ppforest2::JsonReader Class Reference

A small DSL for extracting-and-validating values out of a JSON object with path-aware error messages. More...

#include <JsonReader.hpp>

Public Member Functions
	JsonReader (nlohmann::json const &j, std::string path)
JsonReader	at (std::string const &key) const
	Descend into a required sub-object.
bool	contains (std::string const &key) const
	Whether key is present on the wrapped object.
void	forbid_key (std::string const &key, std::string const &reason) const
	Throw if key is present and non-null on the wrapped object.
nlohmann::json const &	json () const
	The raw underlying JSON (escape hatch for unusual cases).
void	only_keys (std::initializer_list< char const * > allowed) const
	Assert the wrapped object contains only the given keys.
template<typename T>
T	optional (std::string const &key, T fallback) const
	Extract an optional typed value, falling back to fallback.
std::string const &	path () const
	The path prefix used in error messages.
template<typename T>
T	require (std::string const &key) const
	Extract a required typed value.
nlohmann::json const &	require_array (std::string const &key) const
	Return a const reference to a required array field.
std::string	require_enum (std::string const &key, std::initializer_list< char const * > allowed) const
	Extract a required string constrained to allowed values.
long long	require_int (std::string const &key, long long min=std::numeric_limits< long long >::min(), long long max=std::numeric_limits< long long >::max()) const
	Extract a required integer value, optionally constrained.
void	require_keys (std::initializer_list< char const * > keys) const
	Assert every key in keys is present on the wrapped object.
double	require_number (std::string const &key, double min=-std::numeric_limits< double >::infinity(), double max=std::numeric_limits< double >::infinity()) const
	Extract a required number (double), optionally constrained.
void	require_object () const
	Assert the wrapped value is a JSON object.
void	require_string_array (std::string const &key, bool non_empty=false) const
	Assert key is an array of strings, optionally non-empty.
Discard-result siblings of <tt>require_*</tt> for validators that don't need the value.
void	expect_enum (std::string const &key, std::initializer_list< char const * > allowed) const
void	expect_int (std::string const &key, long long min=std::numeric_limits< long long >::min(), long long max=std::numeric_limits< long long >::max()) const
void	expect_number (std::string const &key, double min=-std::numeric_limits< double >::infinity(), double max=std::numeric_limits< double >::infinity()) const
void	expect_array (std::string const &key) const

Detailed Description

A small DSL for extracting-and-validating values out of a JSON object with path-aware error messages.

JsonReader wraps a nlohmann::json object and a dotted path (e.g. "config.pp"). Every extraction method throws std::runtime_error with a message that names the offending field:

"config.pp.lambda: expected number, got string" "config.pp: unexpected key 'lamda' (allowed: name, lambda)" "config.stop.min_size: must be in [2, ∞) (got 0)"

Used both at the top level (see serialization::validate_model_export) and inside every strategy's from_json. Unifies what validate_json_keys

• j.at(k).get<T>() used to do piecemeal.

Scope is deliberately small:

• typed field extraction (require<T> / optional<T>)
• enum-like string validation (require_enum)
• numeric range validation (require_number / require_int)
• unknown-key rejection (only_keys)
• nested-object descent (at)
• array access (require_array)

The DSL is not a schema validator and cannot express composition (oneOf, allOf, recursive schemas). If you find yourself wanting those, bring in a real schema library instead of growing this file.

Constructor & Destructor Documentation

JsonReader()

ppforest2::JsonReader::JsonReader	(	nlohmann::json const &	j,
		std::string	path )

Parameters

j	The JSON object this reader wraps.
path	Dotted path used as the prefix for error messages (e.g. "config.pp"). The reader does not verify that j itself is an object at construction — use require_object() explicitly for that.

Member Function Documentation

at()

JsonReader ppforest2::JsonReader::at

(

std::string const &

key

)

const

Descend into a required sub-object.

