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    <p>It is a sort of contract between the programmer and the compiler. The programmer says <code>{1,2,3,4}</code>, and the compiler creates an object of type <code>initializer_list&lt;int&gt;</code> out of it, containing the same sequence of elements in it. This contract is a requirement imposed by the language specification on the compiler implementation.</p> <p>That means, it is not the programmer who creates manually such an object but it is the compiler which creates the object, and pass that object to function which takes <code>initializer_list&lt;int&gt;</code> as argument. </p> <p>The <code>std::vector</code> implementation takes advantage of this contract, and therefore it defines a constructor which takes <code>initializer_list&lt;T&gt;</code> as argument, so that it could initialize itself with the elements in the initializer-list.</p> <p>Now <strong>suppose for a while</strong> that the <code>std::vector</code> doesn't have any constructor that takes <code>std::initializer_list&lt;T&gt;</code> as argument, then you would get this:</p> <pre><code> void f(std::initializer_list&lt;int&gt; const &amp;items); void g(std::vector&lt;int&gt; const &amp;items); f({1,2,3,4}); //okay g({1,2,3,4}); //error (as per the assumption) </code></pre> <p>As per the assumption, since <code>std::vector</code> doesn't have constructor that takes <code>std::initializer_list&lt;T&gt;</code> as argument, which <em>implies</em> you cannot pass <code>{1,2,3,4}</code> as argument to <code>g()</code> as shown above, because the compiler <em>cannot</em> create an instance of <code>std::vector</code> out of the expression <code>{1,2,3,4}</code> <strong>directly</strong>. It is because no such contract is ever made between programmer and the compiler, and imposed by the language. It is <em>through</em> <code>std::initializer_list</code>, the <code>std::vector</code> is able to create itself out of expression <code>{1,2,3,4}</code>. </p> <p>Now you will understand that <code>std::initializer_list</code> can be used wherever you need an expression of the form of <code>{value1, value2, ...., valueN}</code>. It is why other containers from the Standard library also define constructor that takes <code>std::initializer_list</code> as argument. In this way, no container depends on any other container for construction from expressions of the form of <code>{value1, value2, ...., valueN}</code>.</p> <p>Hope that helps.</p>
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