CODEWITHSUNDEEP
c++templates
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There is nothing we can do

Reputation: 24705

Specializing template for enum

Could anyone tell me why this doesn't work? 

enum CompCriteria{ByKey,ByValue,ByeKeyAndValue};

template<class T>
struct X;

template<>
struct X<CompCriteria>
{
};

int _tmain(int argc, _TCHAR* argv[])
{
    X<CompCriteria::ByeKeyAndValue> x;
    return 0;
}

Upvotes: 5

Views: 3973

Answers (4)

Charles Salvia
Charles Salvia

Reputation: 53339

You're conflating the idea of parameterized types and parameterized values. A template parameter can be a type, or a constant. For example:

template <typename T>
struct Foo;

versus..

template <int N>
struct Foo;

It looks like you want to specialize your template based on an enum constant, rather than a type. Meaning, you need to say:

enum CompCriteria{ByKey,ByValue,ByeKeyAndValue};

template<CompCriteria>
struct X;

// Specialization for ByKeyAndValue
//
template<>
struct X<ByeKeyAndValue>
{
};

int main()
{
    X<ByeKeyAndValue> x; // instantiate specialization 
    return 0;
}

Also, you can't refer to enums using the namespace operator. If you want to encapsulate your enums, you need to wrap them in a namespace:

namespace CompCriteria
{
   enum type {ByKey,ByValue,ByeKeyAndValue};
}

Upvotes: 9

Kirill V. Lyadvinsky
Kirill V. Lyadvinsky

Reputation: 99725

You have specialized X for a type, but you're trying to use it with the integer CompCriteria::ByeKeyAndValue.

You can specialize template class for the enum CompCriteria underlying type - int in this case, as follows:

enum CompCriteria{ByKey,ByValue,ByeKeyAndValue};

template<int>
struct X;

template<>
struct X<ByeKeyAndValue>
{
};

int main()
{
    X<ByeKeyAndValue> x;
    return 0;
}

Upvotes: 2

Alexey Malistov
Alexey Malistov

Reputation: 27023

enum CompCriteria{ByKey,ByValue,ByeKeyAndValue};

template<CompCriteria crit_>
struct X
{
    static const CompCriteria crit = crit_;
};


int _tmain(int argc, _TCHAR* argv[])
{
    X<CompCriteria::ByeKeyAndValue> x;
    return 0;
}

Upvotes: 2

Kos
Kos

Reputation: 72329

If you have a template<class T> = template<typename T>, then T is expected to be, well, a type.

enum CompCriteria is a type, so you can instantiate that template with it. A single value of the enum, however, is not a type, so you can't.

Upvotes: 2

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