c++ - g++ 中 ">"之前的预期主表达式，但在微软编译器中没有

Question

此代码无法在 g++ (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3 上编译，出现此错误

test.cpp: In function ‘T mul(V&, V&)’:
test.cpp:38:27: error: expected primary-expression before ‘>’ token
test.cpp:38:29: error: expected primary-expression before ‘)’ token
test.cpp:38:53: error: expected primary-expression before ‘>’ token
test.cpp:38:55: error: expected primary-expression before ‘)’ token

但它可以在 Microsoft C/C++ Optimizing Compiler Version 15.00.21022.08 for x64 上正确编译和执行

#include <iostream>
#include <complex>

template <class T>
class SM
{
public:
    T value;
};

template <class T>
class SC : public SM<T>
{
};

class PSSM {

public:
    template <class T>
    T & getSC() { return sc; }

private:
    SC<double> sc;
};

class USSM {

public:
    template <class T>
    T & getSC() { return sc; }

private:
    SC<std::complex<double> > sc;
};

template <class T, class V>
T mul( V & G, V & S) {
    return (G.getSC<SC<T> >().value * S.getSC<SC<T> >().value); // error is here
}


int main() {
    PSSM p;
    PSSM q;
    p.getSC<SC<double> >().value = 5; 
    q.getSC<SC<double> >().value = 3; 

    std::cout << mul<double>(p,q);

}

我不明白问题出在哪里。任何人都可以理解如何解决它，或者用 g++ 解释问题的本质吗？

Answer 1

问题在于语法。您应该使用 template在这种情况下消除歧义，以便正确解析您对成员函数模板的调用:

return (G.template getSC<SC<T> >().value * S.template getSC<SC<T> >().value);
//        ^^^^^^^^^                          ^^^^^^^^^

此消歧器有助于编译器识别 G. 之后的内容是成员模板特化，而不是，例如，称为 getSC 的数据成员后跟 < (小于)。

template 的标准引用disambiguator 是 C++11 标准的第 14.2/4 段:

When the name of a member template specialization appears after . or -> in a postfix-expression or after a nested-name-specifier in a qualified-id, and the object expression of the postfix-expression is type-dependent or the nested-name-specifier in the qualified-id refers to a dependent type, but the name is not a member of the current instantiation (14.6.2.1), the member template name must be prefixed by the keyword template. Otherwise the name is assumed to name a non-template.[ Example:

struct X {
template<std::size_t> X* alloc();
template<std::size_t> static X* adjust();
};
template<class T> void f(T* p) {
T* p1 = p->alloc<200>(); // ill-formed: < means less than
T* p2 = p->template alloc<200>(); // OK: < starts template argument list
T::adjust<100>(); // ill-formed: < means less than
T::template adjust<100>(); // OK: < starts template argument list
}

—end example ]