c++ - checking data availability before calling std::getline

Question

I would like to read some data from a stream I have using std::getline. Below a sample using the std::cin.

std::string line;
std::getline( std::cin, line );

This is a blocking function i.e. if there is no data or line to read it blocks execution.

Do you know if exists a function for checking data availability before calling std::getline? I don't want to block.

How can I check whether the stream buffer is full of data valid for a successful call to std::getline?

Whatever looks like the code below

if( dataAvailableInStream() )
{
     std::string line;
     std::getline( std::cin, line );
}

Answer 1

There is no standard way to verify if getline will block. You can use:

std::cin.rdbuf()->in_avail()

to see how many characters are definitely available before a read operation may block, but you would have to read the characters one by one before re-checking in_avail as there is no way to know in advance if any of the pending characters is a newline or the actual end of the stream. A getline call might block if this wasn't the case.

Note that although if in_avail() returns a postive number there are guaranteed that at least that many characters are available before the end of the stream, the converse is not true. If in_avail() returns zero there may still be characters available and the stream might not block immediately.