const struct { x } vs struct { const x} [duplicate] - c++

Question

This question already has answers here:
How does a 'const struct' differ from a 'struct'?
(5 answers)
Closed 9 years ago.
This is a rather simple question, but somehow difficult to find a simple answer for.
In C++, what is the difference between an (edit) const-modified struct variable, and a(edit:) non-const struct variable, but with the struct having all-const members? :
typedef struct mystruct {
const int x;
} t1;
const t1 s1;
vs
typedef struct {
int x;
} t2;
const t2 s2;
? (If the answer is "the same as for classes", then please either explain it for classes or link to the explanation.)

Answer 1

There's no such thing as a const struct. You may have seen something like this:
const struct {
int x;
} y;
This is a declaration of a variable y with struct type. The variable y is const, not the struct. You can think of it as similar to:
struct mystruct {
int x;
};
const mystruct y;
without giving the struct type a name.

Answer 2

There's effectively very little difference between the two objects a and b below:
struct A
{
int x, y;
};
struct B
{
const int x, y;
};
const A a; // (plus initialiser)
B b; // (plus initialiser)
(You know, of course, that other instances of A may not be const-qualified and then you have an obvious difference.)
You can't access the members any differently in the one case than in the other. But:
You will have to ensure that you initialise the members, in both cases (which I haven't done here);
const-qualifiying the type (rather than the members) affects reference-binding:
void foo(A& a);
void foo(B& b);
int main()
{
const A a;
B b;
foo(a); // Error!
foo(b); // OK
}
The same is true, of course, if you use a pointer instead of a reference. The const context only spreads to calling code when it's applied to the type rather than encapsulated within the members.

Answer 3

Assuming
const struct mystruct1 {
int x1;
int x2;
} s1;
and
struct mystruct2 {
const int x1;
int x2;
} s2;
For s1 you should not assign values to any members.
For s2 only the member x1 should not be assigned a value. One is free to do so for x2.
To get closer to your example, one could do:
typedef const struct mystruct1 {
int x1;
int x2;
} S1;
typedef struct mystruct2 {
const int x1;
int x2;
} S2;
S1 s1;
S2 s2;
For s1 and s2 here the same rules apply as above.
Upate:
Referring your question literally (implying the corrections as by my examples), there is no practical difference between the two construct in terms constantness of the value they carry.