C++ constructors and computed fields

In C++, constructors must initialize const variables using initialization lists.

What i开发者_开发知识库f my constructor needs to compute the values of those fields? Say via a database lookup or simple computation.

The factory pattern could be applied here, but it seems a bit heavy. I'm considering static methods like X::GetX(param1, param2) that will compute the values and call a private constructor.

Is there a better or more popular pattern that could be used here?

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No need for calling a private constructor, you can call static methods (or in some cases, even non-static ones) directly from the initializer list. For example:

class testclass {
    public:
    testclass::testclass(int n): memberdata(fn(n)) { }

    private:
    int fn(int n) {
        // Various calculations on 'n'
        return 12;
    }

    int memberdata;
};

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You can call static functions within the initialisers, and those functions can contain whatever logic you please.

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You can call the method inside the initializer list itself:

class A {
public:
  A () : t_Const(X::Get(param1, param2)) { }  // constructor can be public
  const int t_Const;  // this is your variable
};

Why you need a private constructor!