Returns a const reference to the element at with specified key key, with bounds checking.
- Parameters
-
[in] | key | key of the element to access |
- Returns
- const 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
-
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 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';
24 const json str =
"I am a string";
25 std::cout << str.at(
"the good") <<
'\n';
29 std::cout << e.what() <<
'\n';
36 std::cout <<
object.at(
"the fast") <<
'\n';
40 std::cout <<
"out of range" <<
'\n';
Output (play with this example online): "il brutto"
[json.exception.type_error.304] cannot use at() with string
out of range
The <a href= https://github.com/nlohmann/json/blob/develop/doc/examples/ at__object_t_key_type_const.cpp>example code above can be translated withg++ -std=c++11 -Isingle_include doc/examples/
at__object_t_key_type_const.cpp -o
at__object_t_key_type_const
Definition at line 17678 of file json.hpp.