std::basic_stringbuf::overflow

protected:
virtual int_type overflow ( int_type c = Traits::eof() );

Appends the character c to the output character sequence.

If c is the end-of-file indicator traits::eq_int_type(c,traits::eof()) == true, then there is no character to append. The function does nothing and returns an unspecified value other than traits::eof().

Otherwise, if the output sequence has a write position available or this function can successfully make a write position available, then calls sputc(c) and returns c.

This function can make a write position available if the stringbuf is open for output (mode & ios_base::out) != 0): in this case, it reallocates (or initially allocates) the buffer big enough to hold the entire current buffer plus at least one more character. If the stringbuf is also open for input ((mode & ios_base::in) != 0, then overflow also increases the size of the get area by moving egptr() to point just past the new write position.

Parameters

(none).

Return value

Traits::eof() to indicate failure, c if the character c was successfully appended, or some value other than Traits::eof() if called with Traits::eof() as the argument.

Notes

This function is different from a typical overflow() which moves the contents of the buffer to the associated character sequence because for a std::basic_stringbuf, the buffer and the associated sequence are one and the same.

Example

#include <sstream>
#include <iostream>
 
struct mybuf : std::stringbuf
{
    mybuf(const std::string& new_str,
          std::ios_base::openmode which = std::ios_base::in|std::ios_base::out)
           : std::stringbuf(new_str, which) {}
    int_type overflow(int_type c = EOF) override
    {
        std::cout << "stringbuf::overflow('" << char(c) << "') called\n"
                  << "Before: size of get area: " << egptr() - eback() << '\n'
                  << "        size of put area: " << epptr() - pbase() << '\n';
        int_type ret = std::stringbuf::overflow(c);
        std::cout << "After : size of get area: " << egptr() - eback() << '\n'
                  << "        size of put area: " << epptr() - pbase() << '\n';
        return ret;
    }
};
 
int main()
{
    std::cout << "read-write stream:\n";
    mybuf sbuf("   "); // read-write stream
    std::iostream stream(&sbuf);
    stream << 1234;
    std::cout << sbuf.str() << '\n';
 
    std::cout << "\nread-only stream:\n";
    mybuf ro_buf("   ", std::ios_base::in); // read-only stream
    std::iostream ro_stream(&ro_buf);
    ro_stream << 1234;
 
    std::cout << "\nwrite-only stream:\n";
    mybuf wr_buf("   ", std::ios_base::out); // write-only stream
    std::iostream wr_stream(&wr_buf);
    wr_stream << 1234;
}

read-write stream:
stringbuf::overflow('4') called
Before: size of get area: 3
        size of put area: 3
After : size of get area: 4
        size of put area: 512
1234
 
read-only stream:
stringbuf::overflow('1') called
Before: size of get area: 3
        size of put area: 0
After : size of get area: 3
        size of put area: 0
 
write-only stream:
stringbuf::overflow('4') called
Before: size of get area: 0
        size of put area: 3
After : size of get area: 0
        size of put area: 512

See also