std::basic_stringbuf<CharT,Traits,Allocator>::overflow

 

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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 std::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 std::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

c

-

the character to store in the put area

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

In the implementation used to execute this example (e.g. GCC-4.9), overflow() over-allocates the put area to 512 bytes: a call to str() would only return the four initialized bytes, but the next 508 calls to sputc() would not require new calls to overflow().

Run this code

#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;
}

Possible output:

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

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 169	C++98	the buffer (re)allocated could only hold one extra character	allows more extra characters
LWG 432	C++98	overflow moved epptr() to point just past the new write position if the std::stringbuf is open for input	it is not moved

See also

overflow [virtual]	writes characters to the associated output sequence from the put area (virtual protected member function of std::basic_streambuf<CharT,Traits>)
underflow [virtual]	returns the next character available in the input sequence (virtual protected member function)