Why does operator new need constructor to be public?

Question

I was trying to find the relationship between operator new and constructor.

#include <iostream>
using namespace std;
class Foo {
public:
        void* operator new(size_t t) {
                cout << "new()" << endl;
        }
        void operator delete(void* ptr) {
        }
        Foo() {
                cout << "constructor()" << endl;
        }
};
int main() {
        Foo* f = new Foo();
        delete f;
        return 0;
}

Here's the output.

new()
constructor()

It seems like constructor is called by operator new right? Then I set constructor private.

#include <iostream>
using namespace std;
class Foo {
public:
    void* operator new(size_t t) {
        cout << "new()" << endl;
    }
    void operator delete(void* ptr) {
    }
private:
    Foo() {
        cout << "constructor()" << endl;
    }
};
int main() {
    Foo* f = new Foo();
    delete f;
    return 0;
}

And I got a compilation error

root# g++ main.cpp 
main.cpp: In function ‘int main()’:
main.cpp:13: error: ‘Foo::Foo()’ is private
main.cpp:19: error: within this context

Why can't operator new call private constructor, just like Singleton?

Answer 1

It seems like constructor is called by operator new right?

No. new Foo() is new expression, which attempts to allocate storage (via operator new) and then attempts to construct the object (via the constructor of Foo). So the object is constructed from the context of main(), which can't access the constructor of Foo and causes the error. For instance if you make main() the friend of Foo it'll compile fine.