Passing this pointer and arguments of class method to local lambda function at compile time

Question

Suppose you have a scenario when you want to create a constexpr lambda inside a method for calculating something at compile time.

struct A {
    int a;
    constexpr A(int a) : a(a) {}
    constexpr auto operator+(const A& rhs) {
        constexpr auto l = [&]() {
            return A(this->a + rhs.a);
        };
        return l();
    }
};

This code doesn't compile because compiler said that this and rhs are not constant expressions. Is there a way for passing this and rhs to a local constexpr lambda?

What is the point of using the lambda at all? Why not just `constexpr auto operator+(const A& rhs) { return a + rhs.a; }`? — Remy Lebeau, Aug 31 '21 at 23:13
@RemyLebeau Perhaps the posted code is just a *minimal* example of what is being sought? In OP's 'real-world' code, there may be a more justifiably reason to want a lambda. — Adrian Mole, Aug 31 '21 at 23:28

Adrian Mole · Accepted Answer · 2021-08-31T22:59:26.300

You can't capture the a members of this and rhs (by reference) and maintain constexpr validity¹; however, you can pass those members as by (const) reference arguments to your lambda:

struct A {
    int a;
    constexpr A(int a) : a(a) { }
    constexpr auto operator+(const A rhs) {
        constexpr auto l = [](const int& ta, const int& ra) {
            return A(ta + ra);
        };
        return l(a, rhs.a); // Or return l(this->a, rhs.a) if you prefer
    }
};

¹ Or maybe you can, but it's messy: Lambda capture as const reference?

Passing this pointer and arguments of class method to local lambda function at compile time

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