No legal conversion to 'this' pointer

Please take a look at this code and run it:

I'm getting very strange error:开发者_如何学编程

Error 1 error C2663: 'Allocator::allocate_help' : 2 overloads have no legal conversion for 'this' pointer

template<class FailureSignal>
class Allocator
{
private:
    template<class Exception,class Argument>
    void allocate_help(const Argument& arg,Int2Type<true>)
    {
    }

    template<class Exception,class Argument>
    std::nullptr_t allocate_help(const Argument& arg,Int2Type<false>)
    {
        return nullptr;
    }

public:
    template<class T>
    void Allocate(signed long int nObjects,T** ptr = 0)const
    {
    allocate_help<std::bad_alloc>(1,Int2Type<true>());  
    }

};

int _tmain(int argc, _TCHAR* argv[])
{
    Allocator<int> all;
    all.Allocate<int>(1);
    return 0;
}  

I absolutely do not understand this err msg. Hope someone can help me with this. Thank you.

---

I noticed Allocate is declared const but allocate_help is not - could that be related to the issue?

---

I had the same error which was also caused by const but in a bit different way.

I have two virtual functions (overloads), one was const and the other was not. This was causing the problem. Turns out if you want to overload a function, they both need to match if they are const or not.

virtual void value() const = 0;
virtual void value(MyStruct & struct) = 0;

The above code will cause this error. The fix is to change declaration of 2nd to:

virtual void value(MyStruct & struct) const = 0;

---

I had this problem, when i tried to change member data in a const declared method.

As we all know, const methods are supposed not to change member data. The compiler gcc/clang doesn't return good error message.

It successfully compiled after removing const specifier from

void I_am_a_const_method_but_trying_to_modify_member() const{
        amap.emplace(std::make_tuple(1, 1, "a"), 1);
    }

compiler explorer link