定义变量与声明变量

Define v.s. Declare 

A variable is defined if, and only if, the lvalue column for that variable in the symbol table has a memory address assigned to it. When you declare a variable, the lvalue column in the symbol table for that variable does not contain a memory address.

The lvalue of a variable was an abbreviation for its "location value."

定义变量与声明变量

当且仅当为变量分配了内存地址之后，我们才说这是定义该变量；若该变量尚未被分配内存地址时，只能称之为声明变量。