深入理解sizeof

深入理解sizeof2010-09-14房秉毅最近在论坛里总有人问关于sizeof的问题，并且本人对这个问题也一直没有得到很好的解决，索性今天对它来个较为详细的总结，同时结合strlen进行比较，如果能对大家有点点帮助，这是我最大的欣慰了。

一、好首先看看sizeof和strlen在MSDN上的定义：

首先看一MSDN上如何对sizeof进行定义的：

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.

然后再看一下对strlen是如何定义的：

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.