default template arguments in c++ - c++

Question

Suppose i have a function template StrCompare
template<typename T=NonCaseSenCompare>//NonCaseSenCompare is a user defined class look at the detailed code below.
int StrCompare(char* str1, char* str2)
{
...
}
now in the main function i write a line
char* str1="Zia";
char* str2="zia";
int result=StrCompare(str1,str2);
it should work because we have provided a default template argument, but it does'nt compiler gives the following error
no matching function for call to `StrCompare(char*&, char*&)'
Now the detailed code is given by
#include<iostream.h>
class CaseSenCompare
{
public:
static int isEqual(char x, char y)
{
return x==y;
}
};
class NonCaseSenCompare
{
public:
static int isEqual(char x,char y)
{
char char1=toupper(x);
char char2=toupper(y);
return char1==char2;
}
};
template<typename T=NonCaseSenCompare>
int StrCompare(char* str1, char* str2)
{
for(int i=0;i < strlen(str1)&& strlen(str2);i++)
{
if(!T::isEqual(str1[i],str2[i]))
return str1[i]-str2[i];
}
return strlen(str1)-strlen(str2);
}
main()
{
char* ptr1="Zia ur Rahman";
char* ptr2="zia ur Rahman";
int result=StrCompare(ptr1,ptr2);//compiler gives error on this line
cout<<result<<endl;
system("pause");
}
If I write
int result=StrCompare<>(ptr1,ptr2);
compiler gives the same error message.

Answer 1

As gf and AndreyT already wrote, you can't have default template arguments with function templates. However, if you turn your comparators into function objects, you can still use default function arguments:
template<typename Comp>
int StrCompare(char* str1, char* str2, Comp = NonCaseSenCompare())
{
...
}
You can now call StrCompare() like this
StrCompare("abc","aBc",CaseSenCompare());
or like this:
StrCompare("abc","aBc"); // uses NonCaseSenCompare
A comparator would then have to look like this:
struct CaseSenCompare {
bool operator()(char x, char y) const {return x==y;}
};
Adjust StrCompare() accordingly.

Answer 2

§14.1/9:
A default template-argument shall not
be specified in a function template
declaration or a function template
definition, nor in the
template-parameter-list of the
definition of a member of a class
template.
A simple work-around would be to move it into a class:
template<typename T=NonCaseSenCompare>
struct StrCompare {
static int compare(char* str1, char* str2) { /* ... */ }
};

Answer 3

Firstly, function templates do not support default template arguments, only class templates do.
Secondly, even when all class template parameters have default arguments, you still have to specify an empty <> to refer to that class template.

Answer 4

What i use is next trick;
lets say you want to have function like this
template <typename E, typename ARR_E = MyArray_t<E> > void doStuff(ARR_E array)
{
E one(1);
array.add( one );
}
you will not be allowed, but i do next way:
template <typename E, typename ARR_E = MyArray_t<E> >
class worker {
public:
/*static - as you wish */ ARR_E* parr_;
void doStuff(); /* do not make this one static also, MSVC complains */
};
template <typename E, typename ARR_E>
void worker::doStuff<E, ARR_E>::getChunks()
{
E one(1);
parr_->add( one );
}
so this way you may use it like this.
MyArray_t my_array;
worker<int> w;
w.parr_ = &arr;
w.doStuff();
as we can see no need to explicitly set second parameter.
maybe it will be useful for someone.