The output is:

812|813 815
8.31E+02 8.00E+02

Format control requires calling a stream's member functions. Each such call interrupts the respective shift expression. But what if you need to change formats within a shift expression? This is possible in iostreams. In the preceding example, instead of writing:

std::cout << 812 << '|';
std::cout.setf(std::ios_base::left, std::ios_base::adjustfield);
std::cout.width(10);
std::cout << 813 << 815 << '\n';

you can write:

std::cout << 812 << '|' << std::left << std::setw(10) << 813 << 815
 << std::endl;

In this example, objects like left, setw, and endl are called manipulators. There are overloaded versions of the insertion and extraction operators for manipulators, so that manipulators can be extracted from or inserted into a stream together with other objects that have the shift operators defined. (Section 28.3.2 explains in greater detail how manipulators work and how you can implement your own manipulators.)

The effect of a manipulator need not be an actual input to or output from the stream. Most manipulators set just one of the above described format flags, or do some other kind of stream manipulation. For example, an expression like:

std::cout << std::left;

is equivalent to:

std::cout.setf (std::ios_base::left, std::ios_base::adjustfield);

Nothing is inserted into the stream. The only effect is that the format flag for adjusting the output to the left is set.

On the other hand, the manipulator std::endl inserts the newline character to the stream, and flushes to the underlying stream buffer. The expression:

std::cout << std::endl; 

is equivalent to:

(std::cout << '\n').flush();

Some manipulators take arguments, like std::setw(int). The setw manipulator sets the field width. The expression:

std::cout << std::setw(10);

is equivalent to:

std::cout.width(10);

In general, you can think of a manipulator as an object you can insert into or extract from a stream, in order to manipulate that stream. Some manipulators can be applied only to output streams, others only to input streams. Most manipulators change format bits only in one of the stream base classes, ios_base or basic_ios. These can be applied to input and output streams.

Table 29 gives an overview of all manipulators defined by iostreams. The first column, Manipulator, lists its name. All manipulators are defined in the namespace ::std. The second column, Use, indicates whether the manipulator is intended to be used with istreams (i), ostreams (o), or both (io). The third column, Effect, summarizes the effect of the manipulator. The last column, Equivalent, lists the corresponding call to the stream's member function.

Note that the second column indicates only the intended use of a manipulator. In many cases, it is possible to apply an output manipulator to an input stream, and vice versa. Generally, this kind of non-intended manipulation is harmless in that it has no effect. For instance, if you apply the output manipulator showpoint to an input stream, the manipulation is simply ignored. However, if you use an output manipulator on a bidirectional stream during input, the manipulation does not affect current input operations, but subsequent output operations.

Table 29: Manipulators