size_t c++

Alias of one of the fundamental unsigned integer types.
It represents the size of any object in bytes and returned
by sizeof operator. It is used for array indexing and counting.
It can never be negative.

// size_t is a type definition for unsigned long long
// This means that writing size_t is the excact same as
// writing unsigned long long

// C program to demonstrate that size_t or 
// any unsigned int type should be used  
// carefully when used in a loop. 
#include<stdio.h> 
  
#define N 10 
  
int main() 
{ 
    int a[N]; 
  
    // This is fine. 
    for (size_t n = 0; n < N; ++n) { 
        a[n] = n; 
    } 
          
    // But reverse cycles are tricky for unsigned  
    // types as they can lead to infinite loops. 
    for (size_t n = N-1; n >= 0; --n) 
        printf("%d ", a[n]); 
} 

size_t is an unsigned integral data type which is defined in various header files such as:

<stddef.h>, <stdio.h>, <stdlib.h>, <string.h>, <time.h>, <wchar.h> 
It’s a type which is used to represent the size of objects in bytes and is therefore used as the return type by the sizeof operator. It is guaranteed to be big enough to contain the size of the biggest object the host system can handle. Basically the maximum permissible size is dependent on the compiler; if the compiler is 32 bit then it is simply a typedef(i.e., alias) for unsigned int but if the compiler is 64 bit then it would be a typedef for unsigned long long. The size_t data type is never negative.