Are there any commonly used C++ compilers that would allow this syntax?

Question

I am new to C++ and trying to get this open source program (developed for/in linux) to compile and run in xcode on OS X.

When I compiler and run the code I get a LOT of errors (more than xcode is willing to count) like this use of undeclared identifier 'x' or this use of undeclared identifier 'y'

Here is a sample of the code throwing the error:

template<typename T>
struct TVector2 {
    T x, y;
    TVector2(T _x = 0.0, T _y = 0.0)
        : x(_x), y(_y)
    {}
    double Length() const {
        return sqrt(static_cast<double>(x*x + y*y));
    }
    double Norm();
    TVector2<T>& operator*=(T f) {
        x *= f;
        y *= f;
        return *this;
    }
    TVector2<T>& operator+=(const TVector2<T>& v) {
        x += v.x;
        y += v.y;
        return *this;
    }
    TVector2<T>& operator-=(const TVector2<T>& v) {
        x -= v.x;
        y -= v.y;
        return *this;
    }
};
struct TVector3 : public TVector2<T> {
    T z;
    TVector3(T _x = 0.0, T _y = 0.0, T _z = 0.0)
    : TVector2<T>(_x, _y), z(_z)
    {}
    double Length() const {
        return sqrt(static_cast<double>(x*x + y*y + z*z)); //use of undeclared identifier x
    }
    double Norm();
    TVector3<T>& operator*=(T f) {
        x *= f;
        y *= f;
        z *= f;
        return *this;
    }

To my eye as an inexperienced C++ programmer, it looks like x and y are simply undeclared local variables. I can get the compiler to get rid of the errors by simply declaring the variables, like this...

struct TVector3 : public TVector2<T> {
    T z;
    T x;
    T y;

However, the sheer number of these errors makes me think that

There might be (reasonably common) versions of C++ compilers that allow you to declare a variable x as _x. That would explain why the source I downloaded has so many compiler errors.
Maybe I got a "bad batch" of the source and I should not waste my time getting it to compile because the source is screwy somehow.

Can an experienced C++ developer explain what might be going on?

@chris exactly - looks like not enough of the original code is being included in the compilation for it to succeed. — Brad Peabody, Oct 03 '13 at 00:49
@chris you appear to have retired from answer posting to leisurely commenting only?? ;) — goji, Oct 03 '13 at 00:51
I'm not overly sure whether that's enough for the OP to solve the problem and commenting is enough procrastination on writing this proposal :p — chris, Oct 03 '13 at 00:54

score 4 · Accepted Answer · answered Oct 03 '13 at 01:01

4

x and y are data members of the base class, TVector2<T>.
Because the base class is a type that is dependent on a template parameter T, it is not searched when looking up unqualified names.
I believe MSVC used to compile this code, not sure whether it still does in C++11 mode. The reason is that MSVC didn't do name resolution properly in templates.
The fix is usually to say this->x instead of x.

answered Oct 03 '13 at 01:01

Steve Jessop

273,490
39
460
699

I've seen some references to `using x;` working for functions, would that also work for variables? – Adam Oct 03 '13 at 01:03
@Adam, `using TVector2::x;` should. – chris Oct 03 '13 at 01:05
@Steve: Visual Studio compiler is not doing two phase lookup *yet*, although I saw in a presentation that it is planned for the future, although most of C++14 features had higher priority than implementing two phase lookup. – David Rodríguez - dribeas Oct 03 '13 at 02:05

Are there any commonly used C++ compilers that would allow this syntax?

1 Answers1