CODEWITHSUNDEEP
c++templates
Alexander Mihailov
Alexander Mihailov

Reputation: 1152

Default template parameters

I want to build a class with configurable type and difference type template parameters. The first solution compiles fine at VS2012Nov and g++4.7.2:

template <typename T,
    typename DT = decltype(T()-T())>
class A { };

But when I hides decltype(T()-T()) to additional template, VS still compiles it, but g++ not.

template < typename T >
struct Delta {
    typedef decltype( T() - T() ) Value;
};

template <typename T,
    typename DT = Delta<T>::Value >
class A { };

Why g++ doesn't support such syntax?

Upvotes: 0

Views: 171

Answers (1)

Angew is no longer proud of SO
Angew is no longer proud of SO

Reputation: 171177

You're missing the typename keyword, to tell the compiler that the dependent name is a type:

template < typename T >
struct Delta {
    typedef decltype( T() - T() ) Value;
};

template <typename T,
    typename DT = typename Delta<T>::Value >
class A { };

Live example

For more info, see Where and why do I have to put the “template” and “typename” keywords?.

Upvotes: 1

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