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◆ front() [1/2]
template<template< typename, typename, typename... > class ObjectType = std::map, template< typename, typename... > 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 > class AllocatorType = std::allocator, template< typename, typename=void > class JSONSerializer = adl_serializer>
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.214 | when called on null value |
- Example
- The following code shows an example for
front() . 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"; 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'; 26 std::cout << j_array.front() << '\n'; 28 std::cout << j_string.front() << '\n'; basic_json<> json default JSON class
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
- See also
- back() – access the last element
- Since
- version 1.0.0
Definition at line 10576 of file json.hpp.
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