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>

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 compound value (array or object), a reference to the first element is returned. In cast of number, string, or boolean values, a reference to the value is returned.

Complexity

Constant.

Note

Calling front on an empty container is undefined.

Exceptions

std::out_of_range

when called on null value

Example

The following code shows an example for front.

#include <json.hpp>

using namespace nlohmann;

int main()
{
    // create JSON values
    json j_null;
    json j_boolean = true;
    json j_number_integer = 17;
    json j_number_float = 23.42;
    json j_object = {{"one", 1}, {"two", 2}};
    json j_object_empty(json::value_t::object);
    json j_array = {1, 2, 4, 8, 16};
    json j_array_empty(json::value_t::array);
    json j_string = "Hello, world";

    // call front()
    //std::cout << j_null.front() << '\n';          // would throw
    std::cout << j_boolean.front() << '\n';
    std::cout << j_number_integer.front() << '\n';
    std::cout << j_number_float.front() << '\n';
    std::cout << j_object.front() << '\n';
    //std::cout << j_object_empty.front() << '\n';  // would throw
    std::cout << j_array.front() << '\n';
    //std::cout << j_array_empty.front() << '\n';   // would throw
    std::cout << j_string.front() << '\n';
}

Output:

true
17
23.42
1
1
"Hello, world"

The example code above can be translated with

g++ -std=c++11 -Isrc doc/examples/front.cpp -o front 

.