CODEWITHSUNDEEP
c++templates
gpvos
gpvos

Reputation: 2802

Define template variables without a class

I want to define template variables without a class, but MSVC++ does not accept it, and Googling around it seems to be incorrect according to the C++ standard:

template<CharType> static CharType hexDigits[17];
template<> char hexDigits[17] = "0123456789ABCDEF";
template<> wchar_t hexDigits[17] = L"0123456789ABCDEF";

These specialized variables will then be used from within a (non-specialized) template function.

So I'm forced to write it like this:

template<typename CharType> class dummyclass {
    static CharType hexDigits[17];
};
template<> char dummyclass<char>::hexDigits[17] = "0123456789ABCDEF";
template<> wchar_t dummyclass<wchar_t>::hexDigits[17] = L"0123456789ABCDEF";

Is there any way I can define these two variables without defining a dummy class?

Also, is there any good reason why the C++ standard does not allow the first piece of code? After all, template functions outside a class are permitted.

Upvotes: 2

Views: 158

Answers (2)

Useless
Useless

Reputation: 67723

Also, is there any good reason why the C++ standard does not allow the first piece of code? After all, template functions outside a class are permitted.

Note that this:

template<CharType> static CharType hexDigits[17];
template<> char hexDigits[17] = "0123456789ABCDEF";
template<> wchar_t hexDigits[17] = L"0123456789ABCDEF";

has two symbols with different types but the same name: this can't possibly work, so the compiler would have to start mangling/decorating the names of variables like it already does for functions and classes.

In terms of implementing this cleanly, this looks like a trait to me ... if you don't mind getting a link error instead of a compile error, you can even skip the specialization and only declare the appropriate static members:

template <typename CharType> struct my_char_traits {
    static CharType hex_digits[17];
};

template<> char my_char_traits<char>::hex_digits[17] = "0123456789ABCDEF";
template<> wchar_t my_char_traits<wchar_t>::hex_digits[17] = L"0123456789ABCDEF";

Upvotes: 2

Tony The Lion
Tony The Lion

Reputation: 63190

You can only create a class template or struct or a function template.

Template variable's like you're trying to do is illegal in C++.

You'll have to create a class template and then use that.

Upvotes: 0

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