decltype error C2440 cannot convert from 'int *' to 'int *&'

Question

The following is a contrived example of the actual code:

int** Ptr = 0;
decltype(Ptr[0]) Test = (int*)0;

I get the error:

Answer 1

If we go to the draft C++ standard section 7.1.6.2 Simple type specifiers [dcl.type.simple] and see what he cases are. For decltype it starts out saying:

For an expression e, the type denoted by decltype(e) is defined as follows:

We see that in this case the expression is not a id-expression nor a class member access, which would give the result you expected(emphasis mine):

• if e is an unparenthesized id-expression or an unparenthesized class member access (5.2.5), decltype(e) is the type of the entity named by e. If there is no such entity, or if e names a set of overloaded functions, the program is ill-formed;

but the result is an lvalue:

• otherwise, if e is an lvalue, decltype(e) is T&, where T is the type of e;

which results in a reference.

As M.M point out std::remove_reference could be used to obtain the result you want:

std::remove_reference::type Test = (int*)0;

As T.C. points out std::decay is also an option and is shorter:

std::decay::type Test = (int*)0;