预期在g ++中“>”之前的primary-expression，但不在microsoft编译器中

Question

This code fails to compile on g++ (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3, with this error

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

but it compiles and executes correctly on Microsoft C/C++ Optimizing Compiler Version 15.00.21022.08 for x64

#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);

}

I don't understand where the problem is. Can anyone understand how to work around it, or explain the nature of the problem in g++?

Answer 1

The problem is syntactic. You should use the template disambiguator in this case, so that your invocation of a member function template will be correctly parsed:

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

This disambiguator helps the compiler recognizing that what follows G. is a member template specialization and not, for instance, a data member called getSC followed by a < (less than).

The Standard reference for the template disambiguator is Paragraph 14.2/4 of the C++11 Standard:

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 ]