linux-kernel-module-cheat/userland/cpp/most_vexing_parse.cpp

// https://cirosantilli.com/linux-kernel-module-cheat#cpp

#include "common.hpp"

int main() {

    struct C {
        int i;
        C() : i(1) {}
        C(int i) : i(i) {}
    };

    struct D {
        D() {}
    };

    // Declares *FUNCTION* called `c` that returns `C` inside function main.
    //
    // This is the same as in C, where it is possible to declare a function from inside another function,
    // but not define it.
    //
    // Therefore there would be not way for C++ to distinguish between the two,
    // and still be backwards compatible with C.
    {
        C c();

#if 0
        // ERROR: function definition is not possible inside another function.
        C c() {return C();}
#endif

        //c.i;
    }

    // If you want to call a default constructor, use:
    {
        C c;
        assert(c.i == 1);
    }

    // For non-default constructors, literal arguments disambiguate
    // things as this syntax could not possibly be a function declaration.
    {
        C c(2);
        assert(c.i == 2);
    }

    // But sometimes even arguments are not enough: here D()
    // could matn that the declared `c`
    {
        C c(D());
#if 0
        // error: request for member ‘i’ in ‘c’, which is of non-class type ‘main()::C(main()::D (*)())’
        assert(c.i == 2);
#endif
    }
}