make member variable immutable after constractor

[Kallinteris Andreas]

In other langs like Java there are keywords like 'final' where a member variable suffixed by 'final' is immutable after the construction of the object

I know C++ has the 'Const' but it required a variable's value to be set during declaration (not creation)

Note: the Variable should also be mutable during the deconstractor (do not want to be forced to leak memory)

struct X{
	const int f;
	X create(int i){
		X x;
		x.f = i;
		return x;
	}
};

int main() {
	X x.(5);
}

}

[keskiverto]

Umm, what?

struct X{
	const int f;
	X( int i ) : f(i) {}
};

int main() {
	X x(5);
}

[Kallinteris Andreas]

@keskiverto is it possible to set the value of f in the curly braces ({})
for example if I want to set the value f based on a branch operation or i through reading a file, how would I go about it
Thanks

[Ganado]

You can use ternary operator, or factor out more complicated logic into a function.

int foo(int i)
{
    if (i % 2 == 0)
    {
        return 42;
    }
    else
    {
        return 1729;   
    }
}

struct X{
	const int f;
	X( int i ) : f(foo(i)) {}
};

int main() {
	X x(5);
}

[seeplus]

As an example of using ternary:

struct X {
	const int f;
	X(int i, bool b) : f(b ? i : i * i) {}
};

int main() {
	X x(5, true);
}

as an example of using an immediate lambda:

#include <iostream>
#include <string>
#include <fstream>

struct X {
	const int f;
	X(const std::string& fn) : f([&fn]() {std::ifstream ifs(fn); int i {}; ifs >> i; return i; }()) {}
};

int main() {
	X x("test.txt");
}

[Kallinteris Andreas]

Is there a Reason as to why const variable can not be altered inside the constructor's {} also how to free it if I have to in the destructor's {}

P.S. I have previously marked this thread as solved, but I realized I have a few more questions

[seeplus]

cause it's const! const variables can only be initialised as they are defined.

Just use delete as would normally. However, having a pointer to const isn't often that helpful as the memory to which it points can't be changed. You can have a const pointer to memory.

struct myclass {
	int* const ptr {};  // const pointer to amendable memory

	myclass() : ptr(new int[10]) {}
	~myclass() { delete[] ptr; }
};

[lastchance]

Just make the member variable private. Then your member access arrangements will control what you can do to that variable (setting it in the constructor; doing whatever you want to do in the destructor).

[Kallinteris Andreas]

is there a way make a variable immutable after it's initialized and not it's declaration

[seeplus]

No.

A variable can only be set as const when it is defined. You can't define, set it's value with say an assignment and then set it as const.