Why can't I initialize an auto deduced pointer with nullptr?

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Category:Languages

I was trying to do something similar to this:

auto foo = int*(nullptr); 

Which with VC++ doesn't compile with the error message:

Type name is not allowed

And with GCC 8.2 doesn't compile with:

Expected primary expression before 'int'

I was really curious as to why this appears to be an illegal syntax. In my mind it should be fine since literals can be initialized like this.

auto foo = int(2); 

The only way I could think off to get this to work was to either make a type alias or do this:

auto foo = std::add_pointer_t<int>(nullptr); 

I tried googling for this but frankly I don't even know how to properly formulate this question since my standardese is weak. Any insight would be appreciated!

 


int* is a "derived declarator type"(not a standard term, but useful for reasoning about this). A functional style cast notation (which is what int(2) is) can only contain a "A simple-type-specifier or typename-specifier". A derived declarator type doesn't fall under either category.

You have to either:

  • write it out in the form of a C-style cast:

    auto foo = (int*) nullptr; 
  • or in the form of a C++-style cast:

    auto foo = static_cast<int*>(nullptr); 
  • or alias the pointer type:

    using iptr = int*; auto foo = iptr(nullptr); 

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