Accessing non-const data members from constexpr member function-CodePudding

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?

CodePudding user response：

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