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PORework for loop over STL container to use functional techniques
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  1. PORework for loop over STL container to use functional techniques
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    copied!<p>I have a <code>std::vector</code> of pointers <code>Person</code> objects, which have a member function <code>std::string getName() const</code>. Using STL algorithms I want to count all the <code>Person</code> objects in the vector where <code>getName()</code> returns "Chad".</p> <p>The behaviour simply iterating over the loop would be:</p> <pre><code>int num_chads = 0; for(std::vector&lt;Person *&gt;::const_iterator it = vec.begin(); it != vec.end(); ++it) { if((*it)-&gt;getName() == "Chad") ++num_chads; } </code></pre> <p>I want to rework this so it uses all STL algorithms and functors etc (make it more functional-oriented). This is what I think I need to do:</p> <pre><code>const int num_chads = std::count_if(vec.begin(), vec.end(), std::bind1st(std::bind2nd(std::equal_to, mem_fun(Person::getName)), "Chad")); </code></pre> <p>As you can probably tell this doesn't work. Firstly, as I understand it, you can't use bind1st/bind2nd on binder1st/binder2nd objects as they are specifically designed to work with <code>std::binary_functions</code>. Secondly, and much more importantly, I don't think I am using the correct technique. I <em>do</em> want to bind one of the arguments to "Chad", but with the iterator argument I actually just want to transform the iterator value to a string before calling the bound version of <code>equals_to</code>.</p> <p>I think it is possible to do this using Boost, but is it possible using just the core C++03 (i.e. no C++0x lambas!)?</p> <p>EDIT: Can anyone come up with an example which does not use a user-defined predicate (i.e. just using the tools provided in the std toolkit)?</p> <p>EDIT: While Matthieu's answer is a textbook answer for how to use functors in STL algorithms, Cubbi's answer came from the approach I was looking for (although Mathieu did answer before I edited the question to make it more specific, so apologies there!). </p>
 

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