std::basic_stringbuf<CharT,Traits,Allocator>::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 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().
#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: 512Defect 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 newwrite position if the std::stringbuf is open for input
|
it is not moved |
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] |
returns the next character available in the input sequence (virtual protected member function) |