C++调用Base类的模板函数

Question

以下是两个案例.

案例1)Base-> BaseIndirect-> DerivedIndirect

案例2)Base-> Derived

在案例2)中,我能够使用3种符号调用Base类的模板函数.在案例1)中,我只能使用其中一个符号来调用Base类的模板函数.并且,我无法使用任何符号调用BaseIndirect的模板函数:(.如何解决此问题？谢谢.

struct Base {
  template<bool R> inline void fbase(int k) {};
};

template<class ZZ> struct BaseIndirect : Base {
  template<bool R> inline void fbaseIndirect(int k) {};
};


template<class ZZ>
struct DerivedIndirect : BaseIndirect<ZZ> {
  DerivedIndirect() {
    this->fbase<true>(5);         // gives error, line 13
    fbase<true>(5);               // gives error, line 14
    Base::fbase<true>(5);           // WORKS, line 15
    this->fbaseIndirect<true>(5); // gives error, line 16
    fbaseIndirect<true>(5);       // gives error, line 17
    BaseIndirect<ZZ>::fbaseIndirect<true>(5);   // gives error, line 18
  }
};

template<class ZZ>
struct Derived : Base {
  Derived() {
    this->fbase<true>(5); //  WORKS
    fbase<true>(5);       // WORKS
    Base::fbase<true>(5); // WORKS
  }
};


int main() {
  Derived<int> der;
  DerivedIndirect<int> derIndirect;
};                              

编译错误

test.cpp: In constructor 'DerivedIndirect<ZZ>::DerivedIndirect()':
test.cpp:14: error: 'fbase' was not declared in this scope
test.cpp:17: error: 'fbaseIndirect' was not declared in this scope
test.cpp: In constructor 'DerivedIndirect<ZZ>::DerivedIndirect() [with ZZ = int]':
test.cpp:34:   instantiated from herep 
test.cpp:13: error: invalid operands of types '<unresolved overloaded function type>' and 'bool' to binary 'operator<'
test.cpp:16: error: invalid operands of types '<unresolved overloaded function type>' and 'bool' to binary 'operator<'
test.cpp:18: error: invalid operands of types '<unresolved overloaded function type>' and 'bool' to binary 'operator<'

Answer 1

许多这些调用失败的原因是,您需要使用template关键字最简单的单一解析来解决语法歧义.而不是写作

this->fbase<true>(5);

你需要写

this->template fbase<true>(5);

原因是如果没有template关键字,编译器会将其解析为

(((this->fbase) < true) > 5)

这是荒谬的.template关键字明确消除了这种歧义.将template关键字添加到您提到的其他案例中应该可以解决这些问题.

我实际上不确定为什么这适用于直接基类,所以如果有人能回答问题的这一部分,我很乐意看到答案是什么.