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Showing content from https://en.cppreference.com/w/cpp/language/../io/basic_stringbuf/../basic_istream/readsome.html below:

std::basic_istream<CharT,Traits>::readsome - cppreference.com

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.

The number of characters actually extracted.

if an error occurred (the error state flag is not

) and

is set to throw for that state.

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

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.

#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';
}

Output:

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