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POUsing an std::string as a key for an std::map
 

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    <p>I would like to have an std::map (int .NET 4.0). We of course know that a map is a tree and requires an operator&lt; that string does not define for us. </p> <blockquote> <p>Error 24 error C2676: binary '&lt;' : 'const std::string' does not define this operator or a conversion to a type acceptable to the predefined operator c:\program files\microsoft visual studio 10.0\vc\include\xfunctional 125 1 FXCMMarketDataServer</p> </blockquote> <p>So I put my google-foo to work and found this solution:</p> <pre><code>struct StringComparerForMap { public: bool operator()(const std::string x, const std::string y) { // Add compare logic here } }; ... std::map&lt;std::string, CustomObject, StringComparerForMap&gt; myMap; </code></pre> <p>This worked fine for a while, and now I'm encountering a bug that I believe is due to this. Somewhere deep down in the STL framework it would seem that it ignores the above definition and defaults to operator&lt;.</p> <p>Is there a way in VS2010 .NET 4.0 to use a string as the key of a map?</p> <p>I understand that I can take that string and write a function to hash it to an int, but where's the fun in that?</p> <p><strong>EDIT</strong></p> <p>I will try and explain this as best I can for David. When the map uses the comparer struct, it crashes in release and fails a debug assertion in debug. The assert that fails is in xtree line 1746. </p> <blockquote> <p>Expression: invalid operator&lt;</p> <p>|Abort| |Retry| |Ignore|</p> </blockquote> <p>That is what leads me to believe that despite giving map a comparer, it still down certain paths defaults to operator&lt; for comparison. The line in my code that causes this is:</p> <pre><code>CustomObject o = stringObjectMap[key]; </code></pre>
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    1. USSteve H.
    1. USSteve H.
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    1. COstd::string does have a suitable definition of the required operator. Are you certain you have included the <string> header? I think I have seen similar issues when I failed to include it; forward declares of std::string mean that its name itself is known.
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      1. USKeith
    2. CO.NET has nothing to do with std::map or std::string
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      1. USChris Card
    3. COWhat makes you think that deep inside the library layers it defaults to `<`? If it were so, it would be broken and you should file a bug report, but most probably you are misinterpreting the data --that error would get triggered in many other situations, someone would probably have detected it before... When I told my first boss that the compiler was wrong he smiled and said: *99% of the time, the problem is located between the chair and the keyboard and not in the compiler*
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      1. USDavid Rodríguez - dribeas
 

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