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POunexpected std::bad_function_call in recursion
 

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  1. POunexpected std::bad_function_call in recursion
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    <p>I have written the following code, which plays with functions of type <code>function&lt;int(int)&gt;</code>. The functions <code>compose</code>, <code>print</code>, <code>inc</code> and <code>guarded</code> are helpers which combine other functions or produce some external effect. Then I use them to build my programs:</p> <pre><code>/* start of the program */ function&lt;int(int)&gt; recursion(); function&lt;int(int)&gt; go = compose(guarded(10, recursion()), compose(inc, print("go"))); function&lt;int(int)&gt; recursion() { return compose(go, print("recursion")); } </code></pre> <p>However, when calling <code>recursion()(0)</code>, an exception <code>std::bad_function_call</code> was thrown when <code>go</code> was reached the second time but I don't see why. Is there any dangling reference or empty <code>std::function</code>? Moreover, eta-expanding <code>go</code> works:</p> <pre><code>function&lt;int(int)&gt; go = [](int n) -&gt; int { return compose(guarded(10, recursion()), compose(inc, print("go")))(n); }; </code></pre> <p>What's wrong with the original code? Why does the alternative one work?</p> <p>Full code:</p> <pre><code>#include &lt;string&gt; #include &lt;iostream&gt; #include &lt;functional&gt; using namespace std; /* helper functions, some combinators */ //composing two functions, f1 . f2 function&lt;int(int)&gt; compose(const function&lt;int(int)&gt;&amp; f1, const function&lt;int(int)&gt;&amp; f2) { return [f1,f2](int n) -&gt; int { return f1(f2(n)); }; } function&lt;int(int)&gt; print(const string&amp; msg) { return [msg](int n) -&gt; int { cout &lt;&lt; "print: " &lt;&lt; msg &lt;&lt; endl; return n; }; } function&lt;int(int)&gt; inc = [](int n) -&gt; int { cout &lt;&lt; "in inc lambda: " &lt;&lt; n &lt;&lt; endl; return n+1; }; //calls the given function `f` only when `n` is less then `m` function&lt;int(int)&gt; guarded(int m, function&lt;int(int)&gt; f) { auto g = [m,f](int n) -&gt; int { return n&lt;m? f(n) : m; }; return compose(g, print("guarded")); } /* start of the program */ function&lt;int(int)&gt; recursion(); function&lt;int(int)&gt; go = compose(guarded(10, recursion()), compose(inc, print("go"))); function&lt;int(int)&gt; recursion() { return compose(go, print("recursion")); } int main() { try { recursion()(0); } catch (bad_function_call e) { cout &lt;&lt; "bad_function_call: " &lt;&lt; e.what() &lt;&lt; endl; } return 0; } </code></pre>
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