JSON for Modern C++  3.7.0

◆ front() [2/2]

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>
const_reference nlohmann::basic_json::front ( ) const
inline

Returns a reference to the first element in the container. For a JSON container c, the expression c.front() is equivalent to *c.begin().

Returns
In case of a structured type (array or object), a reference to the first element is returned. In case of number, string, or boolean values, a reference to the value is returned.
Complexity
Constant.
Precondition
The JSON value must not be null (would throw std::out_of_range) or an empty array or object (undefined behavior, guarded by assertions).
Postcondition
The JSON value remains unchanged.
Exceptions
invalid_iterator.214when called on null value
Example
The following code shows an example for front().
1 #include <iostream>
2 #include <nlohmann/json.hpp>
3 
4 using json = nlohmann::json;
5 
6 int main()
7 {
8  // create JSON values
9  json j_null;
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";
18 
19  // call front()
20  //std::cout << j_null.front() << '\n'; // would throw
21  std::cout << j_boolean.front() << '\n';
22  std::cout << j_number_integer.front() << '\n';
23  std::cout << j_number_float.front() << '\n';
24  std::cout << j_object.front() << '\n';
25  //std::cout << j_object_empty.front() << '\n'; // undefined behavior
26  std::cout << j_array.front() << '\n';
27  //std::cout << j_array_empty.front() << '\n'; // undefined behavior
28  std::cout << j_string.front() << '\n';
29 }
basic_json<> json
default JSON class
Definition: json.hpp:2355

Output (play with this example online):
true
17
23.42
1
1
"Hello, world"
The example code above can be translated with
g++ -std=c++11 -Isingle_include doc/examples/front.cpp -o front 
See also
back() – access the last element
Since
version 1.0.0

Definition at line 17963 of file json.hpp.