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POUsing variadic macros or templates to implement a set of functions
 

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  1. POUsing variadic macros or templates to implement a set of functions
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    <p>I have a set of methods used to instanciate and initialize a set of objects. They all look pretty much the same, except for the number of arguments that are passed to the <strong>Init</strong> function :</p> <pre><code>ObjectType* CreateObjectType(Arg1 a1, Arg2 arg2, ... ArgN aN) { ObjectType* object = new ObjectType(); [...] object-&gt;Init(this, a1, a2, ..., aN); [...] return object; } </code></pre> <p>Note that the arguments are not to be used anywhere except to be passed to the <strong>Init</strong> function.</p> <p>I would like to find a way to implement all of those without having to duplicate the code for each object type. </p> <hr> <p>I tried using variadic macros, with the following (invalid) result :</p> <pre><code>#define CREATE_OBJECT_IMPL(ObjectType, ...) \ ObjectType* Create##ObjectType##(__VA_ARGS__) \ { \ ObjectType* object = new ObjectType(); \ [...] object-&gt;Init(this, ##__VA_ARGS__); \ [...] return object; \ } // This is the result I am trying to achieve : CREATE_OBJECT_IMPL(MyFirstObject, bool, float) CREATE_OBJECT_IMPL(MySecondObject, int) CREATE_OBJECT_IMPL(MyThirdObject) </code></pre> <p>Now, in this implementation, I used VA_ARGS twice, both times incorrectly :</p> <ul> <li><p>In the first case, I want to have a list of arguments with the types I specified (Arg1 a1, Arg2 a2...)</p></li> <li><p>In the second case, I want to call these arguments by their names ( Init(a1, a2...) ). </p></li> </ul> <hr> <p>I tried using variadic templates :</p> <pre><code>template&lt; typename ObjectType, typename... Args &gt; ObjectType* CreateObject(Args args) { ObjectType* object = new ObjectType(); [...] object-&gt;Init(this, args); [...] return object; } #define CREATE_OBJECT_IMPL(ObjectType, ...) \ ObjectType* Create##ObjectType##(__VA_ARGS__) \ { \ return CreateObject&lt;ObjectType, __VA_ARGS__&gt;(__VA_ARGS__); \ } </code></pre> <p>...but this doesn't seem to work as well, I get the following error on the template definition line :</p> <blockquote> <p>error C2143: syntax error : missing ',' before '...'</p> <p>error C2065: 'Args' : undeclared identifier</p> </blockquote> <p>I am using VS2012.</p> <p>I could still write N similar macros for each number of arguments, however I was wondering if there was a way to obtain the same result without duplicating code?</p>
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    1. USTshepang
    1. USuser2683028
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    1. COThe signature of variadic template should be `template< typename ObjectType, typename... Args > ObjectType* CreateObject(Args... args);`. But you'll need some context to expand the argument pack to.
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      1. USjrok
    2. CODynamic allocation, dumb pointers, and two-stage initialisation? This is a recipe for nightmarish debugging sessions. You should run away right now.
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      1. USMike Seymour
 

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