Why C++ doesn't allow me to use typeof?

Question

I'm using following code with C++11 and getting a error that I'm not allowed to use typeof !

What is the problem and how to fix this ?

The error :

Error   10  error C2923: 'typeof' is not a valid template type argument for parameter 'C'

Here is my code :

#define HIBERLITE_NVP(Field) hiberlite::sql_nvp< typeof(Field) >(#Field,Field)


class Person{
friend class hiberlite::access;
template<class Archive>
void hibernate(Archive & ar)
{
    ar & HIBERLITE_NVP(name); //ERROR
    ar & HIBERLITE_NVP(age);  //ERROR
    ar & HIBERLITE_NVP(bio);  //ERROR
}
public:
string name;
double age;
vector<string> bio;
};

sql_nvp is like this :

template<class C>
 class sql_nvp{
public:
    std::string name;
    C& value;
    std::string search_key;

    sql_nvp(std::string _name, C& _value, std::string search="") :    name(_name), value(_value), search_key(search) {}
 };

Alternatively, you could write a function template which can infer the type, e.g. `make_nvp(#Field,Field)`. This will also work in older versions of the language without `decltype`. — Mike Seymour, Apr 18 '13 at 10:21
Agree with @MikeSeymour. To clarify: class templates do not have Template Argument Deduction, which is why the standard has a `std::make_pair(T, U)` to create a `std::pair`. Clearly, `make_nvp` would be a template function that returns a `sql_nvp`, using Template Argument Deduction to figure out the `Field` type. — MSalters, Apr 18 '13 at 11:40
In defence of OP a lot of compilers had typeof with variable semantic, some with underscores, etc. Hence the start-over with decltype. Plus Javascript, C# spell it this way. — emsr, Apr 19 '13 at 12:14
see http://stackoverflow.com/questions/1986418/typeid-and-typeof-in-c — Marius Bancila, Apr 29 '13 at 08:29
technically, `typeid` in C++ is more like `typeof` operator found in other languages like C# but certainly here the author was looking for `decltype` — , Apr 29 '13 at 08:21

score 30 · Accepted Answer · answered Apr 18 '13 at 10:16

30

What you are looking for is decltype():

#define HIBERLITE_NVP(Field) hiberlite::sql_nvp< decltype(Field) >(#Field,Field)
//                                               ^^^^^^^^

C++ does not have an operator called typeof.

answered Apr 18 '13 at 10:16

Andy Prowl

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It might be worth mentioning that there are/were differences between `typeof` and `decltype`. Depending on which compiler and version provided `typeof(X)`, it is more like `std::decay::type` than raw `decltype(X)`. – Daniel Frey Apr 18 '13 at 15:42
2

@blue: No need. The OP is explicitly writing: "*I'm using following code with C++11*" – Andy Prowl Apr 18 '13 at 22:10

Why C++ doesn't allow me to use typeof?

1 Answers1