Accessing public members of base class fails

Question

might be a bit of a coward-ish question: I've got two classes, and declared all variables public. Why can't I access the variables from derived class??

g++ tells me: vec3d.h:76:3: error: ‘val’ was not declared in this scope

template<typename TYPE>
class vec{
public:
        TYPE *val;
        int dimension;
public:
        vec();
        vec( TYPE right );
        vec( TYPE right, int _dimension );

[etc]


template<typename TYPE>
class vec3d : public vec<TYPE>{
public:
        vec3d() : vec<TYPE>( 0, 3 ){};
        vec3d( TYPE right ) : vec<TYPE>( right, 3 ){};
        vec3d( TYPE X_val, TYPE Y_val, TYPE Z_val ) : vec<TYPE>( 0, 3 ){
                val[0] = X_val; //// <----------THIS ONE FAILS!
                val[1] = Y_val;
                val[2] = Z_val;
        };
[etc]

can you remove all the comments in that code, it makes it really hard to read... — Tony The Lion, Sep 02 '11 at 08:45
possible duplicate of [Access protected member of a class in a derived class](http://stackoverflow.com/questions/1624564/access-protected-member-of-a-class-in-a-derived-class) — CB Bailey, Sep 02 '11 at 08:49
For future reference this question would have been easier to read if you'd reduced it to a 10 line example, without the full blown vector and '[etc]'. See [Short, Self Contained, Correct (Compilable), Example](http://sscce.org/). — Flexo, Sep 02 '11 at 08:50
yeah now it's a bit more clear...i thought it got something to do with the public declaration...all tutorials do protected... as in the duplicate. Since this is not the issue I could've used the other thread - sorry guys ;) — 0815ZED, Sep 02 '11 at 08:52

score 5 · Accepted Answer · answered Sep 02 '11 at 08:46

5

This is purely a lookup issue and nothing to do with access control.

Because vec3d is a template and its base class depends on the template parameter, the members of the base class are not automatically visible in the derived class in expression that are non-dependent. The simplest fix is to use a dependent expression such as this->X_val to access members of the base class.

answered Sep 02 '11 at 08:46

CB Bailey

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Ok that answers my other question ^^ Thanks – 0815ZED Sep 02 '11 at 08:47
Oh...So this concept would work if the class is not-templated?? – 0815ZED Sep 02 '11 at 08:48
1

It would be fine if either the derived class was not a class template or if the base class was not dependent on any template parameter of the derived class template. – CB Bailey Sep 02 '11 at 08:50

score 3 · Answer 2 · edited May 23 '17 at 12:19

3

You will need to refer to them via this->val or vec<TYPE>::val. There's a good explanation in this answer to a similar question.

edited May 23 '17 at 12:19

Community

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1

answered Sep 02 '11 at 08:44

Flexo

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Thanks...so they're not actual members of the new class although defined public?? – 0815ZED Sep 02 '11 at 08:45
They are still members, it's a name lookup issue. They need to be looked up later on than they would if they were left unqualified. – Flexo Sep 02 '11 at 08:48

Accessing public members of base class fails

2 Answers2

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