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整理自 Thinking in C++

sizeof tells you #byte used by any particular variable. It can also give the size of a data type (with no variable name).

By definition, the size of any type of char (signed, unsigned or plain) is always 1, regardless of whether the underlying storage for a char is actually 1 byte (我不清楚什么平台下 char 会占 2 bytes;也不明白为什么要规定 char 的 size 一定要是 1,即使它占 2 bytes). For all other types, the result is the size in bytes.

Note that sizeof is an operator, not a function. If you apply it to a type, it must be used with the parentheses, but if you apply it to a variable you can use it without parentheses.

s = sizeof(int); // OK
// s = sizeof int; // Error

int i;
s = sizeof(i); // OK
s = sizeof i; // OK