output vector in template function

Question

This section of code gives an error:

template <class T>
void print_vector(vector<T>& v, string sep)
{
    std::ostream_iterator<T> ostr_it(std::cout, sep) ;
    std::copy(begin(v), end(v), ostr_it);
}

main.cpp:17:30: error: no matching constructor for initialization of 'std::ostream_iterator<float>' std::ostream_iterator<T> ostr_it(std::cout, sep);

I am confused because if I do it outside the template function and output the vector directly there is no error:

vector<float> result(elements);
std::copy(begin(result), end(result), ostream_iterator<float>(cout, ", "));

What is wrong? Do I need to specialize each template function?

`std::ostream_iterator` does not seem to have a constructor taking `string` object. But there is a one that takes `const char*`. http://en.cppreference.com/w/cpp/iterator/ostream_iterator/ostream_iterator — zahir, May 06 '18 at 19:53
Yikes, that works. That seems an omission in the language to me. Explicit conversions? — Ivan, May 06 '18 at 19:55

Kostas · Accepted Answer · 2018-05-08T02:52:24.570

3

Since no answer was posted I guess I'll go ahead.

The signature for ostream_iterator accept's a C-string, and not a C++ string:

std::ostream_iterator(ostream_type& stream, const CharT* delim)

It has been chosen that implicit cast to char * from std::string is not desirable, as said here, so you get an error.

To make it work, you can simply cast the std::string yourself:

std::ostream_iterator<T> ostr_it(std::cout, sep);         // DOES NOT WORK
std::ostream_iterator<T> ostr_it(std::cout, sep.c_str()); // WORKS

edited May 08 '18 at 02:52

answered May 08 '18 at 00:08

Kostas

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It would be good to also show some alternative correct code (I suppose `sep.c_str()`) – M.M May 08 '18 at 01:46

output vector in template function

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