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◆ at() [3/6]
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, class BinaryType = std::vector<std::uint8_t>>
reference nlohmann::basic_json< ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType >::at |
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const typename object_t::key_type & |
key | ) |
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inline |
Returns a reference to the element at with specified key key, with bounds checking.
- Parameters
-
[in] | key | key of the element to access |
- Returns
- reference to the element at key key
- Exceptions
-
type_error.304 | if the JSON value is not an object; in this case, calling at with a key makes no sense. See example below. |
out_of_range.403 | if the key key is is not stored in the object; that is, find(key) == end() . See example below. |
- Exception safety
- Strong guarantee: if an exception is thrown, there are no changes in the JSON value.
- Complexity
- Logarithmic in the size of the container.
- See also
- operator[](const typename object_t::key_type&) for unchecked access by reference
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value() for access by value with a default value
- Since
- version 1.0.0
- Example
- The example below shows how object elements can be read and written using
at() . It also demonstrates the different exceptions that can be thrown.
2 #include <nlohmann/json.hpp>
11 { "the good", "il buono"},
12 { "the bad", "il cattivo"},
13 { "the ugly", "il brutto"}
17 std::cout << object.at( "the ugly") << '\n';
20 object.at( "the bad") = "il cattivo";
23 std::cout << object << '\n';
30 json str = "I am a string";
31 str.at( "the good") = "Another string";
35 std::cout << e.what() << '\n';
42 object.at( "the fast") = "il rapido";
46 std::cout << e.what() << '\n';
Output (play with this example online): "il brutto"
{"the bad":"il cattivo","the good":"il buono","the ugly":"il brutto"}
[json.exception.type_error.304] cannot use at() with string
[json.exception.out_of_range.403] key 'the fast' not found
The example code above can be translated withg++ -std=c++11 -Isingle_include doc/examples/at__object_t_key_type.cpp -o at__object_t_key_type
Definition at line 19896 of file json.hpp.
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