Why can I not call templated method of templated class from a templated function [duplicate]

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Possible Duplicate:

Confusing Template error

开发者_高级运维

I have a templated class with a templated method. Now I have another function, with 2 template arguments creating the class with the first template argument and calling the function with the second. Consider this example:

template<class S>
class A {
public:
    template<class T>
            T f1() {
                    return (T)0.0;
            }

};

template<class T,class CT>
void function() {
    A<T> a;
    a.f1<CT>(); // gcc error in this line
}

gcc gived me:

error: expected primary-expression before ‘>’ toke

in the line marked above. Why does this not work and how can I fix it? Thanks! Nathan

---

A<T> is a dependent type (i.e. it depends on the template parameter T), so you have to specify that you're referring to a template member:

a.template f1<CT>();

---

You can, but you need to help the compiler.

Because a has a dependent type, the compiler cannot assume what a.f1 refers to. It could be a type, a constant, a function... whatever.

Because the template syntax uses < it is easily confused with the operator<.

Therefore the Standard requires that you disambiguates the nature of elements within a dependent type. This applies both to:

• types, with the use of typename, such as typedef typename A<T>::SomeType type;
• functions, with the use of template, such as a.template f1<CT>();

It can be argued that since it is known that CT is a type, such a disambiguation is meaningless, but things get hairy when it involves functions being passed as type parameters.

For example a.f1<CT()>(); can be interpreted either as:

• Compare a.f1 and a default constructed CT using operator<
• Call a.f1 with a CT() as template parameter

The C++ syntax is (for once) uniform and requires disambiguation in all cases.

A quality compiler will suggest to you the appropriate fix when it can makes sense of your construct.