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json/tests/src/unit-json_view_from_json.cpp
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Niels Lohmann 8ac510b8d3 json_view: exceptions name the path and the bytes of the value 
With JSON_DIAGNOSTICS and JSON_DIAGNOSTIC_POSITIONS, the exceptions of views
now carry the same context as those of basic_json: "(/path) " and
"(bytes a-b) " in front of the message, in basic_json's format (with
JSON_DIAGNOSTICS, no context for the root). Before, views threw without any
context, and since get<T>() reads the view directly, conversions had lost
even the relative path they used to get through materialize().

The throw helpers of detail/view/errors.hpp take an error_site (document
and node; empty without the two macros). Only on the error path, the path is
found by descending from the root (pruned by subtree extent before the first
edit, a full walk through edited sequences after), and the bytes as
materialize() sets them (position_source; a container's end by following the
chain of last elements and scanning to the closing brackets). from_json() for
views and the strict enum macro put the same context in front through
exception_prefix(). Iterators keep their container only under the two
macros, for key(). Without the macros, the accessors compile to the same
code as before.

Tests compare every value's exceptions with those of the parsed or
materialized basic_json value, including edited documents; the checks that
were disabled under the two macros run again.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
2026-10-30 21:51:23 +01:00

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// __ _____ _____ _____
// __| | __| | | | JSON for Modern C++ (supporting code)
// | | |__ | | | | | | version 3.12.0
// |_____|_____|_____|_|___| https://github.com/nlohmann/json
//
// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
// SPDX-License-Identifier: MIT
#include "doctest_compatibility.h"
#include <nlohmann/json_view.hpp>
using nlohmann::json;
using nlohmann::json_document;
using nlohmann::json_view;
using nlohmann::json_editable_document;
using nlohmann::ordered_json_document;
#include "json_view_test_helpers.hpp"
#if !defined(JSON_NOEXCEPTION)
using json_view_test::exception_of_call;
#endif
#include <array>
#include <cstddef>
#include <cstdint>
#include <deque>
#include <forward_list>
#include <list>
#include <map>
#include <set>
#include <string>
#include <tuple>
#include <type_traits>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <valarray>
#include <vector>
#ifdef JSON_HAS_CPP_17
#include <optional>
#endif
// NLOHMANN_JSON_SERIALIZE_ENUM uses a static std::pair; same() compares floating-point values on purpose
DOCTEST_CLANG_SUPPRESS_WARNING_PUSH
DOCTEST_CLANG_SUPPRESS_WARNING("-Wexit-time-destructors")
DOCTEST_CLANG_SUPPRESS_WARNING("-Wfloat-equal")
DOCTEST_GCC_SUPPRESS_WARNING_PUSH
DOCTEST_GCC_SUPPRESS_WARNING("-Wfloat-equal")
// The from_json() functions of the NLOHMANN_DEFINE_TYPE_* and
// NLOHMANN_JSON_SERIALIZE_ENUM macros read views directly; other from_json()
// functions get the value of materialize().
namespace view_from_json
{
struct point
{
int x = 0;
int y = 0;
NLOHMANN_DEFINE_TYPE_INTRUSIVE(point, x, y)
friend bool operator==(const point& a, const point& b)
{
return a.x == b.x && a.y == b.y;
}
};
struct shape
{
// initialized for GCC -Weffc++
// NOLINTNEXTLINE(readability-redundant-member-init)
std::string name{};
// NOLINTNEXTLINE(readability-redundant-member-init)
std::vector<point> points{};
point center{};
// NOLINTNEXTLINE(readability-redundant-member-init)
std::map<std::string, point> named{};
NLOHMANN_DEFINE_TYPE_INTRUSIVE(shape, name, points, center, named)
};
struct with_defaults
{
int a = 1;
std::string b = "default";
std::vector<int> c = {1, 2};
};
// NOLINTNEXTLINE(misc-use-internal-linkage): found by ADL, like the from_json() functions of users
NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT(with_defaults, a, b, c)
