CODEWITHSUNDEEP
c++templates
A M
A M

Reputation: 15277

Template parameter "function type" and "function pointer". When to apply what?

I am trying to understand the difference between a "Function Type" and a "Function Pointer" as template parameter and when to apply what. Or, what's the difference?

Simple example:

template <int Function(int)>
struct S1 {
    //....
};
template <int (*Function)(int)>
struct S2 {
    //....
};

int always42(int) {
    return 42;
}

int main() {

    S1<always42> s1;
    S2<always42> s2;
    //....
    return 0;
}

Or will the "function type" decay to a "function pointer"?

Can somebody please shed some light on this?

Upvotes: 3

Views: 68

Answers (2)

jfMR
jfMR

Reputation: 24738

Both template parameters are int (*)(int) as there is type decay for int(int). That is, the type of a non-type template parameter specified as int(int) decays into int(*)(int).

You can see that both template parameters correspond to the same type by using the incomplete type_shower class template below:

template<typename> struct type_shower;

template <int Function(int)>
struct S1 {
   type_shower<decltype(Function)> _;
};

template <int (*Function)(int)>
struct S2 {
   type_shower<decltype(Function)> _;
};

For both, the compiler issues the same error message, which shows the type of Function:

error: implicit instantiation of undefined template 'type_shower<int (*)(int)>' type_shower<decltype(Function)> _;

Note that no type decay occurs if the template parameter is specified as a reference to a function, i.e., int(&)(int):

template <int (&Function)(int)>
struct S3 {
   type_shower<decltype(Function)> _;
};

error: implicit instantiation of undefined template 'type_shower<int (&)(int)>' type_shower<decltype(Function)> _;

Upvotes: 2

songyuanyao
songyuanyao

Reputation: 172924

Yes, function type is adjusted to function pointer for non-type template parameter.

Array and function types may be written in a template declaration, but they are automatically replaced by pointer to object and pointer to function as appropriate.

[temp.param]/10

A non-type template-parameter of type “array of T” or of function type T is adjusted to be of type “pointer to T”.

Upvotes: 2

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