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POHow can I provide template specializations for typedefs of the same type?
 

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  1. POHow can I provide template specializations for typedefs of the same type?
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    <p>A 3rd party SDK defines several typedefs, e.g.:</p> <pre><code>typedef unsigned char SDK_BYTE typedef double SDK_DOUBLE typedef unsigned char SDK_BOOLEAN </code></pre> <p>It also defines a variant type SdkVariant:</p> <pre><code>class SdkVariant { public: enum SdkType { SdkByte, SdkDouble, SdkBoolean }; bool toByte(SDK_BYTE&amp;); bool toDouble(SDK_DOUBLE&amp;); bool toBool(SDK_BOOLEAN&amp;); SdkType type(); }; </code></pre> <p>Retrieving a value from such a variant looks like this (presuming, we know the type of the contained value):</p> <pre><code>SdkVariant variant(foobar()); double value; bool res = variant.toDouble(value); if (!res) diePainfully(); else doSomethingWith(value); </code></pre> <p>This is quite verbose and therefore I want to provide a variant_cast-function-class that can perform the value retrieval and error handling:</p> <pre><code>// general interface: template&lt;class T&gt; class variant_cast { public: T operator()(const SdkVariant&amp; variant); }; // template specializations: template&lt;&gt; SDK_DOUBLE variant_cast&lt;SDK_DOUBLE&gt;::operator()(const SdkVariant&amp; variant) { SDK_DOUBLE value; bool res = variant.toDouble(value); if (!res) diePainfully(); return value; } template&lt;&gt; SDK_BYTE variant_cast&lt;SDK_BYTE&gt;::operator()(const SdkVariant&amp; variant) { SDK_BYTE value; bool res = variant.toByte(value); if (!res) diePainfully(); return value; } template&lt;&gt; SDK_BOOLEAN variant_cast&lt;SDK_BOOLEAN&gt;::operator()(const SdkVariant&amp; variant) { SDK_BOOLEAN value; bool res = variant.toByte(value); if (!res) diePainfully(); return value; } </code></pre> <p>This does not compile (C2995: Function template already defined), because SDK_BYTE and SDK_BOOLEAN are the same types (unsigned char). My idea is now to let the preprocessor check that SDK_BYTE and SDK_BOOLEAN are the same and if so, define a single template specialization for both. If they're different, it should use the two separate specializations from above. Like this:</p> <pre><code>#if SDK_BYTE == SDK_BOOLEAN template&lt;&gt; SDK_BYTE variant_cast&lt;SDK_BYTE&gt;::operator()(const SdkVariant&amp; variant) { SDK_BYTE value; bool res; if (variant.type() == SdkByte) res = variant.toByte(value); else res = variant.toBool(value); if (!res) diePainfully(); return value; } #else // code from above #endif </code></pre> <p>The problem with the above code is, that it appears to be impossible for the preprocessor to resolve the two typedefs. Is there a way to compare two typedefs (correctly) during preprocessing? If not, is there a way to hinder the compiler from resolving the typedefs, so that it would accept two different template specializations for SDK_BYTE and SDK_BOOLEAN? If not, I can still provide the single template specialization and use BOOST_STATIC_ASSERT to make the compiler fail if SDK_BYTE and SDK_BOOLEAN are unequal, but is there a better way to solve my problem?</p>
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    1. USAlexander Tobias Bockstaller
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      1. POHow can I provide template specializations for typedefs of the same type?
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      1. POHow can I provide template specializations for typedefs of the same type?
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    1. COUnfortunately, the preprocessor does not understand `typedefs`.
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      1. POHow can I provide template specializations for typedefs of the same type?
      1. USLyubomir Vasilev
    2. CO[`BOOST_STRONG_TYPEDEF`](http://www.boost.org/doc/libs/1_50_0/boost/strong_typedef.hpp)
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      1. POHow can I provide template specializations for typedefs of the same type?
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    3. COCould you use C++11 and `std::enable_if`, combined with `std::is_same`?
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      1. USjogojapan
 

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