struct with_names
{
int value = 0;
// initialized for GCC -Weffc++
// NOLINTNEXTLINE(readability-redundant-member-init)
std::string text{};
};
// NOLINTNEXTLINE(misc-use-internal-linkage): found by ADL, like the from_json() functions of users
NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_NAMES(with_names, "Value", value, "Text", text)
struct empty
{
NLOHMANN_DEFINE_TYPE_INTRUSIVE(empty)
};
enum class color
{
red,
green,
blue
};
// NOLINTNEXTLINE(misc-use-internal-linkage): found by ADL, like the from_json() functions of users
NLOHMANN_JSON_SERIALIZE_ENUM(color,
{
{color::red, "red"},
{color::green, "green"},
{color::blue, nullptr},
})
enum class strict_color
{
red,
green
};
// NOLINTNEXTLINE(misc-use-internal-linkage): found by ADL, like the from_json() functions of users
NLOHMANN_JSON_SERIALIZE_ENUM_STRICT(strict_color,
{
{strict_color::red, "red"},
{strict_color::green, 1},
})
struct base
{
int id = 0;
NLOHMANN_DEFINE_TYPE_INTRUSIVE(base, id)
};
struct derived : base
{
color c = color::red;
NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE(derived, base, c)
};
struct derived_with_defaults : base
{
std::string label = "none";
NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_DEFAULT(derived_with_defaults, base, label)
};
// a base whose from_json() is written for json only: the derived type reads
// its own members from the view and the base part from materialize()
struct legacy_base
{
int id = 0;
};
// NOLINTNEXTLINE(misc-use-internal-linkage): found by ADL, like the from_json() functions of users
inline void from_json(const json& j, legacy_base& b)
{
b.id = j.at("id").get<int>();
}
struct derived_from_legacy : legacy_base
{
int extra = 0;
};
// NOLINTNEXTLINE(misc-use-internal-linkage): found by ADL, like the from_json() functions of users
NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE(derived_from_legacy, legacy_base, extra)
// from_json() for json only: converted through materialize()
struct legacy
{
int v = 0;
};
// NOLINTNEXTLINE(misc-use-internal-linkage): found by ADL, like the from_json() functions of users
inline void from_json(const json& j, legacy& l)
{
l.v = j.at("v").get<int>();
}
// an unconstrained from_json() template, as in the documentation of
// arbitrary type conversions: converted through materialize()
struct unconstrained
{
int v = 0;
};
template<typename BasicJsonType>
// NOLINTNEXTLINE(misc-use-internal-linkage): found by ADL, like the from_json() functions of users
void from_json(const BasicJsonType& j, unconstrained& u)
{
u.v = j.at("v").template get<int>();
}
// a from_json() constrained by the user to views and json: reads the view
struct constrained
{
int v = 0;
};
template < typename J, typename std::enable_if < nlohmann::is_json_view<J>::value || std::is_same<J, json>::value, int >::type = 0 >
// NOLINTNEXTLINE(misc-use-internal-linkage): found by ADL, like the from_json() functions of users
void from_json(const J& j, constrained& c)
{
c.v = j.at("v").template get<int>();
}
} // namespace view_from_json
namespace
{
template<typename T, typename View = json_view>
struct reads_view : nlohmann::detail::has_view_from_json<nlohmann::adl_serializer<T, void>, View, T> {};
// whether a member converts to the same T from the view and from the materialized value
template<typename T>
bool same(const json_view& v, const json& j, const char* key)
{
return v[key].get<T>() == j[key].get<T>();
}
} // namespace
TEST_CASE("json_view from_json")
{
using namespace view_from_json; // NOLINT(google-build-using-namespace)
SECTION("which from_json() functions read views")
{
static_assert(reads_view<point>::value, "macro types read views");
static_assert(reads_view<shape>::value, "macro types read views");
static_assert(reads_view<with_defaults>::value, "macro types read views");
static_assert(reads_view<with_names>::value, "macro types read views");
static_assert(reads_view<empty>::value, "macro types read views");
