Deep understanding of sizeof

Recently in the Forum has always been asked about sizeof, and I have not been a very good solution to this problem, simply today to it to a more detailed summary, and combined with strlen for comparison, if you have a bit of help, this is my greatest comfort.

First, let's take a look at the definition of sizeof and strlen on MSDN:

First look at how the sizeof is defined on an MSDN:

sizeof Operator
sizeof expression
The sizeof keyword gives the amount of storage, in bytes, associated with a variable or a type
(including aggregate types). This keyword returns a value of type size_t.
The expression is either an identifier or a type-cast expression (a type specifier enclosed in
parentheses).
When applied to a structure type or variable, sizeof returns the actual size, which may include
padding bytes inserted for alignment. When applied to a statically dimensioned array, sizeof
returns the size of the entire array. The sizeof operator cannot return the size of dynamically
allocated arrays or external arrays.

And then look at how the definition of strlen is defined:

strlen
Get the length of a string.
Routine Required Header：
strlen <string.h>
size_t strlen( const char *string );
Parameter
string：Null-terminated string
Libraries
All versions of the C run-time libraries.
Return Value
Each of these functions returns the number of characters in string, excluding the terminal
NULL. No return value is reserved to indicate an error.
Remarks
Each of these functions returns the number of characters in string, not including the
terminating null character. wcslen is a wide-character version of strlen; the argument of
wcslen is a wide-character string. wcslen and strlen behave identically otherwise.