Why do I get a compilation error?

Question

I am using GCC 4.8 to compile the following code:

#include <memory>

template<typename T, typename ...Args>
    std::unique_ptr<T> make_unique(Args&& ...args) {
    return std::unique_ptr<T>(new T{std::forward<Args>(args)...});
}

struct S {
    template<class... Args>
    static std::unique_ptr<S> create(Args&&... args) {
        return make_unique<S>(std::forward<Args>(args)...);
    }
private: // if I remove this line, then the compilation is OK
    S(int) {}
    S() = default;
};

int main() {
    auto s1 = S::create(); // OK
    auto s2 = S::create(0); // Compilation error
}

Can anyone explain me the reason of this error from the compiler?

main.cpp: In instantiation of 'std::unique_ptr make_unique(Args&& ...) [with T = S; Args = {int}]':

main.cpp:11:58: required from 'static std::unique_ptr S::create(Args&& ...) [with Args = {int}]'

main.cpp:20:26: required from here

main.cpp:14:5: error: 'S::S(int)' is private

 S(int) {}
 ^

main.cpp:5:65: error: within this context return std::unique_ptr(new T{std::forward(args)...});

                                                             ^

Answer 1

Can anyone explain me the reason of this error from the compiler?

The constructor which takes int is declared to be private which is why it is giving compilation error. Note that the constructor is getting invoked from make_unique (which doesn't have access to private members), not from create.

However, you're probably wondering why the first call to to create() compiles fine, I think it is because GCC has bug. It should not compile even in this case, because the default constructor is declared to be private as well. Clang correctly gives error for both calls (see this).

Anyway, if you want to keep them private, then make make_unique a friend of the class.