What's the Difference Between Size_T and Int in C++

Difference between size_t and unsigned int?

if it is use to represent non negative value so why we not using unsigned int instead of size_t

Because unsigned int is not the only unsigned integer type. size_t could be any of unsigned char, unsigned short, unsigned int, unsigned long or unsigned long long, depending on the implementation.

Second question is that size_t and unsigned int are interchangeable or not and if not then why?

They aren't interchangeable, for the reason explained above ^^.

And can anyone give me a good example of size_t and its brief working ?

I don't quite get what you mean by "its brief working". It works like any other unsigned type (in particular, like the type it's typedeffed to). You are encouraged to use size_t when you are describing the size of an object. In particular, the sizeof operator and various standard library functions, such as strlen(), return size_t.

Bonus: here's a good article about size_t (and the closely related ptrdiff_t type). It reasons very well why you should use it.

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

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.

Difference between usage of type size_t and int when used as template type?

std::size_t is an unsigned type that is at least as large as an unsigned int.

int is a signed type, whose upper bound is less than that of an unsigned int.

There are going to be values which cannot be represented by int that size_t can represent.

Passing -1 as an int results in a negative value. Passing -1 as a size_t results in a large positive value.

Overflow on int is undefined behavior; undefined behavior at compile time makes expressions non-constexpr in some contexts.

Overflow on size_t is defined behavior, it is mathematics modulo 2^n for some (unspecified) n.

Many containers in C++ use size_t for their indexing, and tuple uses it for the index of its get.

There are disadvantages to unsigned values, in that they behave strangely (unlike "real integers") "near zero", while int behaves strangely far from zero, and being far from zero is a rarer case than being near zero.

size_t cannot be negative, which is seemingly makes sense to use to represent values that cannot be negative, but the wrap-around behavior can sometimes cause big problems. I find this happens less so with compile-time code however.

You could use ptrdiff_t, which is basically the signed equivalent of size_t, as another choice.

There are consequences to both choices. Which of these consequences you want to deal with is up to you. Which is better, a matter of opinion.

comparing int with size_t

It's safe provided the int is zero or positive. If it's negative, and size_t is of equal or higher rank than int, then the int will be converted to size_t and so its negative value will instead become a positive value. This new positive value is then compared to the size_t value, which may (in a staggeringly unlikely coincidence) give a false positive. To be truly safe (and perhaps overcautious) check that the int is nonnegative first:

/* given int i; size_t s; */
if (i>=0 && i == s)

and to suppress compiler warnings:

if (i>=0 && (size_t)i == s)

size_t vs int in C++ and/or C

In general, size_t should be used whenever you are measuring the size of something. It is really strange that size_t is only required to represent between 0 and SIZE_MAX bytes and SIZE_MAX is only required to be 65,535...

The other interesting constraints from the C++ and C Standards are:

• the return type of sizeof() is size_t and it is an unsigned integer
• operator new() takes the number of bytes to allocate as a size_t parameter
• size_t is defined in <cstddef>
• SIZE_MAX is defined in <limits.h> in C99 but not mentioned in C++98?!
• size_t is not included in the list of fundamental integer types so I have always assumed that size_t is a type alias for one of the fundamental types: char, short int, int, and long int.

If you are counting bytes, then you should definitely be using size_t. If you are counting the number of elements, then you should probably use size_t since this seems to be what C++ has been using. In any case, you don't want to use int - at the very least use unsigned long or unsigned long long if you are using TR1. Or... even better... typedef whatever you end up using to size_type or just include <cstddef> and use std::size_t.

unsigned int vs. size_t

The size_t type is the unsigned integer type that is the result of the sizeof operator (and the offsetof operator), so it is guaranteed to be big enough to contain the size of the biggest object your system can handle (e.g., a static array of 8Gb).

The size_t type may be bigger than, equal to, or smaller than an unsigned int, and your compiler might make assumptions about it for optimization.

You may find more precise information in the C99 standard, section 7.17, a draft of which is available on the Internet in pdf format, or in the C11 standard, section 7.19, also available as a pdf draft.

Why do I need to use `size_t` in C++?

Its main advantage is that it's the right tool for the job.

size_t is literally defined to be big enough to represent the size of any object on your platform. The others are not. So, when you want to store the size of an object, why would you use anything else?

You can use int if you like, but you'll be deliberately choosing the inferior option that leads to bugs. I don't quite understand why you'd want to do so, but hey it's your code.

If you choose to use float, though, please tell us what program you're writing so we can avoid it. :)

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