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POC++ unordered_map with char* as key
 

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    <p>I feel exhausted when trying to use the container <code>unordered_map</code> with <code>char*</code> as the key (on Windows, I am using VS 2010). I know that I have to define my own compare function for <code>char*</code>, which inherits from <code>binary_function</code>. The following is a sample program.</p> <pre><code>#include&lt;unordered_map&gt; #include &lt;iostream&gt; #include &lt;string&gt; using namespace std; template &lt;class _Tp&gt; struct my_equal_to : public binary_function&lt;_Tp, _Tp, bool&gt; { bool operator()(const _Tp&amp; __x, const _Tp&amp; __y) const { return strcmp( __x, __y ) == 0; } }; typedef unordered_map&lt;char*, unsigned int, ::std::tr1::hash&lt;char*&gt;, my_equal_to&lt;char*&gt; &gt; my_unordered_map; //typedef unordered_map&lt;string, unsigned int &gt; my_unordered_map; my_unordered_map location_map; int main(){ char a[10] = "ab"; location_map.insert(my_unordered_map::value_type(a, 10)); char b[10] = "abc"; location_map.insert(my_unordered_map::value_type(b, 20)); char c[10] = "abc"; location_map.insert(my_unordered_map::value_type(c, 20)); printf("map size: %d\n", location_map.size()); my_unordered_map::iterator it; if ((it = location_map.find("abc")) != location_map.end()) { printf("found!\n"); } return 0; } </code></pre> <p>I insert the same C string <code>abc</code> twice and look it up. The second insertion should fail and there will be only one <code>abc</code> in the unordered_map. However, the output size is 3. It seems that the compare function does not work properly here.</p> <p>Moreover, I get another strange result about the <code>find</code> function, by running the program for many times, the finding result even changes! Sometimes the string <code>abc</code> is found, while the other times <code>abc</code> is not found!</p> <p>Could anyone help me on this? Your help is very much appreciated!</p> <p>++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++</p> <p>Edit: After defining a hash function for <code>char*</code> by my own, the program works properly. The full program code is listed below. Thank you all.</p> <pre><code>#include&lt;unordered_map&gt; #include &lt;iostream&gt; using namespace std; template &lt;class _Tp&gt; struct my_equal_to : public binary_function&lt;_Tp, _Tp, bool&gt; { bool operator()(const _Tp&amp; __x, const _Tp&amp; __y) const { return strcmp( __x, __y ) == 0; } }; struct Hash_Func{ //BKDR hash algorithm int operator()(char * str)const { int seed = 131;//31 131 1313 13131131313 etc// int hash = 0; while(*str) { hash = (hash * seed) + (*str); str ++; } return hash &amp; (0x7FFFFFFF); } }; typedef unordered_map&lt;char*, unsigned int, Hash_Func, my_equal_to&lt;char*&gt; &gt; my_unordered_map; int main(){ my_unordered_map location_map; char a[10] = "ab"; location_map.insert(my_unordered_map::value_type(a, 10)); char b[10] = "abc"; location_map.insert(my_unordered_map::value_type(b, 20)); char c[10] = "abc"; location_map.insert(my_unordered_map::value_type(c, 20)); printf("map size: %d\n", location_map.size()); my_unordered_map::iterator it; if ((it = location_map.find("abc")) != location_map.end()) { printf("found!\n"); } return 0; } </code></pre> <p><strong>Note: Using <code>char</code>* as the key type for an unordered_map or other STL containers may be dangerous, a safe way (seems to be the only way) is: in the main function, <code>new</code> or <code>malloc</code> a block (e.g. an array of c strings) on heap and fill it with c strings. Insert these c strings into unordered_map. The allocated block of memory is freed at the end of of main function (by <code>delete</code> or <code>free</code>).</strong></p>
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    1. COYou do not need to inherit from `binary_function`. It might even be deprecated; I can't look it up just now.
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    2. COThis isn't the problem, but names that contain two consecutive underscores (`__x`, `__y`) and names that begin with an underscore followed by a capital letter (`_Tp`) are reserved to the implementation (the compiler and its library). Don't use them.
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      1. USPete Becker
 

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