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

std::basic_stringbuf<CharT,Traits,Allocator>::overflow - cppreference.com

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.

[edit] Parameters c - the character to store in the put area [edit] 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.

[edit] 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.

[edit] 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().

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

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

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