29 Input/output library [input.output]

Returns: true if the previous state of the standard iostream objects was synchronized and otherwise returns false.

The first time it is called, the function returns true.

Effects: If any input or output operation has occurred using the standard streams prior to the call, the effect is implementation-defined.

Otherwise, called with a false argument, it allows the standard streams to operate independently of the standard C streams.

When a standard iostream object str is synchronized with a standard stdio stream f, the effect of inserting a character c by

fputc(f, c);

is the same as the effect of

str.rdbuf()->sputc(c);

for any sequences of characters; the effect of extracting a character c by

c = fgetc(f);

is the same as the effect of

c = str.rdbuf()->sbumpc();

for any sequences of characters; and the effect of pushing back a character c by

ungetc(c, f);

is the same as the effect of

str.rdbuf()->sputbackc(c);

for any sequence of characters.288