问题
Possible Duplicate:
Why doesn't a derived template class have access to a base template class' identifiers?
Translating of the following program
A.h
#ifndef A_H
#define A_H
template <class T>
class A
{
protected :
T a;
public:
A(): a(0) {}
};
#endif
B.h
#ifndef B_H
#define B_H
template <class T>
class A;
template <class T>
class B: public A <T>
{
protected:
T b;
public:
B() : A<T>(), b(0) {}
void test () { b = 2 * a;} //a was not declared in this scope
};
#endif
causes an error: "a was not declared in this scope". (Netbeans 6.9.1).
But the construction
void test () { b = 2 * this->a;}
is correct... Where is the problem?
Is it better to use forward declaration or file include directive?
B.h
template <class T>
class A;
vs.
#include "A.h"
回答1:
A<T>::a is a dependent name, so you can't use it unqualified.
Imagine that there was a specialization of A<int> somewhere:
template<> class A<int> { /* no a defined */ };
What should the compiler do now? Or what if A<int>::a was a function instead of a variable?
Qualify your access to a, as you've already discovered this->a, and things will work right.
来源：https://stackoverflow.com/questions/4540106/c-template-compiler-error