Why must I declare template above each function in a template?

Question

I'm writing a template, and I'm a bit new to doing that. I've noticed that every tutorial I run into declares the template above each function that is implemented. I'm curious if this is for a reason, if there is a way around it, and how it is useful.

Example: Within some file (we will call it template.h):

template <class T> class MyClass
{
public:
    MyClass(T parameter);
    T getParameter() const;
    void setParameter();
    void doSomethingUseless() const;
private:
    T mParameter;
}


template<class T>
MyClass<T>::MyClass(T parameter):mParameter{parameter}
    {}

template<class T>
T MyClass<T>::getParameter() const
    {
        return mParameter;
    }

template<class T>
void MyClass<T>::setParameter(T parameter)
    {
        mParameter = parameter;
    }
template<class T>
void MyClass<T>::doSomethingUseless() const
    {
        for (int i = 0; i < 10000; i++)
    }

• Why must every function be templated if it has nothing to do with the template type? (for example, the doSomethingUseless method)
• Why are both the template<class T> and the MyClass<T> required above each function for this to compile?
• Am I approaching this incorrectly/Is there an easier way to do this?

Anyways, thanks for looking. As a note, I did not compile the example above, I generalized a different template class that I wrote (without a bunch of busy functions in there) If there are mistakes, I apologize.

Answer 1

The member functions of a template class are themselves also templates. Because of this, they need to be defined with any required template parameters (in your case T). If you did not do this, you might write a function like:

void MyClass::memberFunc(T var) {}

What is T in this case? There's no T defined within this scope.

So what do people do? Many code bases will simply define their functions inline with the class. However others insist on separating them, like you have done. The unfortunate reality is that it's simply a requirement of the language.