From cppreference.com
(See also type for type system overview and the list of type-related utilities that are provided by the C++ library)
Tóm Tắt
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Void type
-
void
– type with an empty set of values. It is an incomplete type that cannot be completed (consequently, objects of type
void
are disallowed). There are no arrays ofvoid
, nor references tovoid
. However, pointers tovoid
and functions returning typevoid
(procedures in other languages) are permitted.
<cstddef>
Defined in header
typedef
decltype
(
nullptr
)
nullptr_t
;
(since C++11)
std::nullptr_t is the type of the null pointer literal, nullptr. It is a distinct type that is not itself a pointer type or a pointer to member type. Its values are null pointer constant (see NULL), and may be implicitly converted to any pointer and pointer to member type.
sizeof(std::nullptr_t) is equal to sizeof(void *).
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Data models
The choices made by each implementation about the sizes of the fundamental types are collectively known as data model. Four data models found wide acceptance:
32 bit systems:
-
- LP32 or 2/4/4 (int is 16-bit, long and pointer are 32-bit)
-
- Win16 API
- ILP32 or 4/4/4 (int, long, and pointer are 32-bit);
-
- Win32 API
- Unix and Unix-like systems (Linux, macOS)
64 bit systems:
-
- LLP64 or 4/4/8 (int and long are 32-bit, pointer is 64-bit)
-
- Win64 API
- LP64 or 4/8/8 (int is 32-bit, long and pointer are 64-bit)
-
- Unix and Unix-like systems (Linux, macOS)
Other models are very rare. For example, ILP64 (8/8/8: int, long, and pointer are 64-bit) only appeared in some early 64-bit Unix systems (e.g. UNICOS on Cray).
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Signed and unsigned integer types
-
int
– basic integer type. The keyword
int
may be omitted if any of the modifiers listed below are used. If no length modifiers are present, it’s guaranteed to have a width of at least 16 bits. However, on 32/64 bit systems it is almost exclusively guaranteed to have width of at least 32 bits (see below).
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Modifiers
Modifies the basic integer type. Can be mixed in any order. Only one of each group can be present in type name.
Signedness
-
signed
– target type will have signed representation (this is the default if omitted)
-
unsigned
– target type will have unsigned representation
Size
-
short
– target type will be optimized for space and will have width of at least 16 bits.
-
long
– target type will have width of at least 32 bits.
-
long
long
– target type will have width of at least 64 bits.
(since C++11)
Note: as with all type specifiers, any order is permitted: unsigned long long int and long int unsigned long name the same type.
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Properties
The following table summarizes all available integer types and their properties in various common data models:
Type specifier
Equivalent type
Width in bits by data model
C++ standard
LP32
ILP32
LLP64
LP64
short
short
int
at least
16
16
16
16
16
short
int
signed
short
signed
short
int
unsigned
short
unsigned
short
int
unsigned
short
int
int
int
at least
16
16
32
32
32
signed
signed
int
unsigned
unsigned
int
unsigned
int
long
long
int
at least
32
32
32
32
64
long
int
signed
long
signed
long
int
unsigned
long
unsigned
long
int
unsigned
long
int
long
long
long
long
int
(C++11)
at least
64
64
64
64
64
long
long
int
signed
long
long
signed
long
long
int
unsigned
long
long
unsigned
long
long
int
(C++11)
unsigned
long
long
int
Note: integer arithmetic is defined differently for the signed and unsigned integer types. See arithmetic operators, in particular integer overflows.
std::size_t is the unsigned integer type of the result of the sizeof
operator as well as the sizeof...
operator and the alignof
operator (since C++11).
See also Fixed width integer types.
(since C++11)
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Boolean type
-
bool
– type, capable of holding one of the two values: true or false. The value of
sizeof
(
bool
)
is implementation defined and might differ from 1.
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Character types
-
signed
char
– type for signed character representation.
-
unsigned
char
– type for unsigned character representation. Also used to inspect object representations (raw memory).
-
char
– type for character representation which can be most efficiently processed on the target system (has the same representation and alignment as either
signed
char
or
unsigned
char
, but is always a distinct type). Multibyte characters strings use this type to represent code units.
