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    <p>You've provided a <em>template</em> where a <em>type</em> is expected.</p> <p>For what it's worth, GCC 4.8 gives a decent diagnostic:</p> <pre><code>template &lt;typename&gt; struct Foo; template &lt;typename&gt; struct Bar; int main() { Foo&lt;Bar&gt; x; } </code></pre> <p>Diagnostic:</p> <pre class="lang-none prettyprint-override"><code>In function ‘int main()’: error: type/value mismatch at argument 1 in template parameter list for ‘template&lt;class&gt; struct Foo’ Foo&lt;Bar&gt; x; ^ error: expected a type, got ‘Bar’ </code></pre> <p>(GCC got a lot better since Clang started producing very nice error messages, so this may be recent; I imagine Clang would also give a useful error here.)</p> <hr> <p>Concerning your updated question: The typical way to make a container adapter, or generally a class that needs a certain container which you want to be customizable, is to pass the entire container as a template parameter:</p> <pre><code>template &lt;typename XContainer, typename YContainer&gt; struct Database { XContainer xstorage; YContainer ystorage; }; Database&lt;std::map&lt;int, double&gt;, std::unordered_map&lt;std::string, double&gt;&gt; db; db.xstorage[1] = 2.5; db.ystorage["foo"] = 4.5; </code></pre> <p>This is the most general solution since you can use <em>any</em> class that fits your needs, without requiring that class to have any particular structure.</p> <p>As an alternative (<em>which I do not recommend</em>), if you want all your containers to be template specializations of the same template, you can pass the template in directly as a template parameer (a so-called template template parameter):</p> <pre><code>template &lt;template &lt;typename, typename, typename...&gt; class CTmpl, typename XType, typename YType&gt; struct Database { CTmpl&lt;XType, double&gt; x; CTmpl&lt;YType, double&gt; y; }; Database&lt;std::map, int, std::string&gt; db; // as above </code></pre> <p>This is a lot more restrictive: not only does your database now have to use the same data structure for everything, but the data structure also <em>has</em> to be given as a template with fixed first two parameters "key type" and "mapped type".</p>
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    1. POtemplate rebinding vs. late template binding or how to call piggy backing a template in a type
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    1. USKerrek SB
    1. USKerrek SB
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    1. COActually I was happy with the name *template rebinding* which I did not know and as an answer I would have hooked it as correct. Your last solution however does absolutely not apply (one template argument for each map). Note that my *database* was a simplified example code. Imagine there are 1000 different maps used inside my database and I want to switch from binary trees to hashes. Your most helpful advise was to use clang which showed me that I can actually pass a template (not a type) as an argument in one case and thus I could fix my last example.
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      1. USPatrick Fromberg
    2. CO@PatrickFromberg This answer does not (currently) mention the word "rebind," as it shouldn't. That term applies to a template which allows transforming one specialization into another, such as `std::allocator` which provides a `rebind` member template. I think "wrapper class" suffices.
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      1. USPotatoswatter
    3. CO@Potatoswatter, I see your point regarding the rebinding which is only one way out of many to pass kind of template template argument [see here](http://stackoverflow.com/q/12362363/2712726). But a "wrapper class" is to general because there are many wrapping idioms for very different purposes totally unrelated to the question. After your comment *late template binding* would be my personal favorite as name for the idiom.
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