diff. between template<class T,class U> & template<class T>template<class U>

[Artur]

Hi all;

Can u give an example please to represent the difference between

template <class T,class U>
//some function or class 

and

template <class T> 
template <class U>
//some function or class 

Thank you in advance

[guestgulkan]

Example 1;
This is an illegal declaration - you cannot declare the template parameters more than once

template <typename T>
template <typename U>
class MyClass
{
    
};

If you have 2 template parameters for the class, then:

template <typename T, typename U>
class MyClass
{
    
};

Example 2: A template function in a template class

//declaration
template <typename T>
class MyClass
{
    template <typename U>
    U some_func(U param);
    
};
//definition
template <typename T>
template <typename U>
U MyClass<T>::some_func(U param) {return param *2;}

Note when doing the definition, you have to get the template lines the right way around - this would be wrong:

template <typename T>
class MyClass
{
    template <typename U>
    U some_func(U param);
    
};

//Error - template parameters are not in the right order here
template <typename U>
template <typename T>
U MyClass<T>::some_func(U param) {return param *2;}