Exceptions

std::runtime_error

if key is missing or not an object.

contains()

bool ppforest2::JsonReader::contains

(

std::string const &

key

)

const

Whether key is present on the wrapped object.

expect_array()

void ppforest2::JsonReader::expect_array ( std::string const & key ) const

inline

expect_enum()

void ppforest2::JsonReader::expect_enum ( std::string const & key, std::initializer_list< char const * > allowed ) const

inline

expect_int()

void ppforest2::JsonReader::expect_int ( std::string const & key, long long min = std::numeric_limits<long long>::min(), long long max = std::numeric_limits<long long>::max() ) const

inline

expect_number()

void ppforest2::JsonReader::expect_number ( std::string const & key, double min = -std::numeric_limits<double>::infinity(), double max = std::numeric_limits<double>::infinity() ) const

inline

forbid_key()

void ppforest2::JsonReader::forbid_key	(	std::string const &	key,
		std::string const &	reason ) const

Throw if key is present and non-null on the wrapped object.

Mode-homogeneous schemas use this to express "this field must be absent / null in this variant" — e.g. classification's meta.groups is required, regression's must be null.

json()

nlohmann::json const & ppforest2::JsonReader::json ( ) const

inline

The raw underlying JSON (escape hatch for unusual cases).

only_keys()

void ppforest2::JsonReader::only_keys

(

std::initializer_list< char const * >

allowed

)

const

Assert the wrapped object contains only the given keys.

Exceptions

std::runtime_error

naming the first unexpected key it finds (plus the list of allowed keys for context).

optional()

template<typename T>

T ppforest2::JsonReader::optional	(	std::string const &	key,
		T	fallback ) const

Extract an optional typed value, falling back to fallback.

path()

std::string const & ppforest2::JsonReader::path ( ) const

inline

The path prefix used in error messages.

require()

template<typename T>

T ppforest2::JsonReader::require

(

std::string const &

key

)

const

Extract a required typed value.

Template Parameters

T	Target type (e.g. int, float, std::string).

Parameters

key	Field name within the wrapped object.

Exceptions

`std::runtime_error`

with the dotted path on missing keys or type mismatch.

require_array()

nlohmann::json const & ppforest2::JsonReader::require_array

(

std::string const &

key

)

const

Return a const reference to a required array field.

Exceptions

std::runtime_error

if the key is missing or the value is not a JSON array.

require_enum()

std::string ppforest2::JsonReader::require_enum	(	std::string const &	key,
		std::initializer_list< char const * >	allowed ) const

Extract a required string constrained to allowed values.

Exceptions

std::runtime_error

if the key is missing, not a string, or the value is not in the allowed set.

require_int()

long long ppforest2::JsonReader::require_int	(	std::string const &	key,
		long long	min = std::numeric_limits< long long >::min(),
		long long	max = std::numeric_limits< long long >::max() ) const

Extract a required integer value, optionally constrained.

Accepts any JSON number whose value is an integer — both literals written without a fractional part (5) and equivalent numeric forms (5.0, 5e0). Non-integer fractions (5.5), non-finite numbers (NaN, Inf), and non-numeric types are rejected.

This matches JSON Schema's type: integer semantics, which is value-based: the spec (RFC 8259 §6) does not distinguish integer from non-integer JSON numbers at the format level, and JSON Schema defines integer as "any JSON number with zero fractional part." nlohmann's is_number_integer() is a parse-level tag (whether the literal had a decimal point) that's stricter than the spec — we deliberately don't inherit it. This means values round-tripped through languages that serialize all numbers uniformly (R, Python with default json.dumps) validate correctly.

Parameters

key	Field name within the wrapped object.
min	Inclusive lower bound (default: no lower bound).
max	Inclusive upper bound (default: no upper bound).

require_keys()

void ppforest2::JsonReader::require_keys

(

std::initializer_list< char const * >

keys

)

const

Assert every key in keys is present on the wrapped object.

Mirror of only_keys: where that one rejects extra keys, this one rejects missing ones. Throws on the first absent key it finds.

require_number()

double ppforest2::JsonReader::require_number	(	std::string const &	key,
		double	min = -std::numeric_limits< double >::infinity(),
		double	max = std::numeric_limits< double >::infinity() ) const

Extract a required number (double), optionally constrained.

require_object()

void ppforest2::JsonReader::require_object

(

)

const

Assert the wrapped value is a JSON object.

require_string_array()

void ppforest2::JsonReader::require_string_array	(	std::string const &	key,
		bool	non_empty = false ) const

Assert key is an array of strings, optionally non-empty.

Exceptions

std::runtime_error

if the key is missing, not an array, empty (when non_empty is true), or contains non-string elements (path includes [i] for the offending index).

---

The documentation for this class was generated from the following file:

• core/src/utils/JsonReader.hpp