Boost C++ Libraries

#include <boost/phoenix/statement/if.hpp>

The syntax is

if_(conditional_expression)
[
    sequenced_statements
]
.else_
[
    sequenced_statements
]

Take note that else has a leading dot and a trailing underscore: .else_

Example: This code prints out all the elements and appends " > 5", " == 5" or " < 5" depending on the element's actual value:

std::for_each(c.begin(), c.end(),
    if_(arg1 > 5)
    [
        cout << arg1 << " > 5\n"
    ]
    .else_
    [
        if_(arg1 == 5)
        [
            cout << arg1 << " == 5\n"
        ]
        .else_
        [
            cout << arg1 << " < 5\n"
        ]
    ]
);

Notice how the if_else_ statement is nested.