[Error] missing template arguments before 'aSt'

This error message indicates that there are missing template arguments before the variable name "aSt".

In C++, templates are used to create generic functions or classes that can work with different data types. When declaring a template function or class, you need to specify the template parameters inside angle brackets "< >".

For example, if you want to create a template function that takes two arguments of type T and returns their sum, you would write:

template <typename T>
T sum(T a, T b) {
    return a + b;
}

In this case, "typename T" is the template parameter.

If you forget to specify the template parameters when declaring a variable of a templated type, you will get the "missing template arguments" error.

For example, if you have a class template called "MyClass" that takes two template parameters, and you try to declare a variable of type "MyClass" without specifying the template arguments, you will get this error:

template <typename T1, typename T2>
class MyClass {
    // class implementation
};

int main() {
    MyClass aSt; // error: missing template arguments before 'aSt'
    return 0;
}

To fix this error, you need to specify the template arguments when declaring the variable. For example:

int main() {
    MyClass<int, double> aSt; // OK
    return 0;
}

In this case, we are declaring a variable of type "MyClass" with the template arguments "int" and "double".