CODEWITHSUNDEEP
c++templates
user3140486
user3140486

Reputation: 19

how create c++ Template?

I am currently trying to write a template specialization for a function that showes the power of different values. my code is :

#include <iostream>
using namespace std;
template <class T>
T myfunc(T x)
{
    return x*x;
}
template<>
string myfunc<string>(string ss)
{
    return (ss+ss); 
}
int main()
{
cout<<myfunc("test")<<endl;
return 0;
}

Compilation fails with:

error C2784: 'std::_String_iterator<_Mystr> std::operator +(_String_iterator<_Mystr>::difference_type,std::_String_iterator<_Mystr>)' : could not deduce template argument for 'std::_String_iterator<_Mystr>' from 'std::string'

Can you please help me detect where exactly is the problem?

Upvotes: 1

Views: 99

Answers (3)

user3125280
user3125280

Reputation: 2829

You need to pass the function a string, not a char array. Try myfunc(string("test"))

To expand a little - the choice of which template / template specialisation to use when a function is called is called template argument deduction.

You want myfunc to be called, however the type of test is char[5] (null character on the end). Therefore the template argument is 'deduced' to be char[5].

Often, char[] can be cast to string implicitly (this is deprecated i believe) in a call to a fully specialised function (not a template function). This implicit cast is because, as user1274223 pointed out, a std::string has a constructor with signature string(const char *) and this constructor is not explicit! That is helpful for when we want to pretend c strings (char*) are easily interchangeable with cplusplus strings (std::string), however it can be confusing in these types of situations.

The other work around is myfunc<string>("test"), because here the template argument is passed directly, and char[] can be cast implicitly.

Upvotes: 5

Chen
Chen

Reputation: 58

As others already said. in addition, I think you need to

#include <string>

in which the operator+ for strings and string itself declared the error you get is your compiler try to match all the other operator+ in and failed.

Upvotes: 0

sth
sth

Reputation: 229593

A template specialization is unnecessary in this case, you should just overload the function:

std::string myfunc(const std::string &ss)
{
    return (ss+ss); 
}

This way calling it as myfunct("test") will work.

For a all the details why function template specializations are usually not usefu you can have a look at Herb Sutter's article Why Not Specialize Function Templates?

Upvotes: 1

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