Initialize complex map in constructor with initializer list

Question

I was wondering the other day whether it is possible in C++ (any standard) to initialize a map in an initializer list of a constructor with a loop or a more complex procedure than literals such that I can make it a const member variable?

class MyClass {
public:
    const int myInt;
    const std::unordered_map<int, int> map;
    MyClass(int i) : myInt(i), /* initialize map like: for(int i = 0; i < myInt; ++i) { map[i] = whatever; } { }
}

I suppose you can always call a (freestanding or static) function that returns a map that depends on the run-time value of i? You won't be able to do that with literals, or (even recursive ) constexpressions, or (even recursive) templates because i is not known at compile time. The problem is the overhead in passing a map by value with pre-move standards/implementations. — Peter - Reinstate Monica
– Peter - Reinstate Monica, Commented Jan 14, 2021 at 10:36

alex_noname · Accepted Answer · 2021-01-14 11:41:43Z

3

You could use a lambda function inside constructor initializer list. Like so:

class MyClass {
public:
    const int myInt;
    const std::unordered_map<int, int> umap;
    MyClass(int i) : myInt(i), umap( [i]() -> decltype(umap) {
      std::unordered_map<int, int> tu;
      for(int j=0; j<i; j++)
        tu[j]=j;
      return tu; 
    }()) {}
};

answered Jan 14, 2021 at 11:41

alex_noname

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