsonj_object={{"one",1},{"two",2}};
jsonj_object_empty(json::value_t::object);
jsonj_array={1,2,4,8,16};
jsonj_array_empty(json::value_t::array);
jsonj_string="Hello, world";
// call empty()
std::cout<<std::boolalpha;
std::cout<<j_null.empty()<<'\n';
std::cout<<j_boolean.empty()<<'\n';
std::cout<<j_number_integer.empty()<<'\n';
std::cout<<j_number_float.empty()<<'\n';
std::cout<<j_object.empty()<<'\n';
std::cout<<j_object_empty.empty()<<'\n';
std::cout<<j_array.empty()<<'\n';
std::cout<<j_array_empty.empty()<<'\n';
std::cout<<j_string.empty()<<'\n';
}

Output:

true
false
false
false
false
true
false
true
false

Version history¶

Added in version 1.0.0.
Extended to return false for binary types in version 3.8.0.