Thread

1. StilesCrisis · 1791036041 · focus · HN ↗

   I wish the readme example showed a lambda capture. Now that's where compiler magic REALLY shows up.

   1. sltkr · 1791059716 · focus · HN ↗

      C++ lambdas are just structs that overload operator(); once you realize that they become a lot less mysterious. But it's a great example of how this tool is useful to learn about those things (of course you could also read the C++ standard but who has time for that?)

      Here&#x27;s an example: <a href="https:&#x2F;&#x2F;cppinsights.io&#x2F;s&#x2F;f04b6896" rel="nofollow">https:&#x2F;&#x2F;cppinsights.io&#x2F;s&#x2F;f04b6896

      (That also shows how C++&#x27;s range-based for-loops work, which is similarly some syntactic suger for a fairly mundane for-loop, but details are interesting: unlike a typical for-loop, the loop variable is declared inside the loop body.)

      1. StilesCrisis · 1791064803 · focus · HN ↗

         I said shows a lambda _capture_. This involves the compiler automatically determining which variables cross the scope boundary, and injecting code for each one.

         1. frabert · 1791125522 · focus · HN ↗

            I&#x27;m curious to understand which part you think is particularly interesting. The compiler clearly knows which variables are declared in what scope, so what&#x27;s mysterious about figuring out which values need to be captured?
      2. Someone · 1791093439 · focus · HN ↗

         &gt; That also shows how C++&#x27;s range-based for-loops work, which is similarly some syntactic suger for a fairly mundane for-loop, but details are interesting: unlike a typical for-loop, the loop variable is declared inside the loop body

         That’s not true in C++, is it? There, this does not compile

           for(int i = 0; i < 3; ++i)
           {}
           i = i + 1;
         

         (<a href="https:&#x2F;&#x2F;cppinsights.io&#x2F;s&#x2F;26405687" rel="nofollow">https:&#x2F;&#x2F;cppinsights.io&#x2F;s&#x2F;26405687: error: use of undeclared identifier &#x27;i&#x27;)

         <a href="https:&#x2F;&#x2F;en.cppreference.com&#x2F;cpp&#x2F;language&#x2F;for" rel="nofollow">https:&#x2F;&#x2F;en.cppreference.com&#x2F;cpp&#x2F;language&#x2F;for:

         “A for statement equivalent to:

           {
             init-statement
             while ( condition )
             {
               statement
               expression ;
             }
           }
         

         Except that […]”

         and

         “While in C names declared in the scope of init-statement and condition can be shadowed in the scope of statement, it is forbidden in C++:

           for (int i = 0;;)
           {
             long i = 1;   // valid C, invalid C++
             // ...
           }

         “

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