Why does a template have to be declared before every function that uses one?

Question

Why is the following code valid

template<class T>
T AddThree(T input)
{
    return input + 3;
}

template<class T>    // If I remove this line it won't compile
T SubtractThree(T input)
{
    return input - 3;
}

But if I comment out the line indicated it won't compile? How come the compiler doesn't still know about template<class T> from the first time it is declared (like anything else declared in the body of the file)?

Answer 1

You can think of it as part of the function signature. Maybe it's easier to see the connection if you write the full declaration on one line:

template<class T> T AddThree(T input)
{
    return input + 3;
}

It is like how you need to declare the parameters for each function. You would not expect this to work:

std::string AddThree(std::string input)
{
    return input + "3";
}

std::string SomethingElse(input)
{
    // ...
}

Here, like with the template parameters, you need to declare input in the second function as well as the first. It is the scoping rules of the language :)

Answer 2

How would the compiler know that SubtractThree is a templated function? Clearly you don't want a single template declaration to mean everything afterword is a template. You could also imagine designing the C++ spec such that every unrecognized class (T in this case) is seen as a template, but then you risk making template functions when you misspell classes.