C语言中结构体的直接赋值

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aihouhen 2016-04-30 10:39:17

比如：
struct A{
char v1[20];
int v2;} a,b;

a = b;

这是没有任何问题的赋值.

struct B{
char *v1;
int v2;} c,d;
c = d;
这种结构体赋值，就需要注意（包括在C++里）。对于指针赋值，它不是数据进行了复制保存而是多了一个指针指向而已，这样一旦b对象释放，a的指向就成了非法的垃圾数据。
所以在 C/C++ 中，有些需要要另外自己定义复制函数的。

C语言直接支持结构体变量的赋值，它是按值逐元素赋值的。

参看K&R A.7.17 Assignment Expressions

In the simple assignment with =, the value of the expression replaces that of the object referred to by the lvalue. One of the following must be true: both operands have arithmetic type, in which case the right operand is converted to the type of the left by the assignment; or both operands are structures or unions of the same
                            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
type; or one operand is a pointer and the other is a pointer to void, or the left
~~~~
operand is a pointer and the right operand is a constant expression with value 0; or both operands are pointers to functions or objects whose types are the same except for the possible absence of const or volatile in the right operand.

进一步，C99标准§6.7.2.1上说：
  struct s { int n; double d[]; };
……
  struct s *s1;
  struct s *s2;
……
The assignment:
  *s1 = *s2;
only copies the member n and not any of the array elements.

而C99标准的TC2 Committee Draft则进一步指出：
if any of the array elements are within the first sizeof (struct s) bytes
of the structure, they might be copied or simply overwritten with indeterminate values.