unique_ptr constructor with custom deleter is deleted

Question

This example compiles and runs well with gcc 4.8.3:

#include <memory>
#include <functional>
#include <iostream>

int main() {
    auto str = new const char[6]{'h', 'e', 'l', 'l', 'o', '\0'};
    std::unique_ptr<const char[], std::function<void(const char *)>> u_ptr(str, [](const char *s){ delete[] s; });
    std::cout << u_ptr.get() << std::endl;
}

But when I try it with Visual Studio Professional 2013 it doesn't compile (complains about a deleted function). Is this not yet possible with Visual Studio 2013? Or is my sample code wrong and gcc ignores my mistake?

Error is:

main.cpp(8) : error C2280: 'std::unique_ptr>::unique_ptr>(_Ptr2,_Dx2)' : attempting to reference a deleted function with [ _Ptr2=const char * , _Dx2=main:: ] C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\INCLUDE\memory(16 16) : see declaration of 'std::unique_ptr>::unique_ptr'

There should be a FAQ on "why is using std::function as a unique_ptr deleter a terrible idea?" — T.C.
Shouldn't T be char instead of char const[]? This works on Visual C++ 2015 RC. — isanae
std::unique_ptr already supports delete[] there is no reason to write your own here. — Mgetz
@T.C.: Can you elaborate or give some link about why is that a bad idea? — rodrigo
@rodrigo Exceptions. unique_ptr requires that all operations associated with its deleter must not throw; std::function's constructors can throw, because it uses type erasure and may need to allocate memory under the hood. (In the OP's code it's even worse, because the std::function would be constructed before unique_ptr's constructor is called, and if that throws then the pointer is leaked regardless of what unique_ptr requires.) — T.C.

isanae isanae · Accepted Answer · 2015-06-01T17:08:12

This appears to be a defect in the Visual C++ 2013 standard library. I cannot reproduce the problem on 2015.

The unique_ptr class has this constructor for taking a pointer and a deleter:

unique_ptr(pointer _Ptr,
    typename _If<is_reference<_Dx>::value, _Dx,
        const typename remove_reference<_Dx>::type&>::type _Dt) _NOEXCEPT
    : _Mybase(_Ptr, _Dt)
    {   // construct with pointer and (maybe const) deleter&
    }

However, the unique_ptr<T[]> specialization also has a catch-all constructor:

template<class _Ptr2,
    class _Dx2>
    unique_ptr(_Ptr2, _Dx2) = delete;

This version is preferred over the previous one.

However, because the non-specialized unique_ptr doesn't have it at all, changing u_ptr to a const char instead of const char[] fixes the problem.

Using the array version with a deleter like you're doing is also unnecessary:

If you want to call delete[] on your pointer, there's already a specialization for arrays. You don't need a custom deleter.
If you want to do something else, you should use the non-specialized version.

unique_ptr constructor with custom deleter is deleted

1 Answers