Because there is no operator >> on std::stringstream and std::string_view (and std::istream_iterator requires this operator).
As @tkausl points out in the comments, it's not possible for >> to work on std::string_view because it's not clear who would own the memory pointed to by the std::string_view.
In the case of your program, ss << inputStr copies the characters from inputStr into ss, and when ss goes out of scope its memory would be freed.
Here is a possible implementation that uses C++20's std::ranges::views::split instead of std::stringstream. It only supports a single space as the delimiter.
#include <iostream>
#include <sstream>
#include <string>
#include <string_view>
#include <vector>
#include <iterator>
#include <algorithm>
#include <ranges>
[[ nodiscard ]] std::vector< std::string_view >
tokenize( const std::string_view inputStr, const size_t expectedTokenCount )
{
constexpr std::string_view delim { " " };
std::vector< std::string_view > foundTokens { };
if ( inputStr.empty( ) ) [[ unlikely ]]
{
return foundTokens;
}
foundTokens.reserve( expectedTokenCount );
for ( const auto token : std::views::split( inputStr, delim ) )
{
foundTokens.emplace_back( token.begin( ), token.end( ) );
}
return foundTokens;
}
int main( )
{
using std::string_view_literals::operator""sv;
constexpr auto text { "Today is a nice day."sv };
const auto tokens { tokenize( text, 4 ) };
std::cout << tokens.size( ) << '\n';
std::ranges::copy( tokens, std::ostream_iterator< std::string_view >{ std::cout, "\n" } );
}
This works with gcc 12.1 (compile with -std=c++20), but it doesn't work with clang 14.0.0 because clang hasn't implemented P2210 yet.