Re: Invalid Template Argument?

From:

Victor Bazarov <v.Abazarov@comAcast.net>

Newsgroups:

comp.lang.c++

Date:

Sat, 14 Aug 2010 19:15:49 -0400

Message-ID:

<i4783h$7nj$1@news.datemas.de>

On 8/14/2010 1:57 PM, Immortal Nephi wrote:

Why do I get compiler error message? It explains that I am not
allowed to create variable in the template parameter, but constant
variable is acceptable.
How can I fix the error?

Try initializing your static integral consts right in their respective
class definitions (full 'Traits' template specializations), instead of
later.

c:\test_mycode.cpp(24) : error C2975: 'size' : invalid template
argument for 'Test', expected compile-time constant expression
         c:\test_template.h(41) : see declaration of 'size'

// Test_Template.h

#if !defined( TEST_TEMPLATE_H )
#define TEST_TEMPLATE_H

enum Type {
    Sample_Type1,
    Sample_Type2
};

template< Type>
struct Traits {
};

template<>
struct Traits< Sample_Type1> {
    static const int x;

Try writing

static const int x = 1;

static const int y;
};

template<>
struct Traits< Sample_Type2> {
static const int x;

And here (just in case)

static const int x = 2;

     static const int y;
};

template< typename T, int size>
class Test {
};

#endif // end macro !defined( TEST_TEMPLATE_H )

// Test_Template.cpp

#include "Test_Template.h"

const int Traits< Sample_Type1>::x = 1;
const int Traits< Sample_Type1>::y = 2;

const int Traits< Sample_Type2>::x = 10;
const int Traits< Sample_Type2>::y = 20;

template
class Test< int, Traits< Sample_Type1>::x>;

template
class Test< int, Traits< Sample_Type2>::x>;

// main.cpp

#include "Test_Template.h"

const Type type = Sample_Type1;
const int size = Traits< type>::x;

int main() {
// Test< int, size> test;
    Test< int, Traits< type>::x> test2;

    return 0;
}

V
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