JSON for Modern C++ 3.10.4

◆ operator== [3/3]

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>>
template<typename ScalarType , typename std::enable_if< std::is_scalar< ScalarType >::value, int >::type = 0>
bool operator== ( ScalarType  lhs,
const_reference  rhs 
)
friend

comparison: equal Compares two JSON values for equality according to the following rules:

  • Two JSON values are equal if (1) they are from the same type and (2) their stored values are the same according to their respective operator==.
  • Integer and floating-point numbers are automatically converted before comparison. Note that two NaN values are always treated as unequal.
  • Two JSON null values are equal.
Note
Floating-point inside JSON values numbers are compared with json::number_float_t::operator== which is double::operator== by default. To compare floating-point while respecting an epsilon, an alternative comparison function could be used, for instance
template<typename T, typename = typename std::enable_if<std::is_floating_point<T>::value, T>::type>
inline bool is_same(T a, T b, T epsilon = std::numeric_limits<T>::epsilon()) noexcept
{
return std::abs(a - b) <= epsilon;
}
constexpr value_t type() const noexcept
return the type of the JSON value (explicit)
Definition: json.hpp:19972
Or you can self-defined operator equal function like this:
bool my_equal(const_reference lhs, const_reference rhs) {
const auto lhs_type lhs.type();
const auto rhs_type rhs.type();
if (lhs_type == rhs_type) {
switch(lhs_type)
// self_defined case
case value_t::number_float:
return std::abs(lhs - rhs) <= std::numeric_limits<float>::epsilon();
// other cases remain the same with the original
...
}
...
}
const value_type & const_reference
the type of an element const reference
Definition: json.hpp:17793
NaN values never compare equal to themselves or to other NaN values.
Parameters
[in]lhsfirst JSON value to consider
[in]rhssecond JSON value to consider
Returns
whether the values lhs and rhs are equal
Exception safety
No-throw guarantee: this function never throws exceptions.
Complexity
Linear.
Example
The example demonstrates comparing several JSON types.
1#include <iostream>
2#include <nlohmann/json.hpp>
3
4using json = nlohmann::json;
5
6int main()
7{
8 // create several JSON values
9 json array_1 = {1, 2, 3};
10 json array_2 = {1, 2, 4};
11 json object_1 = {{"A", "a"}, {"B", "b"}};
12 json object_2 = {{"B", "b"}, {"A", "a"}};
13 json number_1 = 17;
14 json number_2 = 17.000000000000001L;
15 json string_1 = "foo";
16 json string_2 = "bar";
17
18 // output values and comparisons
19 std::cout << std::boolalpha;
20 std::cout << array_1 << " == " << array_2 << " " << (array_1 == array_2) << '\n';
21 std::cout << object_1 << " == " << object_2 << " " << (object_1 == object_2) << '\n';
22 std::cout << number_1 << " == " << number_2 << " " << (number_1 == number_2) << '\n';
23 std::cout << string_1 << " == " << string_2 << " " << (string_1 == string_2) << '\n';
24}
basic_json<> json
default JSON class
Definition: json.hpp:3472

Output (play with this example online):
[1,2,3] == [1,2,4] false
{"A":"a","B":"b"} == {"A":"a","B":"b"} true
17 == 17.0 true
"foo" == "bar" false
The example code above can be translated with
g++ -std=c++11 -Isingle_include doc/examples/operator__equal.cpp -o operator__equal 
Since
version 1.0.0

Definition at line 23966 of file json.hpp.