static_assert(reads_view<derived>::value, "macro types read views");
static_assert(reads_view<derived_from_legacy>::value, "macro types read views");
static_assert(reads_view<color>::value, "enum macro types read views");
static_assert(reads_view<strict_color>::value, "enum macro types read views");
static_assert(reads_view<constrained>::value, "constrained from_json() reads views");
static_assert(reads_view<point, nlohmann::json_editable_view>::value, "editable views as well");
static_assert(reads_view<point, nlohmann::ordered_json_view>::value, "views of ordered_json as well");
static_assert(!reads_view<legacy>::value, "from_json() for json");
static_assert(!reads_view<legacy_base>::value, "from_json() for json");
static_assert(!reads_view<unconstrained>::value, "unconstrained from_json()");
static_assert(nlohmann::is_json_view<json_view>::value, "is_json_view");
static_assert(nlohmann::is_json_view<nlohmann::ordered_json_editable_view>::value, "is_json_view");
static_assert(!nlohmann::is_json_view<json>::value, "is_json_view");
static_assert(!nlohmann::is_json_view<json_document>::value, "is_json_view");
static_assert(!nlohmann::is_json_view<int>::value, "is_json_view");
}
SECTION("the library's own conversions read views")
{
static_assert(reads_view<std::list<int>>::value, "std types");
static_assert(reads_view<std::deque<point>>::value, "std types");
static_assert(reads_view<std::set<std::string>>::value, "std types");
static_assert(reads_view<std::unordered_set<int>>::value, "std types");
static_assert(reads_view<std::forward_list<int>>::value, "std types");
static_assert(reads_view<std::valarray<double>>::value, "std types");
static_assert(reads_view<std::array<int, 3>>::value, "std types");
static_assert(reads_view<std::pair<int, std::string>>::value, "std types");
static_assert(reads_view<std::tuple<int, bool, point>>::value, "std types");
static_assert(reads_view<std::map<int, int>>::value, "std types");
static_assert(reads_view<std::unordered_map<int, point>>::value, "std types");
static_assert(reads_view<std::map<color, int>>::value, "std types");
static_assert(reads_view<std::map<std::string, int>>::value, "std types");
static_assert(reads_view<std::vector<point>>::value, "std types");
static_assert(reads_view<std::vector<bool>>::value, "std types");
static_assert(reads_view<std::vector<std::uint8_t>>::value, "std types");
static_assert(reads_view<std::string>::value, "std types");
static_assert(reads_view<int>::value, "std types");
static_assert(reads_view<bool>::value, "std types");
static_assert(reads_view<double>::value, "std types");
static_assert(reads_view<std::nullptr_t>::value, "std types");
static_assert(!reads_view<json>::value, "get<BasicJsonType>() is materialize()");
static_assert(reads_view<json::binary_t>::value, "std types");
#ifdef JSON_HAS_CPP_17
static_assert(reads_view<std::optional<point>>::value, "std types");
#endif
CHECK(true);
}
SECTION("std types: the same values as from the materialized value")
{
const json_document d = json_document::parse(R"({"ints": [3, 1, 2, 1], "strings": ["b", "a", "b"], "pairs": [[1, "x"], [2, "y"]],
"points": [{"x": 1, "y": 2}, {"x": 3, "y": 4}], "object": {"b": 1, "a": 2}, "triple": [1, true, {"x": 5, "y": 6}],
"colors": {"green": 1, "red": 2}, "bytes": [1, 2, 255], "nested": [[1, 2], [3]], "number": 1.5, "flag": true,
"text": "hello", "nothing": null, "mixed": [1, 2.5, -3, 4], "flags": [true, false, true]})");
const json j = d.root().materialize();
const json_view v = d.root();
CHECK(same<std::list<int>>(v, j, "ints"));
CHECK(same<std::deque<int>>(v, j, "ints"));
CHECK(same<std::set<int>>(v, j, "ints"));
CHECK(same<std::multiset<int>>(v, j, "ints"));
CHECK(same<std::unordered_set<std::string>>(v, j, "strings"));
CHECK(same<std::forward_list<int>>(v, j, "ints"));
CHECK(same<std::array<int, 4>>(v, j, "ints"));
CHECK(same<std::array<double, 4>>(v, j, "mixed"));
CHECK((v["pairs"][0].get<std::pair<int, std::string>>() == j["pairs"][0].get<std::pair<int, std::string>>()));
