CODEWITHSUNDEEP
c++visual-studiovisual-studio-2017standards-compliance
Ishita
Ishita

Reputation: 489

Compilation Error with (/permissive-) while creating a 'ClassType<std::unique_ptr<Base>>' from 'std::unique_ptr<Derived>'

I have a code snippet which used to work properly when conformance mode settings were set to off. When I set the conformance mode to (/permissive-). I am facing error in part of the code where template template parameter is involved.

My code is as follows:

template <typename T> using IsNotLValue = std::enable_if_t<!std::is_lvalue_reference_v<T>>;

// Base Class Template
template <typename T> class MyClassBase 
{ 
   protected:   
       template <typename T_ = T, typename = IsNotLValue<T_>>   
       explicit MyClassBase(T &&ptr) : ptr_(std::move(ptr))
       { 
          std::cout << "MyClassBase ::IsNotLValue" << std::endl; 
       }

       T ptr_ = nullptr; 
};


// Derived Class Template
template<typename T> class MyClass : public MyClassBase<T> 
{ 
  public:

    template <typename T_ = T, typename = IsNotLValue<T_>> 
    MyClass(T &&ptr) : MyClassBase<T>(std::move(ptr))   
    {       
      std::cout << "MyClass :: IsNotLValue" << std::endl;   
    }
};

// Base Class
class Base 
{  
 public:    
   int x = 0;   
   Base() : x(10) {}
};

// Derived Class
class Derived : public Base 
{ 
 public:    
    Derived() :Base(), y(20) {}
    int y = 0; 
};

// Function returning MyClass Object to pointer to Base Class Type
MyClass<Base*> ptrToBase()
{   
  return new Derived();    // Line 1 : OK
}

// Function returning uniqueptr to MyClass Object to pointer to Base Class Type
MyClass<std::unique_ptr<Base>> uniquePtrToBase() 
{   
  return std::make_unique<Derived>(); // Line 2 :compilation error here
}

int main() 
{   
  auto pUB = uniquePtrToBase();     
  auto pB = ptrToBase();
  return 0; 
}

Line 2 is giving compilation error when conformance mode is set to ON. It is getting fixed, If I add another constructor as below:

template <typename D, typename = IsNotLValue<D>>
        MyClass(D &&ptr) : MyClassBase<T>(std::move(ptr))   
        { 
          std::cout << "MyClass for uniqueptr" << std::endl;    
        }

However, I want to enable this function only if template parameter for D is derived from template parameter of T or D is derived from T, so I modified the condition to :

 template <typename D, typename T> using IsConvertibleToBase =
     std::enable_if_t<std::is_convertible_v<D*, T*>>;

    template <typename D, typename = IsConvertibleToBase<D, T>>
    MyClass(D &&ptr) : MyClassBase<T>(std::move(ptr))
    {
        std::cout << "MyClass :: IsConvertibleToBase" << std::endl;
    }

This code is giving compilation error because, here D is std::unique_ptr<Derived> and T is std::unique_ptr<Base>. Hence, is_convertible is failing.

How can I ensure that this ctor is enabled only for inherited classes? And Why this error is occurring with conformance mode on, if its the ZC conformance mode, then which one? I tried overriding twoPhase, that didnt work?

Upvotes: 0

Views: 402

Answers (1)

Dinesh Khandelwal
Dinesh Khandelwal

Reputation: 327

template <typename D, typename T> 
using IsConvertibleToBase = std::enable_if_t<std::is_convertible_v<D*, T*>>;

With above definition, D* and T* will be an error when IsConvertibleToBase is used with unique_ptr.

One solution is to use pointer_traits which will work for any pointer-like type:

template <typename D, typename T> 
using IsConvertibleToBase = std::enable_if_t<
    std::is_convertible_v<typename std::pointer_traits<D>::pointer, 
                          typename std::pointer_traits<T>::pointer>>;

Upvotes: 1

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