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Suppose I want to define a class Foo which has a member variable that is also a template Bar<int num> baz. Also suppose that I always want num to be some particular value (say 5) for baz as defined in Foo. Then presumably I could accomplish this as follows:

class Foo
{
private:
    Bar<5> baz;
};

However, this would be introducing a magic number which is typically not good programming practice. I have considered defining a constant in the class definition a la const int num = 5;, but that would create an extra integer for every instance of Foo, which is not ideal if I have lots of instances. I have also considered defining a constant variable outside the class definition but in the header file. This has the advantage of being seen in the .cpp file, but pollutes the global name space.

My question is, what is the most stylistically appropriate way to accomplish this goal? Ideally there would be some variable which is only visible to the header and .cpp file, and which can be used in the template call.

Lucas Myers
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    Does this answer your question? [Difference between \`constexpr\` and \`const\`](https://stackoverflow.com/questions/14116003/difference-between-constexpr-and-const) – 273K Jun 10 '21 at 14:18
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    You might have `static constexpr` member. – Jarod42 Jun 10 '21 at 14:18
  • @S.M. that question halfway answers mine -- the other piece was the `static` keyword to keep from allocating an `int` for every instance of `Foo`. Thanks, though. – Lucas Myers Jun 10 '21 at 14:25

1 Answers1

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You want a single static variable, common to all instances of Foo.

class Foo
{
private:
    static constexpr int num = 5;
    Bar<num> baz;
};

For the same reason magic numbers are considered bad, you also probably want a variable name more informative than num.

Drew Dormann
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