default value initializer in templatized class

Posted on 2002-04-15
Last Modified: 2010-04-02
I want to create a templatizable class which includes a default value for the templatized data type.  The following oversimplified example will work for atomic types like int and BOOL but not for data types that are structures or classes:

template <class DataType, DataType DefaultValue>
class CMyClass
  DataType m_Data;
  CMyClass() {m_Data = DefaultValue;}
}; //end CMyClass

I get several errors if I try to instantiate the following:
CMyClass<CString, "default string value"> MyObject;

I found that CString is not a valid type for a template argument because structures are not allowed.  MSDN (my C++ documentation) says to use a pointer instead if that is the case, but when I have tried that, I get an error like "expected constant value" because I have to instantiate it something like this:

const CString DefaultValue("default string value")
CMyClass<CString*, &DefaultValue> MyObject;

and &DefaultValue is where it expects a constant value and I can't give it one.  I use CString as an example, but I want to be able to use this on custom classes as well (all of which would support operator= so that the templatized constructor code m_Data = DefaultValue would work) otherwise, there are a variety of alternative solutions I could use just for strings.


I want to be able to make a similar class for arrays like this:

template <class DataType, int nNumElements, const DataType* pDefaultValue>
class CMyArrayClass
  DataType m_arData[nNumElements];
    for (int i=0; i<nNumElements; i++)
       m_arData[i] = pDefaultValue[i];
}; //end CMyArrayClass

I have tried the above code using a char array as follows:
CMyArrayClass<char, 4, "test"> MyObject;
but I get an error that "test" is an invalid template argument (even though it is a const char*).  If I use NULL instead, it works, but that doesn't help me because I want to pass in an initial value other than a non-value like NULL.

So what can I do?
Question by:ris
  • 4
LVL 30

Expert Comment

ID: 6942980
Default values are not allowed as a template value.

LVL 30

Expert Comment

ID: 6942986
>>So what can I do?

There is a way around this, but it's not a clean method.  In other words, it's ugly.
I'll post an example code in a follow-up comment.

The basic idea would be to create either a singleton class, or a class with a single static function, that would return the default value.
LVL 30

Accepted Solution

Axter earned 100 total points
ID: 6943012
Here's an example method.

#include <stdlib.h>

#pragma warning (disable:4786)
#include <string>
using namespace std;

template<typename T>
class DefaultValueTemplateClass
     static T DefaultValue();

template<class T, class T_DF = DefaultValueTemplateClass<T> >
class foo
     foo(const T& Src = T_DF::DefaultValue()) : m_data(Src)
     T m_data;

#define MAKE_DEFAULT_TEMPL_VAUE(T,V) T DefaultValueTemplateClass<T>::DefaultValue(){return V;}

MAKE_DEFAULT_TEMPL_VAUE(string,"Hello World")

int main(int argc, char* argv[])
     foo<int, DefaultValueTemplateClass<int> > foo_int;
     printf("foo_int = %i\n", foo_int.m_data);

     foo<int> foo_int_with_default_default;
     printf("foo_int_with_default_default = %i\n", foo_int_with_default_default.m_data);

     foo<long> foo_long;
     printf("foo_long = %i\n", foo_long.m_data);

     foo<string> foo_string;
     printf("foo_string = %s\n", foo_string.m_data.c_str());

     return 0;

LVL 30

Expert Comment

ID: 6943054
Even though the previous macro I posted, compiles and runs on VC++ 6.0, it shouldn't have.
The correct syntax, is the following:

#define MAKE_DEFAULT_TEMPL_VAUE(T,V) template<> T DefaultValueTemplateClass<T>::DefaultValue(){return V;}

Author Comment

ID: 6943070
I see what you are getting at there.  Thanks.

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