Friend function is unable to construct a unique pointer of the class

Question

I have a certain design strategy where the constructor of my class is private and can only be constructed by friends of the class. Inside the friend function, I am trying to cre

Answer 1

Here is another approach I've seen used, apparently known as the passkey idiom : have the public constructor require a private access token.

class Spam {
    struct Token {};
    friend void Foo();
public:
    Spam(Token, int mem) : mem(mem) {}

private:
    int mem;
};

void Foo() {
    std::unique_ptr spam = std::make_unique(Spam::Token{}, 10);
}

void Bar() {
    // error: 'Spam::Token Spam::token' is private
    // std::unique_ptr spam = std::make_unique(Spam::Token{}, 10);
}