Pairs and stringstream - c++

Question

In a previous question I have asked how I could read and comfortably transform a vector of strings into a vector of integers or doubles.
Now I am expanding that vector of simple types to vector of pairs of types (consider int, double, or even std::string, all stream-enabled types) separated by colons.
Example:
time:5, length:10
Should be read as a std::pair<std::string, unsigned int>, with something like this:
// Here declare what to parse: a string and an uint, separated by a colon
get_vector<std::pair<std::string, unsigned int>>(s);
How can I write a std::stringstream operator that does the trick?
I cannot figure how (or even if) I could play with getline (my code follows). I'd really like to leave the get_vector function as it is, if possible.
Thanks!
template <class F, class S>
std::stringstream& operator >> (std::stringstream& in, std::pair<F, S> &out)
{
in >> out.first >> out.second;
return in;
}
template <class T>
auto get_vector(std::string s) -> std::vector<T>
{
std::vector<T> v;
// Vector of strings
auto tmp = split(s);
// Magic
std::transform(tmp.begin(), tmp.end(), std::back_inserter(v),
[](const std::string& elem) -> T
{
std::stringstream ss(elem);
T value;
ss >> value;
return value;
});
return v;
}

Answer 1

Your operator>> receives a std::stringstream which in that moment contains a string like "time:5". The operator needs to parse that string by splitting it up into two tokens, then convert the first token to an F and the second to an S.
One way to do this is to use std::getline with a custom delimiter. You will end up with two std::string tokens. These can be converted, for example, with two internal std::istringstreams.
Here is some code you can try:
template <class F, class S>
std::stringstream& operator >> (std::stringstream& in, std::pair<F, S> &out)
{
std::string first_token;
std::string second_token;
std::getline(in, first_token, ':');
std::getline(in, second_token);
std::istringstream first_converter(first_token);
std::istringstream second_converter(second_token);
first_converter >> out.first;
second_converter >> out.second;
return in;
}
You may want to add some error handling for the case when there is no ':' in the string.
Note that you should not use std::stringstream if you do not want to combine input and output. Most of the times, you will want either std::istringstream or std::ostringstream.

Answer 2

Because I am new in English I am not sure to understand what you want.
I jest seeing that #Christian Hackl do what. So if this answer does not
relate to what you want please tell me and I will delete it.
I assume you need to extract a string from string or stream and then put them into a vector that contains a std::pair< std::string, std::size_t >
Say you have this string that can be an input-stream, too:
std::string string( "one:1, two:20, three:300, four:40000, five:500000" );
There is more than one way to do it, but I say it in this way!
So first you need to split this string and then put what has split, in to vector of pair.
std::string string( "one:1, two:20, three:300, four:40000, five:500000" );
std::stringstream io_stream;
std::basic_regex< char > regex( ", " );
std::regex_token_iterator< std::string::const_iterator > last;
std::regex_token_iterator< std::string::const_iterator > first( string.begin(), string.end(), regex, -1 );
std::vector< std::pair< std::string, std::size_t > > vector_of_pair;
std::string str_pair( "" );
std::size_t ui_pair( 0 );
while( first != last ){
io_stream.clear();
io_stream.seekg( 0 );
io_stream.seekp( 0 );
io_stream << *first << '\n';
std::getline( io_stream, str_pair, ':' );
// error:
// no match function to call: stream and int
// std::getline( io_stream, ui_pair);
io_stream >> ui_pair;
vector_of_pair.emplace_back( str_pair, ui_pair );
++first;
}
test
for( std::size_t index = 0; index < vector_of_pair.size(); ++index ){
std::cout << vector_of_pair[ index ].first << " and " << vector_of_pair[ index ].second << '\n';
}
output
one and 1
two and 20
three and 300
four and 40000
five and 500000
how it work
There is no difficult code to understand. It just by std::regex_token_iterator splits the string, then in a while loop send the value of it to is_stream, then read the line and send it to str_pair, but here because std::getline has not function to extracts int type, I used >> of stringstream itself, then emplace_back to vector and that's it.
NOTE
For:
How can I write a std::stringstream operator that does the trick?
I think now you can do it by yourself. Since I was not sure, I have not written the code with operator>>.