default template arguments in 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.

I don't think we will ever get rid of the "Hay Dear!" :D

Hassan Syed
– Hassan Syed

2010-02-24 18:21:37 +00:00
Commented Feb 24, 2010 at 18:21 — Hassan Syed
– Hassan Syed, Commented Feb 24, 2010 at 18:21

Community · Accepted Answer · 2017-05-23 12:11:49Z

6

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.

edited May 23, 2017 at 12:11

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answered Feb 24, 2010 at 18:44

sbi

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1 Comment

Zia ur Rahman Over a year ago

yes, i have got the point we can not give default template arguments incase of function templates. Thanks

Georg Fritzsche · Accepted Answer · 2010-02-24 18:34:07Z

4

§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) { /* ... */ }
};

edited Feb 24, 2010 at 18:34

answered Feb 24, 2010 at 18:28

Georg Fritzsche

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2 Comments

Hassan Syed Over a year ago

thought so, wasn't sure though.

Zia ur Rahman Over a year ago

then where we specify default template argument

AnT stands with Russia · Accepted Answer · 2010-02-24 18:31:48Z

2

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.

answered Feb 24, 2010 at 18:31

AnT stands with Russia

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2 Comments

Zia ur Rahman Over a year ago

Is it mean that we can not give default template arguments in case of function templates , we can only give default template arguments in case of class member functions?

Zia ur Rahman Over a year ago

i have got the point, yes you are right we can not give default template arguments in case of function templates

alariq · Accepted Answer · 2011-07-23 00:07:36Z

0

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.

answered Jul 23, 2011 at 0:07

alariq

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1 Comment

Kev Over a year ago

Be careful when posting copy and paste boilerplate/verbatim answers to multiple questions, these tend to be flagged as "spammy" by the community.

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