Question

// Compiled by Visual Studio 2012

struct A
{
    bool operator ==(const A& other) const
    {
        for (decltype(this->n) i = 0; i < n; ++i) // OK
        {}

        return true;
    }

protected:
    size_t n;
};

struct B : public A
{
    bool operator ==(const B& other) const
    {
        for (decltype(this->n) i = 0; i < n; ++i) // error C2105: '++' needs l-value
        {}

        return true;
    }
};

Is this a bug of VC++ 2012?

Answer 1

This appears to be a VS2012 compiler bug. The spec is quite clear on this, in section 7.1.6.2, paragraph 4. Indeed, one of the examples given shows an expression that references through a const-pointer a. decltype(a->x) yields double, while decltype((a->x)) yields double const &.

So it’s a bug; the compiler thinks that i is const, and therefore can’t ++ it.