Code shared from run.dlang.io

main.d

import std.stdio;

struct B
{
    int a;
    int* t;
    
    this(int a) immutable
    {
        this.a = 7;
    }

    this(int a)
    {
        this.a = 10;
    }
}

void main()
{
    B a = immutable B(2);
    writeln(a.a);
    a.a = 4;

    immutable B a2 = immutable B(3);
    writeln(a2.a);
    //a2.a = 3;        // error : cannot modify
}