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copied!<p>Concepts are for compile-time polymorphism, That means parametric generic code. Interfaces are for run-time polymorphism. </p> <p>You have to implement an interface as you implement a Concept. The difference is that you don't have to explicitly say that you are implementing a Concept. If the required interface is matched then no problems. In the case of interfaces, even if you implemented <strong>all</strong> the required functions, you have to excitability say that you are implementing it!</p> <hr> <p>I will try to clarify my answer :)</p> <p>Imagine that you are designing a container that accepts any type that has the <strong>size</strong> member function. We formalize the Concept and call it HasSize, of course we should define it elsewhere but this is an example no more.</p> <pre><code>template <class HasSize> class Container { HasSize[10]; // just an example don't take it seriously :) // elements MUST have size member function! }; </code></pre> <p>Then, Imagine we are creating an instance of our <strong>Container</strong> and we call it <strong>myShapes</strong>, Shape is a base class and it defines the <strong>size</strong> member function. Square and Circle are just children of it. If Shape didn't define size then an error should be produced.</p> <pre><code>Container<Shape> myShapes; if(/* some condition*/) myShapes.add(Square()); else myShapes.add(Circle()); </code></pre> <p>I hope you see that Shape can be checked against <strong>HasSize</strong> at <em>compile time</em>, there is no reason to do the checking at run-time. Unlike the elements of <strong>myShapes</strong>, we could define a function that manipulates them :</p> <pre><code>void doSomething(Shape* shape) { if(/* shape is a Circle*/) // cast then do something with the circle. else if( /* shape is a Square */) // cast then do something with the square. } </code></pre> <p>In this function, you can't know what will be passed till run-time a Circle or a Square!</p> <p>They are two tools for a similar job, though Interface-or whatever you call them- can do almost the same job of Concepts at run-time but you lose all benefits of compile-time checking and optimization!</p>

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