[cpp.predefined]

The following macro names shall be defined by the implementation:

(1.1)
__cplusplus

The integer literal 202302L.

[Note 1:
Future revisions of C++ will replace the value of this macro with a greater value.
— end note]
(1.2)
The names listed in Table 22 .

The macros defined in Table 22 shall be defined to the corresponding integer literal.

[Note 2:
Future revisions of C++ might replace the values of these macros with greater values.
— end note]
(1.3)
__DATE__

The date of translation of the source file: a character string literal of the form "Mmm dd yyyy", where the names of the months are the same as those generated by the asctime function, and the first character of dd is a space character if the value is less than 10.

If the date of translation is not available, an implementation-defined valid date shall be supplied.
(1.4)
__FILE__

The presumed name of the current source file (a character string literal).138
(1.5)
__LINE__

The presumed line number (within the current source file) of the current source line (an integer literal).139
(1.6)
__STDC_HOSTED__

The integer literal 1 if the implementation is a hosted implementation or the integer literal 0 if it is a freestanding implementation ([intro.compliance]).
(1.7)
__STDCPP_DEFAULT_NEW_ALIGNMENT__

An integer literal of type std::size_t whose value is the alignment guaranteed by a call to operator new(std::size_t) or operator new[](std::size_t).

[Note 3:
Larger alignments will be passed to operator new(std::size_t, std::align_val_t), etc.

([expr.new]).
— end note]
(1.8)
__STDCPP_FLOAT16_T__

Defined as the integer literal 1 if and only if the implementation supports the ISO/IEC/IEEE 60559 floating-point interchange format binary16 as an extended floating-point type ([basic.extended.fp]).
(1.9)
__STDCPP_FLOAT32_T__

Defined as the integer literal 1 if and only if the implementation supports the ISO/IEC/IEEE 60559 floating-point interchange format binary32 as an extended floating-point type.
(1.10)
__STDCPP_FLOAT64_T__

Defined as the integer literal 1 if and only if the implementation supports the ISO/IEC/IEEE 60559 floating-point interchange format binary64 as an extended floating-point type.
(1.11)
__STDCPP_FLOAT128_T__

Defined as the integer literal 1 if and only if the implementation supports the ISO/IEC/IEEE 60559 floating-point interchange format binary128 as an extended floating-point type.
(1.12)
__STDCPP_BFLOAT16_T__

Defined as the integer literal 1 if and only if the implementation supports an extended floating-point type with the properties of the typedef-name std::bfloat16_t as described in [basic.extended.fp].
(1.13)
__TIME__

The time of translation of the source file: a character string literal of the form "hh:mm:ss" as in the time generated by the asctime function.

If the time of translation is not available, an implementation-defined valid time shall be supplied.

Table 22: Feature-test macros [tab:cpp.predefined.ft]

🔗	Macro name	Value
🔗	__cpp_aggregate_bases	201603L
🔗	__cpp_aggregate_nsdmi	201304L
🔗	__cpp_aggregate_paren_init	201902L
🔗	__cpp_alias_templates	200704L
🔗	__cpp_aligned_new	201606L
🔗	__cpp_attributes	200809L
🔗	__cpp_auto_cast	202110L
🔗	__cpp_binary_literals	201304L
🔗	__cpp_capture_star_this	201603L
🔗	__cpp_char8_t	202207L
🔗	__cpp_concepts	202002L
🔗	__cpp_conditional_explicit	201806L
🔗	__cpp_constexpr	202211L
🔗	__cpp_constexpr_dynamic_alloc	201907L
🔗	__cpp_constexpr_in_decltype	201711L
🔗	__cpp_consteval	202211L
🔗	__cpp_constinit	201907L
🔗	__cpp_decltype	200707L
🔗	__cpp_decltype_auto	201304L
🔗	__cpp_deduction_guides	201907L
🔗	__cpp_delegating_constructors	200604L
🔗	__cpp_designated_initializers	201707L
🔗	__cpp_enumerator_attributes	201411L
🔗	__cpp_explicit_this_parameter	202110L
🔗	__cpp_fold_expressions	201603L
🔗	__cpp_generic_lambdas	201707L
🔗	__cpp_guaranteed_copy_elision	201606L
🔗	__cpp_hex_float	201603L
🔗	__cpp_if_consteval	202106L
🔗	__cpp_if_constexpr	201606L
🔗	__cpp_impl_coroutine	201902L
🔗	__cpp_impl_destroying_delete	201806L
🔗	__cpp_impl_three_way_comparison	201907L
🔗	__cpp_implicit_move	202207L
🔗	__cpp_inheriting_constructors	201511L
🔗	__cpp_init_captures	201803L
🔗	__cpp_initializer_lists	200806L
🔗	__cpp_inline_variables	201606L
🔗	__cpp_lambdas	200907L
🔗	__cpp_modules	201907L
🔗	__cpp_multidimensional_subscript	202211L
🔗	__cpp_named_character_escapes	202207L
🔗	__cpp_namespace_attributes	201411L
🔗	__cpp_noexcept_function_type	201510L
🔗	__cpp_nontype_template_args	201911L
🔗	__cpp_nontype_template_parameter_auto	201606L
🔗	__cpp_nsdmi	200809L
🔗	__cpp_range_based_for	202211L
🔗	__cpp_raw_strings	200710L
🔗	__cpp_ref_qualifiers	200710L
🔗	__cpp_return_type_deduction	201304L
🔗	__cpp_rvalue_references	200610L
🔗	__cpp_size_t_suffix	202011L
🔗	__cpp_sized_deallocation	201309L
🔗	__cpp_static_assert	201411L
🔗	__cpp_static_call_operator	202207L
🔗	__cpp_structured_bindings	201606L
🔗	__cpp_template_template_args	201611L
🔗	__cpp_threadsafe_static_init	200806L
🔗	__cpp_unicode_characters	200704L
🔗	__cpp_unicode_literals	200710L
🔗	__cpp_user_defined_literals	200809L
🔗	__cpp_using_enum	201907L
🔗	__cpp_variable_templates	201304L
🔗	__cpp_variadic_templates	200704L
🔗	__cpp_variadic_using	201611L

2

#

The following macro names are conditionally defined by the implementation:

(2.1)
__STDC__

Whether __STDC__ is predefined and if so, what its value is, are implementation-defined.
(2.2)
__STDC_MB_MIGHT_NEQ_WC__

The integer literal 1, intended to indicate that, in the encoding for wchar_t, a member of the basic character set need not have a code value equal to its value when used as the lone character in an ordinary character literal.
(2.3)
__STDC_VERSION__

Whether __STDC_VERSION__ is predefined and if so, what its value is, are implementation-defined.
(2.4)
__STDC_ISO_10646__

An integer literal of the form yyyymmL (for example, 199712L).

Whether __STDC_ISO_10646__ is predefined and if so, what its value is, are implementation-defined.
(2.5)
__STDCPP_THREADS__

Defined, and has the value integer literal 1, if and only if a program can have more than one thread of execution .

3

#

The values of the predefined macros (except for __FILE__ and __LINE__) remain constant throughout the translation unit.

4

#

If any of the pre-defined macro names in this subclause, or the identifier defined, is the subject of a #define or a #undef preprocessing directive, the behavior is undefined.

Any other predefined macro names shall begin with a leading underscore followed by an uppercase letter or a second underscore.

15 Preprocessing directives [cpp]

15.11 Predefined macro names [cpp.predefined]