CHECK(same<std::vector<std::pair<int, std::string>>>(v, j, "pairs"));
CHECK(same<std::tuple<int, bool, point>>(v, j, "triple"));
CHECK(same<std::map<int, std::string>>(v, j, "pairs"));
CHECK(same<std::unordered_map<int, std::string>>(v, j, "pairs"));
CHECK(same<std::map<std::string, int>>(v, j, "object"));
CHECK(same<std::unordered_map<std::string, int>>(v, j, "object"));
CHECK(same<std::map<color, int>>(v, j, "colors"));
CHECK(same<std::vector<point>>(v, j, "points"));
CHECK(same<std::vector<int>>(v, j, "ints"));
CHECK(same<std::vector<bool>>(v, j, "flags"));
CHECK(same<std::vector<std::uint8_t>>(v, j, "bytes"));
CHECK(same<std::vector<std::vector<int>>>(v, j, "nested"));
CHECK(same<std::vector<json>>(v, j, "mixed"));
CHECK(same<double>(v, j, "number"));
CHECK(same<bool>(v, j, "flag"));
CHECK(same<std::string>(v, j, "text"));
CHECK(same<std::nullptr_t>(v, j, "nothing"));
CHECK(same<json>(v, j, "object"));
const auto va = v["ints"].get<std::valarray<int>>();
CHECK(va.size() == 4);
CHECK(va[0] == 3);
int c_array[2][2] = {{0, 0}, {0, 0}}; // NOLINT(cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays): the conversion to C arrays
v["nested"][0].get_to(c_array[0]);
CHECK(c_array[0][1] == 2);
#ifdef JSON_HAS_CPP_17
const auto first = v["points"][0].get<std::optional<point>>();
CHECK((first.has_value() && first->y == 2));
CHECK(!v["nothing"].get<std::optional<point>>().has_value());
#endif
// containers of views keep views of the elements
const auto views = v["points"].get<std::vector<json_view>>();
CHECK(views.size() == 2);
CHECK(views[1]["x"].get<int>() == 3);
CHECK(v["object"].get<std::map<std::string, json_view>>().at("a").get<int>() == 2);
}
SECTION("std types: duplicate keys keep the last value, as from the materialized value")
{
const json_document d = json_document::parse(R"({"a": 1, "b": 2, "a": 3})");
CHECK((d.root().get<std::map<std::string, int>>() == d.root().materialize().get<std::map<std::string, int>>()));
CHECK(d.root().get<std::map<std::string, int>>().at("a") == 3);
CHECK(d.root().get<std::unordered_map<std::string, int>>().at("a") == 3);
}
#if !defined(JSON_NOEXCEPTION)
SECTION("std types: the same errors as from the materialized value")
{
const json_document d = json_document::parse(R"([[1, "x"], {"a": 1}, "text", [1], 5, null])");
const json j = d.root().materialize();
for (std::size_t i = 0; i < 6; ++i)
{
CAPTURE(i)
const json_view v = d.root()[i];
CHECK(exception_of_call([&] { static_cast<void>(v.get<std::list<int>>()); }) == exception_of_call([&] { static_cast<void>(j[i].get<std::list<int>>()); }));
CHECK(exception_of_call([&] { static_cast<void>(v.get<std::forward_list<int>>()); }) == exception_of_call([&] { static_cast<void>(j[i].get<std::forward_list<int>>()); }));
CHECK(exception_of_call([&] { static_cast<void>(v.get<std::array<int, 2>>()); }) == exception_of_call([&] { static_cast<void>(j[i].get<std::array<int, 2>>()); }));
CHECK(exception_of_call([&] { static_cast<void>(v.get<std::pair<int, std::string>>()); }) == exception_of_call([&] { static_cast<void>(j[i].get<std::pair<int, std::string>>()); }));
CHECK(exception_of_call([&] { static_cast<void>(v.get<std::map<std::string, int>>()); }) == exception_of_call([&] { static_cast<void>(j[i].get<std::map<std::string, int>>()); }));
CHECK(exception_of_call([&] { static_cast<void>(v.get<std::map<int, int>>()); }) == exception_of_call([&] { static_cast<void>(j[i].get<std::map<int, int>>()); }));
CHECK(exception_of_call([&] { static_cast<void>(v.get<std::vector<std::uint8_t>>()); }) == exception_of_call([&] { static_cast<void>(j[i].get<std::vector<std::uint8_t>>()); }));
CHECK(exception_of_call([&] { static_cast<void>(v.get<json::binary_t>()); }) == exception_of_call([&] { static_cast<void>(j[i].get<json::binary_t>()); }));
CHECK(exception_of_call([&] { static_cast<void>(v.get<std::valarray<int>>()); }) == exception_of_call([&] { static_cast<void>(j[i].get<std::valarray<int>>()); }));
}
}
#endif
