std::strstreambuf:: overflow

From cppreference.com
C++
Input/output library
std::strstreambuf
protected :
virtual int_type overflow ( int_type c = EOF ) ;
(deprecated in C++98)
(removed in C++26)

Appends the character c to the put area of the buffer, reallocating if possible.

1) If c == EOF , does nothing.
2) Otherwise, if the put area has a write position available ( pptr ( ) < epptr ( ) ), stores the character as if by * pptr ( ) ++ = c .
3) Otherwise, if the stream buffer mode is not dynamic or the stream buffer is currently frozen, the function fails and returns EOF .
4) Otherwise, the function reallocates (or initially allocates) a dynamic array large enough to hold the contents of the current dynamic array (if any) plus at least one additional write position. If a pointer to the allocating function palloc was used in the constructor, that function is called with ( * palloc ) ( n ) where n is the number of bytes to allocate, otherwise new char [ n ] is used. If a pointer to the deallocating function pfree was used in the constructor, that function is called with ( * pfree ) ( p ) to deallocate the previous array, if needed, otherwise delete [ ] p is used. If allocation fails, the function fails and returns EOF .

Parameters

c - the character to store in the put area

Return value

If c == EOF , returns some value other than EOF . Otherwise, returns ( unsigned char ) ( c ) on success, EOF on failure.

Example

Run this code 
#include <iostream>
#include <strstream>
 
struct mybuf : std::strstreambuf
{
    int_type overflow(int_type c) 
    {
        std::cout << "Before overflow(): size of the put area is " << epptr()-pbase()
                  << " with " << epptr()-pptr() << " write positions available\n";
        int_type rc = std::strstreambuf::overflow(c);
        std::cout << "After overflow(): size of the put area is " << epptr()-pbase()
                  << " with " << epptr()-pptr() << " write positions available\n";
        return rc;
    }
};
 
int main()
{
    mybuf sbuf; // read-write dynamic strstreambuf
    std::iostream stream(&sbuf);
 
    stream << "Sufficiently long string to overflow the initial allocation, at least "
           << " on some systems.";
}

Possible output: 

Before overflow(): size of the put area is 16 with 0 write positions available
After overflow(): size of the put area is 32 with 15 write positions available
Before overflow(): size of the put area is 32 with 0 write positions available
After overflow(): size of the put area is 64 with 31 write positions available
Before overflow(): size of the put area is 64 with 0 write positions available
After overflow(): size of the put area is 128 with 63 write positions available

See also

[virtual]
writes characters to the associated output sequence from the put area
(virtual protected member function of std::basic_streambuf<CharT,Traits> )
[virtual]
appends a character to the output sequence
(virtual protected member function of std::basic_stringbuf<CharT,Traits,Allocator> )
[virtual]
writes characters to the associated file from the put area
(virtual protected member function of std::basic_filebuf<CharT,Traits> )
writes one character to the put area and advances the next pointer
(public member function of std::basic_streambuf<CharT,Traits> )
inserts a character
(public member function of std::basic_ostream<CharT,Traits> )
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