C Question
Can I write to a const member of a non-const struct?
Is this code legal?:
#include <stdio.h>
typedef struct a_d{
int const x;
} a_d;
int a_d__ctor(a_d *X)
{
*(int*)&X->x = 42; //<-- legal or not?
return 0;
}
int main()
{
a_d a;
a_d__ctor(&a);
printf("a.x=%d\n", a.x);
}
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Answer Source
Modyfing an object declared with const qualifier invokes undefined behavior.
According to the Standard (emphasis mine):
C11 6.7.3/6 Type qualifiers
If an attempt is made to modify an object defined with a const-qualified type through use of an lvalue with non-const-qualified type, the behavior is undefined.
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