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POC++: Generate a compile error if user tries to instantiate a class template wihout instantiating a corresponding function template
 

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  1. POC++: Generate a compile error if user tries to instantiate a class template wihout instantiating a corresponding function template
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    <p>In a library that I'm writing I have a class:</p> <pre><code>template&lt;typename T&gt; class my_class {}; </code></pre> <p>and a function:</p> <pre><code>template&lt;typename T&gt; void my_function(...) {}; </code></pre> <p>I want to guarantee that a user of my code has called <code>my_function&lt;T&gt;</code> somewhere in their code if they try to instantiate <code>my_class&lt;T&gt;</code>. In other words, I want to generate a compile error if they try to instantiate the <code>my_class</code> template without also instantiating the corresponding <code>my_function</code> template.</p> <p>Example:</p> <pre><code>int main() { my_func&lt;int&gt;() // cause instantiation of my_func&lt;int&gt; my_class&lt;int&gt; foo; // okay, because my_func&lt;int&gt; exists my_class&lt;char&gt; bar; // compile error! my_func&lt;char&gt; does not exist } </code></pre> <p>Note that <code>my_class&lt;T&gt;</code> does not need to use <code>my_func&lt;T&gt;</code> internally so instantiating it won't automatically cause <code>my_func&lt;T&gt;</code> to become instantiated.</p> <p><strong>Edit[0]:</strong> I can't call <code>my_func&lt;T&gt;</code> myself because I need the user to call it and pass in information that tells the library how to handle <code>my_class&lt;T&gt;</code>. ie. If they want to use a <code>my_class&lt;char&gt;</code> they need to explicitly say so in a particular place in the code. I could make <code>my_class&lt;T&gt;</code> constructor test whether <code>my_func&lt;T&gt;</code> has been called generate a run-time error if it hasn't but I'd prefer to be able to generate a compile-time error since it's knowable at compile-time whether <code>my_func&lt;T&gt;</code> is ever used.</p> <p><strong>Edit[1]:</strong> One less-than-ideal way to do it would be to make a <code>MY_FUNC</code> macro that creates a template specialisation of an otherwise incomplete template class...</p> <pre><code>template&lt;typename T&gt; class incomplete; #define MY_FUNC(T) template&lt;&gt; incomplete&lt;T&gt; { static const bool x = my_func&lt;T&gt;(); }; template&lt;typename T&gt; class my_class { template&lt;size_t i&gt; class empty {}; typedef empty&lt;sizeof(incomplete&lt;T&gt;)&gt; break_everything; } </code></pre> <p>Now the user can only use a <code>my_class&lt;T&gt;</code> if <code>MY_FUNC(T)</code> is used somewhere. However I'd prefer a solution that doesn't use macros and doesn't force the user to use <code>MY_FUNC</code> at global scope rather than in a function.</p> <p><strong>Edit[2]:</strong> Thanks for the answers everyone. I realise it's impossible to guarantee at compile that the user will isn't calling <code>my_func&lt;T&gt;</code> incorrectly. But I can make it pretty unlikely and make it generate a runtime error early on if they do.</p> <p>They're supposed to define a function called, lets say, <code>initialise_library</code>. The function looks like this:</p> <pre><code>void initialise_library() { my_func&lt;int&gt;(); my_func&lt;char&gt;(); etc.. } </code></pre> <p>During the library initialisation, it checks to make sure none of the <code>my_func</code>s have been called. It then calls <code>initialise_library</code>. It then checks to make sure all of the <code>my_funcs</code> have been called. This much is doable. If the user hasn't defined <code>initialise_library</code> they get a linker error. If they haven't called <code>my_func&lt;T&gt;</code> somewhere in their program for a <code>my_class&lt;T&gt;</code> that they use then (idealy) they get a compile error. If they've defined <code>initialise_library</code> <em>and</em> they've used <code>my_func&lt;T&gt;</code> somewhere <em>but</em> they've used it in the wrong place they get a runtime error when their program starts up.</p> <p>Basically I want to stop them from adding a <code>my_class&lt;T&gt;</code> somewhere in their code and forgetting to put <code>my_func&lt;T&gt;()</code> in <code>initialise_library</code>, because that's something they're likely to do. The only way to runtime check this would be during the construction of <code>my_class&lt;T&gt;</code>. This will be an annoyingly late time to make this check if they only use <code>my_class&lt;T&gt;</code> in a deep and rarely-used chunk of code.</p>
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      1. POC++: Generate a compile error if user tries to instantiate a class template wihout instantiating a corresponding function template
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      1. VTUpMod
    1. COerm, if you need the user to pass stuff in, then it seems you don't have enough information at compile time, so doing a check in the constructor seems like the only viable option
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      1. USNicu Stiurca
    2. CO*"I want to guarantee that a user of my code has called my_function<T> somewhere in their code if they try to instantiate my_class<T>"* - That is either A) a poor design, or B) something you need to document. Try to rethink the decision entirely.
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      1. POC++: Generate a compile error if user tries to instantiate a class template wihout instantiating a corresponding function template
      1. USEd S.
    3. CO`int x = 0; cin >> x; if(x == 1) { my_class<whatever> m; } my_function<whatever>();` Can you tell me if I create an instance before calling your function?
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      1. POC++: Generate a compile error if user tries to instantiate a class template wihout instantiating a corresponding function template
      1. USEd S.
 

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