Not being able to instantiate templated class with templated method without arguments - c++

Question

I am trying to develop a class that has to be templated and needs to have some templated methods. I have been looking and, most likely because I do not know how to explain my problem, I have not been able to find the solution. Here is my exmaple:
template<typename T>
class TestClass{
public:
template<typename M>
TestClass(M val): val_(T(val)){}
template<typename N>
N func() {
return N(val_);
}
T get() {
return val_;
}
template<typename N>
N add(N val) {
return N(val_) + val;
}
private:
T val_;
};
This class will be called in templated functions such as this one:
template<typename T>
std::string str(TestClass<T> f)
{
std::ostringstream out;
out << f.func<T>();
out << "\n";
out << "get: ";
out << f.get();
out << "\n";
out << f.add<T>(0.0);
return out.str();
}
An here is an example of usage:
int main(int argc, char** argv){
TestClass<double> t('a');
std::cout<<"Manual output: \n";
std::cout<<"func: "<<t.func<double>()<<std::endl;
std::cout<<"get: "<<t.get()<<std::endl;
std::cout<<"add: "<<t.add<double>(0)<<std::endl;
std::cout<<"operator<< output: \n";
std::cout<<str(t)<<std::endl;
return 0;
}
I have compiled whithout std::string str(TestClass<T> f) function and its usage within the main and I observe the desired behaberou. However, I cannot compile this code with the following error:
error: expected primary-expression before '>' token
out << f.func<T>();
^
expected primary-expression before ')' token
out << f.func<T>();
^
expected primary-expression before '>' token
out << f.add<T>(0.0);
^
The compiler also produces errors regarding the << operator and the fact that f.func<T>() and f.add<T> type has not been resolved. If I remove the templated parts in the calls within str():
template<typename T>
std::string str(TestClass<T> f)
{
std::ostringstream out;
out << f.func();
out << "\n";
out << "get: ";
out << f.get();
out << "\n";
out << f.add(0.0);
return out.str();
}
Then, the compiler error is:
no matching function for call to 'TestClass<double>::func()'
out << f.func();
^
candidate is:template<class N> N TestClass<T>::func() [with N = N; T = double]
N func() {
^
couldn't deduce template parameter 'N'
out << f.func();
^
Which make sense because func() type can not be deduced. Also I have tryied, using f.func<T>() and f.add(0.0) but the error is simillar to the first one.
My question is: How should I do it so the compiler can do its job?

Answer 1

The func template member function must be labeled as a template function when called:
f.template func<T>();
The template keyword is required to indicate the left angle bracket < is NOT a less-than operator. See this explanation of this use of the template keyword.
The add member function picks up its template type from the parameter:
f.add(0.0); // typename N = double