Where do I find the definition of size t

Question

I see variables defined with this type but I don t know where it comes from  nor what is its purpose  Why not use int or unsigned int   What about other  similar  types  Void t  etc

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In minimalistic programs where a size t definition was not loaded  by chance  in some include but I still need it in some context  for example to access std  vector lt double gt    then I use that context to extract the correct type  For example typedef std  vector lt double gt   size type size t    Surround with namespace       if necessary to make the scope limited

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This way you always know what the size is  because a specific type is dedicated to sizes  The very own question shows that it can be an issue  is it an int or an unsigned int  Also  what is the magnitude  short  int  long  etc    Because there is a specific type assigned  you don t have to worry about the length or the signed-ness  The actual definition can be found in the C   Reference Library  which says   Type  size t  Unsigned integral type  Header   lt cstring gt  size t corresponds to the integral data type returned by the language operator sizeof and is defined in the  lt cstring gt  header file  among others  as an unsigned integral type  In  lt cstring gt   it is used as the type of the parameter num in the functions memchr  memcmp  memcpy  memmove  memset  strncat  strncmp  strncpy and strxfrm  which in all cases it is used to specify the maximum number of bytes or characters the function has to affect  It is also used as the return type for strcspn  strlen  strspn and strxfrm to return sizes and lengths

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As for  Why not use int or unsigned int    simply because it s semantically more meaningful not to  There s the practical reason that it can be  say  typedefd as an int and then upgraded to a long later  without anyone having to change their code  of course  but more fundamentally than that a type is supposed to be meaningful  To vastly simplify  a variable of type size t is suitable for  and used for  containing the sizes of things  just like time t is suitable for containing time values  How these are actually implemented should quite properly be the implementation s job  Compared to just calling everything int  using meaningful typenames like this helps clarify the meaning and intent of your program  just like any rich set of types does

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Practically speaking size t represents the number of bytes you can address  On most modern architectures for the last 10-15 years that has been 32 bits which has also been the size of a unsigned int  However we are moving to 64bit addressing while the uint will most likely stay at 32bits  it s size is not guaranteed in the c   standard   To make your code that depends on the memory size portable across architectures you should use a size t  For example things like array sizes should always use size t s  If you look at the standard containers the   size   always returns a size t   Also note  visual studio has a compile option that can check for these types of errors called  Detect 64-bit Portability Issues

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I m not familiar with void t except as a result of a Google search  it s used in a vmalloc library by Kiem-Phong Vo at AT amp T Research - I m sure it s used in other libraries as well    The various xxx t typedefs are used to abstract a type from a particular definite implementation  since the concrete types used for certain things might differ from one platform to another   For example    size t abstracts the type used to hold the size of objects because on some systems this will be a 32-bit value  on others it might be 16-bit or 64-bit  Void t abstracts the type of pointer returned by the vmalloc library routines because it was written to work on systems that pre-date ANSI ISO C where the void keyword might not exist   At least that s what I d guess  wchar t abstracts the type used for wide characters since on some systems it will be a 16 bit type  on others it will be a 32 bit type    So if you write your wide character handling code to use the wchar t type instead of  say unsigned short  that code will presumably be more portable to various platforms

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size t should be defined in your standard library s headers   In my experience  it usually is simply a typedef to unsigned int   The point  though  is that it doesn t have to be  Types like size t allow the standard library vendor the freedom to change its underlying data types if appropriate for the platform   If you assume size t is always unsigned int  via casting  etc   you could run into problems in the future if your vendor changes size t to be e g  a 64-bit type   It is dangerous to assume anything about this or any other library type for this reason

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size t is the unsigned integer type of the result of the sizeof operator  ISO C99 Section 7 17    The sizeof operator yields the size  in bytes  of its operand  which may be an expression or the parenthesized name of a type  The size is determined from the type of the operand  The result is an integer  The value of the result is implementation-de ned  and  its type  an unsigned integer type  is size t  ISO C99 Section 6 5 3 4

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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  lt snip gt  size t which is the unsigned integer type of the result of the sizeof operator

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According to size t description on en cppreference com size t is defined in the following headers    std  size t           Defined in header  lt cstddef gt           Defined in header  lt cstdio gt        Defined in header  lt cstring gt           Defined in header  lt ctime gt         Defined in header  lt cwchar gt

[c++] Where do I find the definition of size_t?

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