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    <p>There is two things in your code that will not work.</p> <ul> <li>First, when passing a lambda function as argument, I suggest using <code>Template</code>. The standard library on Microsoft seems to use this method for <code>std::for_each</code> for example.</li> <li><p>And :</p> <blockquote> <p>When function template has a return type, which cannot be deduced from arguments, or when function template doesn't have any argument, the type cannot be deduced by the compiler. This function will require <em>template type argument specification</em>.</p> </blockquote></li> </ul> <p>Take a look at this example :</p> <pre><code>template&lt;class Tout, class Tin, class Fun&gt; // ^^^^^^^^^^^ // Note that I changed the order of the types std::vector&lt;Tout&gt; map( const std::vector&lt;Tin&gt;&amp; in, Fun mapper ) { // ^^^^^^^^^^ std::vector&lt;Tout&gt; ret; for( auto elem : in ) { ret.push_back( mapper( elem ) ); } return ret; } int main() { std::vector&lt;int&gt; bars /* = ... */; std::vector&lt;float&gt; foos = map&lt;float&gt;( bars, []( int ) { return 1.0f; } ); // ^^^^^^^ Specify the type Tout system( "pause" ); return 0; } </code></pre> <p><strong>EDIT :</strong></p> <p>Like it is said in the comment, we can use <code>decltype</code> and <code>std::decay</code> to not have to explicitly specify the result of the function :</p> <pre><code> template&lt;class Tin, class Fun&gt; // no Tout // ^^^^^^^^^^^ auto map( const std::vector&lt;Tin&gt;&amp; in, Fun mapper ) //^^^^ ^^^^^^^^^^ -&gt; std::vector&lt;typename std::decay&lt; decltype( mapper( in.front() ) )&gt;::type &gt; { std::vector&lt;typename std::decay&lt; decltype( mapper( in.front() ) )&gt;::type &gt; ret; for( auto elem : in ) { ret.push_back( mapper( elem ) ); } return ret; } int main() { std::vector&lt;int&gt; bars /* = ... */; std::vector&lt;float&gt; foos = map( bars, []( int ) { return 1.0f; } ); // No specification system( "pause" ); return 0; } </code></pre> <p>Let's explain a little bit.</p> <p>First we will use the <em>late-specified return type</em> syntax. It will allow us to use the parameter names in the return type specification. We start the line with <code>auto</code> and put the return type specification after the parameters using <code>-&gt;</code>.</p> <p>We will use <code>decltype</code> because the decltype type specifier yields the type of a specified expression. It will be very useful in our case. For example to get the type of the function we passed in parameters, it is just <code>decltype( f( someArg ) )</code>.</p> <p>Let's state what do we want : The return type of the function should be a vector of the return type of the function passed in argument right ? So we can return <code>std::vector&lt; decltype( mapper( in.front() ) )&gt;</code> and that's it ! (Why the <code>in.front()</code> ? We have to pass a parameter to the function to have a valid expression.)</p> <p>But here again, we have a problem : <code>std::vector</code> does not allow references. To be certain that it will not be a problem for us, we will use the <code>std::decay</code> meta-function who <em>applies lvalue-to-rvalue, array-to-pointer, and function-to-pointer implicit conversions to the type T, <strong>removes cv-qualifiers</strong>, <strong>remove references</strong>, and defines the resulting type as the member typedef type.</em>. That is, if the function returns something like <code>const Foo&amp;</code> it will end in <code>Foo</code>.</p> <p>The result of all of that : <code>std::vector&lt; typename std::decay&lt; decltype( mapper( in.front() ) )&gt;::type &gt;</code>.</p> <p>You have to repeat this expression again at the beginning of the function to declare the variable you will return.</p> <p>Some usefull references about that :</p> <ul> <li><a href="http://en.cppreference.com/w/cpp/types/decay" rel="nofollow">http://en.cppreference.com/w/cpp/types/decay</a></li> <li><a href="http://en.wikipedia.org/wiki/Decltype" rel="nofollow">http://en.wikipedia.org/wiki/Decltype</a></li> <li><a href="http://www.cprogramming.com/c++11/c++11-auto-decltype-return-value-after-function.html" rel="nofollow">http://www.cprogramming.com/c++11/c++11-auto-decltype-return-value-after-function.html</a></li> <li><a href="http://msdn.microsoft.com/en-us/library/dd537655.aspx" rel="nofollow">http://msdn.microsoft.com/en-us/library/dd537655.aspx</a></li> </ul> <p>It is not easy to explain, I hope my explanations are understandable.</p>
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    1. CO@KillianDS You are right, edited :)
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      1. USPierre Fourgeaud
    2. COYou should change the return type to `std::vector< decltype( mapper(in.front()) ) >` with maybe some `std::decay` for taste at least in some cases. Maybe have the default type `Tout` be `void`, and if it is `void` use the above expression, and otherwise use `std::vector<Tout>`.
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      1. USYakk - Adam Nevraumont
    3. COI had to re-read your post a couple of times to get it, but now I do. And thanks for the links as well, they were helpful.
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