C++ - Return const unique_ptr

Question

I am wondering why I get an error when compiling:

const std::unique_ptr<int> get() { 
    return std::make_unique<int>(10);
}

int main() { 

    const std::unique_ptr<int> value = get();

    return EXIT_SUCCESS;
}

I get the following error:

main.cpp: In function ‘int main()’:
main.cpp:10:44: error: use of deleted function ‘std::unique_ptr<_Tp, _Dp>::unique_ptr(const std::unique_ptr<_Tp, _Dp>&) [with _Tp = int; _Dp = std::default_delete<int>]’
     const std::unique_ptr<int> value = get();

It compiles correctly when I remove const from the get signature.

Is there any way to return a constant unique_ptr ?

Answer 1

Because the unique_ptr is constant it can not be moved only copied. And copying a unique_ptr is not allowed (otherwise it would not be "unique").

If the data pointed to by the pointer should be constant, then use std::unique_ptr<const int> instead.