std::basic_istream<CharT,Traits>:: readsome

Extracts up to count immediately available characters from the input stream. The extracted characters are stored into the character array pointed to by s .

Behaves as UnformattedInputFunction . After constructing and checking the sentry object,

• If rdbuf ( ) - > in_avail ( ) == - 1 , calls setstate ( eofbit ) and extracts no characters.

• If rdbuf ( ) - > in_avail ( ) == 0 , extracts no characters.

• If rdbuf ( ) - > in_avail ( ) > 0 , extracts std:: min ( rdbuf ( ) - > in_avail ( ) , count ) characters and stores them into successive locations of the character array whose first element is pointed to by s .

Parameters

Return value

The number of characters actually extracted.

Exceptions

If an internal operation throws an exception, it is caught and badbit is set. If exceptions() is set for badbit , the exception is rethrown.

Notes

The behavior of this function is highly implementation-specific. For example, using readsome() with std::ifstream leads to significant, implementation-specific outcomes. Some library implementations fill the underlying filebuf with data as soon as std::ifstream opens a file, which means readsome() always reads data and could even read the entire file. With other implementations, std::ifstream only reads from a file when an input operation is invoked, which means calling readsome() immediately after opening the file never extracts any characters. Similarly, calling std:: cin . readsome ( ) may return all pending, unprocessed console input or may always return zero and extract no characters.

Example

#include <cassert>
#include <iostream>
#include <sstream>
 
int main()
{
    char c[10] = "*********"; // c[9] == '\0'
 
    // std::stringbuf makes its entire buffer available for unblocking read
    std::istringstream input("This is sample text.");
 
    auto r = input.readsome(c, 5); // reads 'This ' and stores in c[0] .. c[4]
    assert(r == 5);
    std::cout << c << '\n';
 
    r = input.readsome(c, 9); // reads 'is sample' and stores in c[0] .. c[8]
    assert(r == 9);
    std::cout << c << '\n';
}

This ****
is sample

See also