比如：
struct A{
char v1[20];
int v2;} a,b;
a = b;
這是沒有任何問題的賦值.
struct B{
char *v1;
int v2;} c,d;
c = d;
這種結構體賦值，就需要注意（包括在C++裏）。對於指針賦值，它不是數據進行了複製保存而是多了一個指針指向而已，這樣一旦b對象釋放，a的指向就成了非法的垃圾數據。
所以在 C/C++ 中，有些需要要另外自己定義複製函數的。
C語言直接支持結構體變量的賦值，它是按值逐元素賦值的。
參看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.
進一步，C99標準§6.7.2.1上說：
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標準的TC2 Committee Draft則進一步指出：
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.
#include <stdio.h>
typedef struct {
int domain;
int set_id;
int ctxt;
}Dcs;
int main(int argc, char **argv)
{
Dcs dcs = {10,20,30};
Dcs dcs1;
dcs1 = dcs;
printf("befor:dcs1 = %d %d %d \n",
dcs1.domain, dcs1.set_id, dcs1.ctxt);
dcs.domain = 20;
dcs.set_id = 20;
dcs.ctxt = 20;
printf("last dcs = %d %d %d \n",
dcs.domain, dcs.set_id, dcs.ctxt);
printf("last dcs1 = %d %d %d \n",
dcs1.domain, dcs1.set_id, dcs1.ctxt);
return 0;
}
轉載：http://bbs.csdn.net/topics/300160344