For every value of type
unsigned
char
in range [0, 255], converting the value to
char
and then back to
unsigned
char
produces the original value.
(since C++11)
The signedness of
char
depends on the compiler and the target platform: the defaults for ARM and PowerPC are typically unsigned, the defaults for x86 and x64 are typically signed.
-
wchar_t
– type for wide character representation (see wide strings). It has the same size, signedness, and alignment as one of the integer types, but is a distinct type. In practice, it is 32 bits and holds UTF-32 on Linux and many other non-Windows systems, but 16 bits and holds UTF-16 code units on Windows. The standard used to require
wchar_t
to be large enough to represent any supported character code point. However, such requirement cannot be fulfilled on Windows, and thus it is considered as a defect and removed by P2460R2.
-
char16_t
– type for UTF-16 character representation, required to be large enough to represent any UTF-16 code unit (16 bits). It has the same size, signedness, and alignment as std::uint_least16_t
-
char32_t
– type for UTF-32 character representation, required to be large enough to represent any UTF-32 code unit (32 bits). It has the same size, signedness, and alignment as std::uint_least32_t
(since C++11)
-
char8_t
– type for UTF-8 character representation, required to be large enough to represent any UTF-8 code unit (8 bits). It has the same size, signedness, and alignment as
unsigned
char
(and therefore, the same size and alignment as
char
and
signed
char
), but is a distinct type.
(since C++20)
Besides the minimal bit counts, the C++ Standard guarantees that
-
1
==
sizeof
(
char
)
<=
sizeof
(
short
)
<=
sizeof
(
int
)
<=
sizeof
(
long
)
<=
sizeof
(
long
long
)
.
Note: this allows the extreme case in which bytes are sized 64 bits, all types (including char) are 64 bits wide, and sizeof returns 1 for every type.
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Floating-point types
The following three types and their cv-qualified versions are collectively called floating-point types.
-
float
– single precision floating-point type. Matches IEEE-754 binary32 format if supported.
-
double
– double precision floating-point type. Matches IEEE-754 binary64 format if supported.
-
long
double
– extended precision floating-point type. Matches IEEE-754 binary128 format if supported, otherwise matches IEEE-754 binary64-extended format if supported, otherwise matches some non-IEEE-754 extended floating-point format as long as its precision is better than binary64 and range is at least as good as binary64, otherwise matches IEEE-754 binary64 format.
- binary128 format is used by some HP-UX, SPARC, MIPS, ARM64, and z/OS implementations.
- The most well known IEEE-754 binary64-extended format is 80-bit x87 extended precision format. It is used by many x86 and x86-64 implementations (a notable exception is MSVC, which implements
long
double
in the same format as
double
, i.e. binary64).
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Properties
Floating-point types may support special values:
- infinity (positive and negative), see INFINITY
- the negative zero,
–
0.0
. It compares equal to the positive zero, but is meaningful in some arithmetic operations, e.g.
1.0
/
0.0
==
INFINITY
1.0
/-
0.0
==
–
INFINITY
sqrt
(std::complex)
- not-a-number (NaN), which does not compare equal with anything (including itself). Multiple bit patterns represent NaNs, see std::nanNANstd::numeric_limits::has_signaling_NaN
Real floating-point numbers may be used with arithmetic operators + – / * and various mathematical functions from <cmath>. Both built-in operators and library functions may raise floating-point exceptions and set errno as described in math errhandling.
Floating-point expressions may have greater range and precision than indicated by their types, see FLT_EVAL_METHOD. Floating-point expressions may also be contracted, that is, calculated as if all intermediate values have infinite range and precision, see
#pragma STDC FP_CONTRACT. Standard C++ does not restrict the accuracy of floating-point operations.
Some operations on floating-point numbers are affected by and modify the state of the floating-point environment (most notably, the rounding direction).
Implicit conversions are defined between real floating types and integer types.
See Limits of floating-point types and std::numeric_limits for additional details, limits, and properties of the floating-point types.
