The MPL Reference Manual: protect

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# protect

### Synopsis

    template<
          typename F
        >
    struct protect
    {
        // unspecified
        // ...
    };

### Description

`protect` is an identity wrapper for a Metafunction Class that prevents its argument from being recognized as a bind expression.

### Header

    #include <boost/mpl/protect.hpp>

### Parameters

Parameter | Requirement | Description
--- | --- | ---
`F` | Metafunction Class | A metafunction class to wrap.

### Expression semantics

For any Metafunction Class `f`:

    typedef protect<f> g;

Return type: | Metafunction Class.
Semantics: | If `f` is a bind expression, equivalent to
    struct g
    {
        template<
              typename U1 = unspecified,... typename Un = unspecified
            >
        struct apply
            : apply_wrapn<f,U1,...Un>
        {
        };
    };
otherwise equivalent to `typedef f g;`.

### Example

    struct f
    {
        template< typename T1, typename T2 > struct apply
        {
            typedef T2 type;
        };
    };

    typedef bind< quote3<if_>,_1,_2,bind<f,_1,_2> > b1;
    typedef bind< quote3<if_>,_1,_2,protect< bind<f,_1,_2> > > b2;

    typedef apply_wrap2< b1,false_,char >::type r1;
    typedef apply_wrap2< b2,false_,char >::type r2;

    BOOST_MPL_ASSERT(( is_same<r1,char> ));
    BOOST_MPL_ASSERT(( is_same<r2,protect< bind<f,_1,_2> > > ));

### See also

Composition and Argument Binding, invocation, bind, quote, apply_wrap