CODEWITHSUNDEEP
c++11
Benjamin Bihler
Benjamin Bihler

Reputation: 2073

How to make a type alias depending on a compile-time constant within a class?

I want to write a class that makes use of numerical quadrature. The quadrature order defines the size of some containers that I will use. I would like to make a type alias for such containers and it has to depend on the quadrature order.

The following code shows my trials. It feels suboptimal in the sense that I have to repeat the order in the type alias definition:

#include <array>

class Quadrature
{
public:

        static constexpr unsigned int getOrder()
        {
                return 3;
        }

        // This line doesn't compile!
        //
        // using WeightsContainer = std::array<double, getOrder()>;
        //
        // g++ says "error: 'static constexpr unsigned int Quadrature::getOrder()'
        // called in a constant expression before its definition is complete"

        // This line compiles, but repeats the order. :-(
        using WeightsContainer = std::array<double, 3>;

private:

        WeightsContainer container;
};

One solution that I have found is introducing a template parameter Order. But actually I wanted to determine the quadrature order and introducing the template parameter would make it variable.

Is there a possibility to make the order a compile-time constant and use it within my type alias definition?

Edit:

For completeness, I could of course use a preprocessor define. But that feels old-fashioned. :-)

Edit 2:

Okay, I have found another possibility. I could add a function outside the class scope like this:

constexpr unsigned int order()
{
    return 3;
}

But that feels wrong, because this is a property of the class and therefore should be within class scope!

Upvotes: 2

Views: 151

Answers (1)

0x5453
0x5453

Reputation: 13589

One thing you can do is to move the value into a member variable:

class Quadrature
{
private:
        static constexpr unsigned int _order = 3;
public:
        static constexpr unsigned int getOrder()
        {
                return _order;
        }    
        using WeightsContainer = std::array<double, _order>;

        // ...
};

If you need more complicated computations instead of just return 3, under C++17 you can use a lambda as @Quentin mentioned:

class Quadrature
{
public:
        static constexpr auto getOrder = []()
        {
            return ...;
        };
        using WeightsContainer = std::array<double, getOrder()>;

        // ...
};

Otherwise, you will need to pull the function outside of class scope for reasons mentioned here.

Upvotes: 1

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