Difference between revisions of "cpp/language/types"

Latest revision as of 22:33, 13 August 2024

(See also type for type system overview and the list of type-related utilities that are provided by the C++ library)

The following types are collectively called fundamental types :

• (possibly cv-qualified) void

• integral types
• floating-point types

[edit] void

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 of void, nor references to void. However, pointers to void and functions returning type void (procedures in other languages) are permitted.

[edit] Integral types

[edit] Standard 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).

[edit] 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.

Note: as with all type specifiers, any order is permitted: unsigned long long int and long int unsigned long name the same type.

[edit] Properties

The following table summarizes all available standard integer types and their properties in various common data models:

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).

[edit] Boolean type

bool — integer 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.

[edit] Character types

Character types are integer types used for a character representation.

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.

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.

[edit] Floating-point types

[edit] Standard floating-point types

The following three types and their cv-qualified versions are collectively called standard floating-point types.

float — single precision floating-point type. Usually IEEE-754 binary32 format.

double — double precision floating-point type. Usually IEEE-754 binary64 format.

long double — extended precision floating-point type. Does not necessarily map to types mandated by IEEE-754.

• IEEE-754 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 x87 80-bit 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).
• On PowerPC double-double can be used.

[edit] 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, but 1.0 / -0.0 == -INFINITY), and for some mathematical functions, e.g. sqrt(std::complex)
• not-a-number (NaN), which does not compare equal with anything (including itself). Multiple bit patterns represent NaNs, see std::nan, NAN. Note that C++ takes no special notice of signalling NaNs other than detecting their support by std::numeric_limits::has_signaling_NaN, and treats all NaNs as quiet.

Floating-point numbers may be used with arithmetic operators +, -, /, and * as well as 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 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.

[edit] 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 the resolution of CWG issue 1759), because a UTF-8 code unit of value 0x80 used in a UTF-8 string literal must be storable in a char type object.

The range for a floating-point type T is defined as follows:

• The minimum guaranteed range is the most negative finite floating-point number representable in T through the most positive finite floating-point number representable in T.
• If negative infinity is representable in T, the range of T is extended to all negative real numbers.
• If positive infinity is representable in T, the range of T is extended to all positive real numbers.

Since negative and positive infinity are representable in ISO/IEC/IEEE 60559 formats, all real numbers lie within the range of representable values of a floating-point type adhering to ISO/IEC/IEEE 60559.

1. ↑ 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.

[edit] 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)

• Win32 API (also called the Windows API) with compilation target 64-bit ARM (AArch64) or x86-64 (a.k.a. x64)

• 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).

[edit] Notes

[edit] Keywords

void, bool, true, false, char, char8_t, char16_t, char32_t, wchar_t, int, short, long, signed, unsigned, float, double

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit] References

[edit] See also

• The C++ type system overview
• Const-volatility (cv) specifiers and qualifiers
• Storage duration specifiers