ArgTraits.h

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// -*- Mode: c++; c-basic-offset: 4; tab-width: 4; -*-

/******************************************************************************
 *
 *  file:  ArgTraits.h
 *
 *  Copyright (c) 2007, Daniel Aarno, Michael E. Smoot .
 *  Copyright (c) 2017 Google LLC
 *  All rights reserved.
 *
 *  See the file COPYING in the top directory of this distribution for
 *  more information.
 *
 *  THE SOFTWARE IS PROVIDED _AS IS_, WITHOUT WARRANTY OF ANY KIND, EXPRESS
 *  OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 *  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
 *  THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 *  LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
 *  FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 *  DEALINGS IN THE SOFTWARE.
 *
 *****************************************************************************/

// This is an internal tclap file, you should probably not have to
// include this directly

#ifndef TCLAP_ARGTRAITS_H
#define TCLAP_ARGTRAITS_H

namespace TCLAP {

// We use two empty structs to get compile type specialization
// function to work

struct ValueLike {
    typedef ValueLike ValueCategory;
    virtual ~ValueLike() {}
};

struct StringLike {
    virtual ~StringLike() {}
};

struct StringLikeTrait {
    typedef StringLike ValueCategory;
    virtual ~StringLikeTrait() {}
};

struct ValueLikeTrait {
    typedef ValueLike ValueCategory;
    virtual ~ValueLikeTrait() {}
};

template<typename T>
class ArgTraits {
    // This is a bit silly, but what we want to do is:
    // 1) If there exists a specialization of ArgTraits for type X,
    // use it.
    //
    // 2) If no specialization exists but X has the typename
    // X::ValueCategory, use the specialization for X::ValueCategory.
    //
    // 3) If neither (1) nor (2) defines the trait, use the default
    // which is ValueLike.

    // This is the "how":
    //
    // test<T>(0) (where 0 is the NULL ptr) will match
    // test(typename C::ValueCategory*) iff type T has the
    // corresponding typedef. If it does not test(...) will be
    // matched. This allows us to determine if T::ValueCategory
    // exists by checking the sizeof for the test function (return
    // value must have different sizeof).
    template<typename C> static short test(typename C::ValueCategory*);
    template<typename C> static long  test(...);
    static const bool hasTrait = sizeof(test<T>(0)) == sizeof(short);

    template <typename C, bool>
    struct DefaultArgTrait {
        typedef ValueLike ValueCategory;
    };

    template <typename C>
    struct DefaultArgTrait<C, true> {
        typedef typename C::ValueCategory ValueCategory;
    };

public:
    typedef typename DefaultArgTrait<T, hasTrait>::ValueCategory ValueCategory;
};

} // namespace

#endif