Accessing non-const data members from constexpr member function

Question

Both GCC and MSVC seem to allow defining constexpr accessor functions for non-const data members:

#include <random>
#include <iostream>

class Foo
{
    int val;

public:
    Foo(int v) : val(v) {}

    constexpr int get_val() { return val; } // OK
};

int main()
{
    std::random_device rd;
    Foo foo((int)rd());
    std::cout << foo.get_val(); // works
}

Is this nonstandard behavior from MSVC and GCC or does the standard actually allow this?

Answer 1

Of course this is allowed! constexpr don't mean const . You can even mutate values in a constexpr function:

class Foo
{
    int val;

public:
    constexpr Foo(int v) : val(v) {} // OK

    constexpr int get_val() { return val; } // OK
    constexpr void set_val(int v) { val = v; } // OK
};

With this you can write constexpr functions that look like normal function, it's just that they may be executed at compile time in the compiler runtime.

constexpr int test() {
    Foo f{};

    f.set_val(2);

    return f.get_val();
}

static_assert(test() == 2); // Checks at compile time