C++ struct template

Question

I tried to use templates and structs, but they don't work. I searched a lot, but I can't find the solution.

#include <iostream>

using namespace std;

template<struct S>
int add(S s) {
    return s.num + s.num2;
}

int main() {

    struct {
        int num = 10;
        int num2 = 20;
    } test;

    cout << add(test) << endl;
    return 0;
}

With gcc the errors are:

test.cpp:5:17: error: ‘struct S’ is not a valid type for a template non-type parameter
test.cpp: In function ‘int add(S)’:
test.cpp:6:5: error: ‘s’ has incomplete type
test.cpp:5:17: error: forward declaration of ‘struct S’
test.cpp: In function ‘int main()’:
test.cpp:13:19: warning: non-static data member initializers only available with -std=c++11 or -std=gnu++11 [enabled by default]
test.cpp:14:20: warning: non-static data member initializers only available with -std=c++11 or -std=gnu++11 [enabled by default]
test.cpp:17:21: error: no matching function for call to ‘add(main()::<anonymous struct>&)’
test.cpp:17:21: note: candidate is:
test.cpp:6:5: note: template<<declaration error> > int add(S)
test.cpp:6:5: note:   template argument deduction/substitution failed:
test.cpp:17:21: note:   cannot convert ‘test’ (type ‘main()::<anonymous struct>’) to type ‘S’

Answer 1

You can't use the keyword struct here. Use class or typename (you can still instantiate your template with a struct though).

EDIT: Your other problems are to do with the anonymous struct in main . Can't remember the rules off the top of my head (perhaps someone else can explain them), but this is a pretty weird way of doing things anyway, so I just offer a working version: http://ideone.com/VGIogH

Somewhat relevant: Can we have an anonymous struct as template argument?

EDIT AGAIN: What you originally wrote, with the anonymous struct in main works (after replacing struct in the template) with --std=c++11 , but not without (ie c++03).

Answer 2

In C++ a struct and a class are basically the same thing, except that the default access specifier is public in the former and private in the latter. On the other hand, the syntax for defining a template type argument requires the use of either the class or typename keywords (Don't confuse class here with a class in the OO sense, it can be any type).

template <typename S>          // equivalently class S
int add(S s) ...

Answer 3

Additionally to replacing struct with typename in the template, my compiler (GCC) says you can't initialize your struct members directly, so you can do it like this:

#include <iostream>

using namespace std;

template<typename S>
int add(S s) {
   return s.num + s.num2;

}

int main() {

   struct {
       int num;
       int num2;
   } test;

   test.num = 10;
   test.num2 = 20;

   cout << add(test) << endl;
   return 0;
}

EDIT: You must let your template know what is your struct:

#include <iostream>

using namespace std;

struct MyStruct {
   int num;
   int num2;
};
template<typename S>
int add(S s) {
   return s.num + s.num2;

}

int main() {

   MyStruct test;

   test.num = 10;
   test.num2 = 20;

   cout << add(test) << endl;
   return 0;
}

Answer 4

Your problem is that you once saw someone write:

template<class A>
bool foo( A* a ) { 
  return *a;
}

or something similar, and thought that class meant that A had to be a class .

What is actually happening here is that C++ syntax sometimes sucks. Back when templates where first written up, they reused a keyword ( class ) in the template argument list to mean "a type", as opposed to the other things you can pass to a template (like an int ).

This is a very confusing bit of syntax. As it happens, this is equivalent to the above:

template<typename A>
bool foo( A* a ) {
  return *a;
}

except it makes it much clearer that A can be any type.

However, there is lots of C++ code hanging around from when class was the proper way to specify "a type" in a template parameter, so C++ continues to allow it.

Answer 5

It won't work with anonomous types and you must specify the type in the function call:

#include <iostream>

using namespace std;

template<typename S>
int add(S s) {
    return s.num + s.num2;
}

int main() {

    struct A {
        int num = 10;
        int num2 = 20;
    } test;

    cout << add<A>(test) << endl;
    return 0;
}