27 Input/output library [input.output]

27.5 Iostreams base classes [iostreams.base]

27.5.3 Class ios_base [ios.base]

27.5.3.4 ios_base static members [ios.members.static]

bool sync_with_stdio(bool sync = true);

1

Returns: true if the previous state of the standard iostream objects ([iostream.objects]) was synchronized and otherwise returns false. The first time it is called, the function returns true.

2

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.

3

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.299

298)

This implies that operations on a standard iostream object can be mixed arbitrarily with operations on the corresponding stdio stream. In practical terms, synchronization usually means that a standard iostream object and a standard stdio object share a buffer.