SECTION("macro types")
{
const json_document d = json_document::parse(R"({"name": "s", "points": [{"x": 1, "y": 2}, {"x": 3, "y": 4}],
"center": {"x": 5, "y": 6}, "named": {"a": {"x": 7, "y": 8}}})");
const auto s = d.root().get<shape>();
CHECK(s.name == "s");
CHECK(s.points.size() == 2);
CHECK(s.points[1].y == 4);
CHECK(s.center.x == 5);
CHECK(s.named.at("a").y == 8);
point p;
d.root()["center"].get_to(p);
CHECK(p.x == 5);
CHECK(p.y == 6);
static_cast<void>(d.root()["center"].get<empty>());
}
SECTION("defaults and names")
{
const json_document d = json_document::parse(R"({"a": 5, "Value": 3, "Text": "t"})");
const auto w = d.root().get<with_defaults>();
CHECK(w.a == 5);
CHECK(w.b == "default");
CHECK(w.c == std::vector<int> {1, 2});
const json_document null_document = json_document::parse("null");
CHECK(null_document.root().get<with_defaults>().a == 1);
const auto n = d.root().get<with_names>();
CHECK(n.value == 3);
CHECK(n.text == "t");
}
SECTION("enums")
{
const json_document d = json_document::parse(R"(["green", null, "purple", 1, "red"])");
CHECK(d.root()[0].get<color>() == color::green);
CHECK(d.root()[1].get<color>() == color::blue);
CHECK(d.root()[2].get<color>() == color::red); // unknown values map to the first entry
CHECK(d.root()[3].get<strict_color>() == strict_color::green);
CHECK(d.root()[4].get<strict_color>() == strict_color::red);
#if !defined(JSON_NOEXCEPTION)
const json j = d.root().materialize();
#if !JSON_DIAGNOSTICS && !JSON_DIAGNOSTIC_POSITIONS
CHECK(exception_of_call([&] { static_cast<void>(d.root()[2].get<strict_color>()); }) == "[json.exception.out_of_range.410] enum value out of range for strict_color: \"purple\"");
#endif
CHECK(exception_of_call([&] { static_cast<void>(d.root()[2].get<strict_color>()); }) == exception_of_call([&] { static_cast<void>(j[2].get<strict_color>()); }));
#endif
}
SECTION("derived types")
{
const json_document d = json_document::parse(R"({"id": 7, "c": "green", "extra": 3})");
const auto x = d.root().get<derived>();
CHECK(x.id == 7);
CHECK(x.c == color::green);
const auto y = d.root().get<derived_with_defaults>();
CHECK(y.id == 7);
CHECK(y.label == "none");
const auto z = d.root().get<derived_from_legacy>();
CHECK(z.id == 7);
CHECK(z.extra == 3);
}
SECTION("other from_json() functions get the value of materialize()")
{
const json_document d = json_document::parse(R"({"v": 4})");
CHECK(d.root().get<legacy>().v == 4);
CHECK(d.root().get<unconstrained>().v == 4);
CHECK(d.root().get<constrained>().v == 4);
legacy l;
d.root().get_to(l);
CHECK(l.v == 4);
}
SECTION("duplicate keys: members are looked up in the view, where the first one wins")
{
const json_document d = json_document::parse(R"({"x": 1, "y": 2, "x": 3})");
CHECK(d.root().get<point>().x == 1);
// materialize() keeps the last value, like parse()
CHECK(d.root().materialize().get<point>().x == 3);
}
SECTION("editable documents and ordered_json")
{
json_editable_document e = json_editable_document::parse(R"({"x": 1, "y": 2})");
e.set(e.root(), "x", 10);
CHECK(e.root().get<point>().x == 10);
const ordered_json_document o = ordered_json_document::parse(R"({"name": "o", "points": [], "center": {"x": 1, "y": 2}, "named": {}})");
CHECK(o.root().get<shape>().center.y == 2);
}
#if !defined(JSON_NOEXCEPTION)
SECTION("errors are those of the same conversion of a json value")
{
const json_document d = json_document::parse(R"([{"x": 1}, {"x": "1", "y": 2}, [1, 2], {"id": "7", "c": "red"}])");
const json j = d.root().materialize();
for (std::size_t i = 0; i < 3; ++i)
{
CAPTURE(i)
CHECK(exception_of_call([&] { static_cast<void>(d.root()[i].get<point>()); }) == exception_of_call([&] { static_cast<void>(j[i].get<point>()); }));
}
CHECK(exception_of_call([&] { static_cast<void>(d.root()[3].get<derived>()); }) == exception_of_call([&] { static_cast<void>(j[3].get<derived>()); }));
}
#endif
}
DOCTEST_CLANG_SUPPRESS_WARNING_POP
DOCTEST_GCC_SUPPRESS_WARNING_POP
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