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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>
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 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.
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";
20 std::cout << j_boolean. front() << '\n';
21 std::cout << j_number_integer. front() << '\n';
22 std::cout << j_number_float. front() << '\n';
23 std::cout << j_object. front() << '\n';
25 std::cout << j_array. front() << '\n';
27 std::cout << j_string. front() << '\n';
reference front() access the first element Definition: json.hpp:2743
a class to store JSON values Definition: json.hpp:130
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
17
23.42
1
1
"Hello, world"
The example code above can be translated withg++ -std=c++11 -Isrc doc/examples/front.cpp -o front
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