libs/phoenix/doc/modules/statement.qbk

[/==============================================================================
    Copyright (C) 2001-2010 Joel de Guzman
    Copyright (C) 2001-2005 Dan Marsden
    Copyright (C) 2001-2010 Thomas Heller

    Distributed under the Boost Software License, Version 1.0. (See accompanying
    file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
===============================================================================/]

[section Statement]

[*/Lazy statements.../]

The expressions presented so far are sufficiently
powerful to construct quite elaborate structures. We have presented lazy-functions
and lazy-operators. How about lazy-statements? First, an appetizer:

Print all odd-numbered contents of an STL container using `std::for_each`
([@../../example/all_odds.cpp all_odds.cpp]):

    std::for_each(c.begin(), c.end(),
        if_(arg1 % 2 == 1)
        [
            cout << arg1 << ' '
        ]
    );

Huh? Is that valid C++? Read on...

Yes, it is valid C++. The sample code above is as close as you can get to the
syntax of C++. This stylized C++ syntax differs from actual C++ code. First, the
`if` has a trailing underscore. Second, the block uses square brackets instead
of the familiar curly braces {}.

[note *C++ in C++?*

    In as much as __spirit__ attempts to mimic EBNF in C++,
    Phoenix attempts to mimic C++ in C++!!!
]

[note Unlike lazy functions and lazy operators, lazy statements always
return void.]

Here are more examples with annotations. The code almost speaks for itself.

[section Block Statement]

[/    #include <boost/phoenix/statement/sequence.hpp>]

Syntax:

    statement,
    statement,
    ....
    statement

Basically, these are comma separated statements. Take note that unlike the C/C++
semicolon, the comma is a separator put *in-between* statements. This is like
Pascal's semicolon separator, rather than C/C++'s semicolon terminator. For
example:

    statement,
    statement,
    statement, // ERROR!

Is an error. The last statement should not have a comma. Block statements can be
grouped using the parentheses. Again, the last statement in a group should not
have a trailing comma.

    statement,
    statement,
    (
        statement,
        statement
    ),
    statement

Outside the square brackets, block statements should be grouped. For example:

    std::for_each(c.begin(), c.end(),
        (
            do_this(arg1),
            do_that(arg1)
        )
    );

Wrapping a comma operator chain around a parentheses pair blocks the
interpretation as an argument separator.  The reason for the exception for
the square bracket operator is that the operator always takes exactly one
argument, so it "transforms" any attempt at multiple arguments with a comma
operator chain (and spits out an error for zero arguments).

[endsect]

[section if_ Statement]

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

We have seen the `if_` statement. The syntax is:

    if_(conditional_expression)
    [
        sequenced_statements
    ]

[endsect]

[section '''if_else_''' Statement]

    #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.

[/note `if_` is one example of a customized actor. See [link phoenix.advanced.extending.extending_actors Extending Actors] for more details]

[endsect]

[section switch_ Statement]

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

The syntax is:

    switch_(integral_expression)
    [
        case_<integral_value>(sequenced_statements),
        ...
        default_<integral_value>(sequenced_statements)
    ]

A comma separated list of cases, and an optional default can be provided. Note unlike
a normal switch statement, cases do not fall through.

Example: This code prints out `"one"`, `"two"` or `"other value"` depending on the
element's actual value:

    std::for_each(c.begin(), c.end(),
        switch_(arg1)
        [
            case_<1>(std::cout << val("one") << '\n'),
            case_<2>(std::cout << val("two") << '\n'),
            default_(std::cout << val("other value") << '\n')
        ]
    );

[endsect]

[section while_ Statement]

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

The syntax is:

    while_(conditional_expression)
    [
        sequenced_statements
    ]

Example: This code decrements each element until it reaches zero and prints out
the number at each step. A newline terminates the printout of each value.

    std::for_each(c.begin(), c.end(),
        (
            while_(arg1--)
            [
                cout << arg1 << ", "
            ],
            cout << val("\n")
        )
    );

[endsect]

[section '''do_while_''' Statement]

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

The syntax is:

    do_
    [
        sequenced_statements
    ]
    .while_(conditional_expression)

Again, take note that `while` has a leading dot and a trailing underscore:
`.while_`

Example: This code is almost the same as the previous example above with a
slight twist in logic.

    std::for_each(c.begin(), c.end(),
        (
            do_
            [
                cout << arg1 << ", "
            ]
            .while_(arg1--),
            cout << val("\n")
        )
    );

[/note `do_` is one example of a customized actor. See [link phoenix.advanced.extending.extending_actors Extending Actors] for more details]

[endsect]

[section:for_statement for_ Statement]

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

The syntax is:

    for_(init_statement, conditional_expression, step_statement)
    [
        sequenced_statements
    ]

It is again very similar to the C++ for statement. Take note that the
init_statement, conditional_expression and '''step_statement''' are separated by the
comma instead of the semi-colon and each must be present (i.e. `for_(,,)` is
invalid). This is a case where the [link phoenix.modules.core.nothing `nothing`]
actor can be useful.

Example: This code prints each element N times where N is the element's value. A
newline terminates the printout of each value.

    int iii;
    std::for_each(c.begin(), c.end(),
        (
            for_(ref(iii) = 0, ref(iii) < arg1, ++ref(iii))
            [
                cout << arg1 << ", "
            ],
            cout << val("\n")
        )
    );

As before, all these are lazily evaluated. The result of such statements are in
fact expressions that are passed on to STL's for_each function. In the viewpoint
of `for_each`, what was passed is just a functor, no more, no less.

[endsect]

[section try_ catch_ Statement]

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

The syntax is:

    try_
    [
        sequenced_statements
    ]
    .catch_<exception_type>()
    [
        sequenced_statements
    ]
    ...
    .catch_all
    [
        sequenced_statement
    ]

Note the usual underscore after try and catch, and the extra parentheses required
after the catch.

[/note `do_` is one example of a customized actor. See [link phoenix.advanced.extending.extending_actors Extending Actors] for more details]

Example: The following code calls the (lazy) function `f` for each element, and
prints messages about different exception types it catches.

    try_
    [
        f(arg1)
    ]
    .catch_<runtime_error>()
    [
        cout << val("caught runtime error or derived\n")
    ]
    .catch_<exception>()
    [
        cout << val("caught exception or derived\n")
    ]
    .catch_all
    [
        cout << val("caught some other type of exception\n")
    ]

[endsect]

[section throw_]

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

As a natural companion to the try/catch support, the statement module provides
lazy throwing and re-throwing of exceptions.

The syntax to throw an exception is:

    throw_(exception_expression)

The syntax to re-throw an exception is:

    throw_()

Example: This code extends the try/catch example, re-throwing exceptions derived from
runtime_error or exception, and translating other exception types to runtime_errors.

    try_
    [
        f(arg1)
    ]
    .catch_<runtime_error>()
    [
        cout << val("caught runtime error or derived\n"),
        throw_()
    ]
    .catch_<exception>()
    [
        cout << val("caught exception or derived\n"),
        throw_()
    ]
    .catch_all
    [
        cout << val("caught some other type of exception\n"),
        throw_(runtime_error("translated exception"))
    ]

[endsect]

[endsect]