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    <p>You don't have to do it on individual (per-class) basis.</p> <p>The standard library already provides support for this approach, by declaring "default" template implementations for operators <code>!=</code>, <code>&gt;</code>, <code>&lt;=</code> and <code>&gt;=</code>, which rely on user-provided operators <code>==</code> and <code>&lt;</code>. These declarations are located in header <code>&lt;utility&gt;</code> and enclosed in namespace <code>std::rel_ops</code>.</p> <p>Since these implementations are declared in namespace <code>std::rel_ops</code>, they are not found by default by name lookup. If you want to "activate" these definitions in your program, do</p> <pre><code>#include &lt;utility&gt; using namespace std::rel_ops; </code></pre> <p>This, for example, will automatically derive operator <code>!=</code> from your implementation of operator <code>==</code> for all classes in the translation unit. You can still override the mechanism by providing an explicit implementation of operator <code>!=</code> for some specific class.</p>
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    1. POC++: Overload != When == Overloaded
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    1. COThe downside (of using `using namespace std::rel_ops;`) on the other hand is that all operators that define `==` will now also have an implicit `!=` even if there original author specifically did not create one (and if they did not create one they probably had a good reason (especially since it is so easy)).
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      1. USMartin York
    2. COBetter than the `using namespace`, you can add `using ::std::rel_ops::operator!=; using ::std::rel_ops::operator>;...` for the operators that you need in the namespace of the type that you are defining. That will enable ADL to find it, but it still has the issue that Loki mentions: you cannot control by class, only by namespace. It is a pity that the standard did not add a tag type in the namespace, as that would have been simpler: `class MyType : ::std::rel_ops::tag {};` would bring all of the templates into scope for `MyType` without interfering with other types in the namespace.
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      1. USDavid Rodríguez - dribeas
    3. CO(The option of using a tag type to enable of templated operators in a namespaces was suggested by Dietmar Kühl, although that was pre-C++11 `rel_ops`). An alternative (without using `std::rel_ops` I have played with in the past was abusing CRTP: `template <typename T> struct comparable { friend bool operator!=(T const& a, T const& b) { return !(a==b); }; friend bool operator> (T const& a, T const& b) { return b<a; }; ... class MyType : comparable<MyType> {}; /* provide op==, op< */` again in a separate namespace, and again ADL will find it by looking in the base class...
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      1. USDavid Rodríguez - dribeas
 

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