Using template parameters as template parameters

Why is the following cod开发者_StackOverflow中文版e invalid?

template <typename S, typename T>
struct B{
    void f(T t, S s) {t.f<S>(s); }
};

gcc 4.3.4 complains that it "expected primary-expression before '>' token", i.e. that "S" wasn't a valid primary-expression.

You need to specify that f is a template:

void f(T t, S s) {
    t.template f<S>(s);
}

C++ doesn’t know this (at this point) since f’s type depends on the type of the template parameter T. Furthermore, the following syntax would be ambiguous: does < mean the start of a template list or a less-than operator? To help C++ figure that out you need to specify that f is a template, otherwise C++ cannot parse the following part because the parse itself depends on the type of T.

You also can rely on type inference to infer the template type instead of explicitly stating it. Then you would have "t.f(s);", which is actually a slightly more generic way to state it: you probably don't care that f is a templated function, you just want it to have some definition for f that accepts an S.