Where Do I Find the Definition of Size_T

Where do I find the definition of size_t?

From Wikipedia

The stdlib.h and stddef.h header files define a datatype called size_t1 which is used to represent the size of an object. Library functions that take sizes expect them to be of type size_t, and the sizeof operator evaluates to size_t.

The actual type of size_t is platform-dependent; a common mistake is to assume size_t is the same as unsigned int, which can lead to programming errors,2 particularly as 64-bit architectures become more prevalent.

From C99 7.17.1/2

The following types and macros are defined in the standard header stddef.h

<snip>

size_t

which is the unsigned integer type of the result of the sizeof operator

Where is size_t Defined?

I presume this technically means that the definition of size_t in the global namespace has been deprecated?

Yes... but.

The Standard only mandates that std::size_t must be defined¹ by <cstddef>, it does not disallow an implementation to define ::size_t², but if the implementation does, the two definitions must match³.

As a conclusion, you should use std::size_t and should neither rely on ::size_t to be defined nor define it.

The following are UB:

// DON'T
using size_t = std::size_t;        // UB
using size_t = decltype(sizeof 1); // UB

---

¹⁾ [cstddef.syn]

namespace std {
    using ptrdiff_­t = see below;
    using size_­t = see below;
    using max_­align_­t = see below;
    using nullptr_­t = decltype(nullptr);

[...]

The contents and meaning of the header <cstddef> are the same as the C standard library header <stddef.h>, except that it does not declare the type wchar_­t, that it also declares the type byte and its associated operations ([support.types.byteops]), and as noted in [support.types.nullptr] and [support.types.layout].

²⁾[extern.types]/1

For each type T from the C standard library (These types are [...] size_­t,[...].), the types ​::​T and std​::​T are reserved to the implementation[.]

³⁾[extern.types]/1

[...] when defined, ​::​T shall be identical to std​::​T.

What is size_t in C?

From Wikipedia:

According to the 1999 ISO C standard
(C99), size_t is an unsigned integer
type of at least 16 bit (see sections
7.17 and 7.18.3).

size_tis an unsigned data type
defined by several C/C++ standards,
e.g. the C99 ISO/IEC 9899 standard,
that is defined in stddef.h.1 It can
be further imported by inclusion of
stdlib.h as this file internally sub
includes stddef.h.

This type is used to represent the
size of an object. Library functions
that take or return sizes expect them
to be of type or have the return type
of size_t. Further, the most
frequently used compiler-based
operator sizeof should evaluate to a
constant value that is compatible with
size_t.

As an implication, size_t is a type guaranteed to hold any array index.

What is size_t in C?

From Wikipedia:

According to the 1999 ISO C standard
(C99), size_t is an unsigned integer
type of at least 16 bit (see sections
7.17 and 7.18.3).

size_tis an unsigned data type
defined by several C/C++ standards,
e.g. the C99 ISO/IEC 9899 standard,
that is defined in stddef.h.1 It can
be further imported by inclusion of
stdlib.h as this file internally sub
includes stddef.h.

This type is used to represent the
size of an object. Library functions
that take or return sizes expect them
to be of type or have the return type
of size_t. Further, the most
frequently used compiler-based
operator sizeof should evaluate to a
constant value that is compatible with
size_t.

As an implication, size_t is a type guaranteed to hold any array index.

Where do I find the definition of size_t?

From Wikipedia

The stdlib.h and stddef.h header files define a datatype called size_t1 which is used to represent the size of an object. Library functions that take sizes expect them to be of type size_t, and the sizeof operator evaluates to size_t.

The actual type of size_t is platform-dependent; a common mistake is to assume size_t is the same as unsigned int, which can lead to programming errors,2 particularly as 64-bit architectures become more prevalent.

From C99 7.17.1/2

The following types and macros are defined in the standard header stddef.h

<snip>

size_t

which is the unsigned integer type of the result of the sizeof operator

Which header should I include for `size_t`?

Assuming I wanted to minimize the functions and types I was importing I'd go with cstddef as it doesn't declare any functions and only declares 6 types. The others focus on particular domains (strings, time, IO) that may not matter to you.

Note that cstddef only guarantees to define std::size_t, that is, defining size_t in namespace std, although it may provide this name also in the global namespace (effectively, plain size_t).

In contrast, stddef.h (which is also a header available in C) guarantees to define size_t in the global namespace, and may also provide std::size_t.

What's the difference between size_t and int in C++?

From the friendly Wikipedia:

The stdlib.h and stddef.h header files define a datatype called size_t which is used to represent the size of an object. Library functions that take sizes expect them to be of type size_t, and the sizeof operator evaluates to size_t.

The actual type of size_t is platform-dependent; a common mistake is to assume size_t is the same as unsigned int, which can lead to programming errors, particularly as 64-bit architectures become more prevalent.

Also, check Why size_t matters

---

Related Topics