Private constructors

[full wasp]

If I want to make a constructor for my class private do I need to provide an implementation? Or can I just put the declaration and no implementation since I’d never use it.

[hidden night]

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[lucid knot]

private constructors ARE used, just not by the user, but by the class itself

;compile

#include <iostream>
class foo {
    foo(int a, int b): x{a}, y{b} {}
public:
    int x, y;
    static foo twice(int a) {
        return foo(a, 2*a);
    }
};

int main() {
    foo f = foo::twice(4);
    std::cout << f.y << std::endl;
}

[peak garnet]

Program Output

8

rosepointer | 30ms | c++ | x86-64 gcc 15.2 | godbolt.org

[trail veldt]

honestly you should just let go of the idea that you don't need to provide an implementation just because you don't use it

if you declare a function, you should provide an implementation (or = default, = delete or = 0 it)

if you already know you're not going to use something, you probably don't want to declare it in the first place, or = delete it