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POboost::bind internal copy/copies?
 

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  1. POboost::bind internal copy/copies?
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    <p>I was hoping to understand what kind of internal copies of a function object does boost::bind do. Since constructors of these objects do not seem to be invoked, I presumed this is a kind of "very shallow copy", so I introduced dynamic memory allocations to produce some errors. However, runtime output of the code below seems to indicate <em>three</em> additional destructor calls for internal copies made by bind.</p> <pre><code>using namespace std; using namespace boost; class M{ int *somedata; public: M(){ somedata=new int[5]; cout&lt;&lt;"from cstr\n"; somedata[1]=0;} ~M(){cout&lt;&lt;"from dstr\n"; delete somedata;} int operator()(int i){ cout&lt;&lt;++somedata[i]&lt;&lt;endl; return 0;} }; int main() { M instM; bind&lt;int&gt;(instM,1)(); //bind&lt;int&gt;(&amp;M::operator(),&amp;instM,1)(); //this works with no errors, of course instM(1); //would not change the order of output return 0; } </code></pre> <p>The output ... presents some additional puzzles - eg. why first dstr event comes before call to operator()? Also notice the "2" before the last failed destructor call. </p> <pre><code>from cstr from dstr 1 from dstr bind_copy(73365) malloc: *** error for object 0x1001b0: double free *** set a breakpoint in malloc_error_break to debug from dstr bind_copy(73365) malloc: *** error for object 0x1001b0: double free *** set a breakpoint in malloc_error_break to debug 2 from dstr bind_copy(73365) malloc: *** error for object 0x1001b0: double free *** set a breakpoint in malloc_error_break to debug </code></pre> <p>So the question is: can anyone explain briefly in what order, and what kind of copies does bind make?</p> <p>... After some thought I realized bind is simply using the (here default) copy constructor. After supplying some custom version of this cstr (with memory allocation and as-deep-as-one-wishes version of the copy) the output gets clean (as it should), but the puzzle remains: there are <em>three</em> invocations of the copy constructor. <strong>So in this case boost::bind makes <em>three</em> copies of the function object. Why and in which order?</strong> (For nested boost::binds this can lead to quite explosive growth of the number of internal copies.)</p> <p>Output with cp-cstr defined, and some "heritage markers" added ("P"=parent, each cp cstr adds "-C"):</p> <pre><code> from cstr P from cp cstr P-C from cp cstr P-C-C from cp cstr P-C-C-C from dstr P-C-C P-C-C-C:1 from dstr P-C-C-C from dstr P-C P:1 from dstr P </code></pre>
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    1. COOh, who knows what happen when you compile with `--std=c++0x`, `using namespace` both `std` and `boost` and a function named `bind`...
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    2. CO@Benoit: granted; this does not influence the above errors in any way though.
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    3. COI would love to know the answer to this too. New and delete in my os are very expensive and I see it slowing the system on binding a member function. I can see no documentation about whether member functions are copied or not.
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