Constexpr classes

Constexpr classes objects can be created and evaluated at compile time. As we have the knowledge of creating constexpr aggregates, let’s apply the same knowledge here and try creating a constexpr class.

#include <iostream>
 
class Point
{
private:
    int m_x{};
    int m_y{};
 
public:
    Point() = default;
    Point(int x, int y)
        : m_x{x}, m_y{y}
    {
    }
 
    void print() const
    {
        std::cout << "Point(" << m_x << ", " << m_y << ")\n";
    }
    constexpr int greaterCoordinate() const
    {
        return m_x < m_y ? m_y : m_x;
    }
};
 
int main(int argc, char const *argv[])
{
    constexpr Point p{1, 2};
    p.print();
 
    constexpr int greater{p.greaterCoordinate()};
    std::cout << "Greater coordinate: " << greater << "\n";
    return 0;
}

We have declared the Point object with constexpr and declared a constexpr greaterCoordinate member function. Let’s compile the program. When I compiled the program, I got following compilation error on my machine:

constexpr_classes.cpp: In function 'int main(int, const char**)':
constexpr_classes.cpp:28:27: error: call to non-'constexpr' function 'Point::Point(int, int)'
   28 |     constexpr Point p{1, 2};
      |                           ^
constexpr_classes.cpp:11:5: note: 'Point::Point(int, int)' declared here
   11 |     Point(int x, int y)

I think is very clear why compiler is crying. It is saying that call to Point(int,int) constructor is made which is non-constexpr function.

Note

On the resource that I am following, author mentioned a different error of class not being a literal type. Not sure why compiler is not throwing the same error for me. I have tried different C++ standards with g++ compiler.

Let’s declare constructor as constexpr.

#include <iostream>
 
class Point
{
private:
    int m_x{};
    int m_y{};
 
public:
    Point() = default;
    constexpr Point(int x, int y)
        : m_x{x}, m_y{y}
    {
    }
 
    void print() const
    {
        std::cout << "Point(" << m_x << ", " << m_y << ")\n";
    }
    constexpr int greaterCoordinate() const
    {
        return m_x < m_y ? m_y : m_x;
    }
};
 
int main(int argc, char const *argv[])
{
    constexpr Point p{1, 2};
    p.print();
 
    constexpr int greater{p.greaterCoordinate()};
    std::cout << "Greater coordinate: " << greater << "\n";
    return 0;
}

Now it should compiler successfully and run.

Note

In C++11, non-static constexpr member functions are const implicitly (except constructors). In C++14 (and onwards), constexpr member functions are not implicitly const. We need to explicitly make them const.

Furthermore, implicitly defined constructors are constexpr. For example,

class Pair
{
};
 
int main(int argc, char const *argv[])
{
    constexpr Pair p{}; // should work
    return 0;
}

We created an empty class and it should have implicit default constructor. So, initializing this empty class in constexpr context works.

Resources

1. https://www.learncpp.com/cpp-tutorial/constexpr-aggregates-and-classes/