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Range of values
The following table provides a reference for the limits of common numeric representations.
Prior to C++20, the C++ Standard allowed any signed integer representation, and the minimum guaranteed range of N-bit signed integers was from \(\scriptsize -(2^{N-1}-1)\)-(2N-1
-1) to \(\scriptsize +2^{N-1}-1\)+2N-1
-1 (e.g. -127 to 127 for a signed 8-bit type), which corresponds to the limits of ones’ complement or sign-and-magnitude.
However, all C++ compilers use two’s complement representation, and as of C++20, it is the only representation allowed by the standard, with the guaranteed range from \(\scriptsize -2^{N-1}\)-2N-1
to \(\scriptsize +2^{N-1}-1\)+2N-1
-1 (e.g. -128 to 127 for a signed 8-bit type).
8-bit ones’ complement and sign-and-magnitude representations for char have been disallowed since C++11 (via CWG 1759), because a UTF-8 code unit of value 0x80 used in a UTF-8 string literal must be storable in a char element object.
Type
Size in bits
Format
Value range
Approximate
Exact
character
8
signed
-128 to 127
unsigned
0 to 255
16
UTF-16
0 to 65535
32
UTF-32
0 to 1114111 (0x10ffff)
integer
16
signed
± 3.27 · 104
-32768 to 32767
unsigned
0 to 6.55 · 104
0 to 65535
32
signed
± 2.14 · 109
-2,147,483,648 to 2,147,483,647
unsigned
0 to 4.29 · 109
0 to 4,294,967,295
64
signed
± 9.22 · 1018
-9,223,372,036,854,775,808 to 9,223,372,036,854,775,807
unsigned
0 to 1.84 · 1019
0 to 18,446,744,073,709,551,615
binary
floating
point
32
IEEE-754
- min subnormal:
± 1.401,298,4 · 10-45 - min normal:
± 1.175,494,3 · 10-38 - max:
± 3.402,823,4 · 1038
- min subnormal:
±0x1p-149 - min normal:
±0x1p-126 - max:
±0x1.fffffep+127
64
IEEE-754
- min subnormal:
± 4.940,656,458,412 · 10-324 - min normal:
± 2.225,073,858,507,201,4 · 10-308 - max:
± 1.797,693,134,862,315,7 · 10308
- min subnormal:
±0x1p-1074 - min normal:
±0x1p-1022 - max:
±0x1.fffffffffffffp+1023
80[note 1]
x86
- min subnormal:
± 3.645,199,531,882,474,602,528
· 10-4951 - min normal:
± 3.362,103,143,112,093,506,263
· 10-4932 - max:
± 1.189,731,495,357,231,765,021
· 104932
- min subnormal:
±0x1p-16446 - min normal:
±0x1p-16382 - max:
±0x1.fffffffffffffffep+16383
128
IEEE-754
- min subnormal:
± 6.475,175,119,438,025,110,924,
438,958,227,646,552,5 · 10-4966 - min normal:
± 3.362,103,143,112,093,506,262,
677,817,321,752,602,6 · 10-4932 - max:
± 1.189,731,495,357,231,765,085,
759,326,628,007,016,2 · 104932
- min subnormal:
±0x1p-16494 - min normal:
±0x1p-16382 - max:
±0x1.ffffffffffffffffffffffffffff
p+16383
- ↑
The object representation usually occupies 96/128 bits on 32/64-bit platforms respectively.
Note: actual (as opposed to guaranteed minimal) limits on the values representable by these types are available in C numeric limits interface and std::numeric_limits.
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Keywords
void,
bool,
true,
false,
char,
wchar_t,
char8_t, (since C++20)
char16_t, char32_t, (since C++11)
int,
short,
long,
signed,
unsigned,
float,
double
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Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR
Applied to
Behavior as published
Correct behavior
CWG 238
C++98
the constraints placed on a floating-point implementation was unspecified
specified as no constraint
CWG 1759
C++11
char
is not guaranteed to be able to represent UTF-8 code unit 0x80
guaranteed
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See also
C documentation
arithmetic types
for