Where to put a member function template

Question

An aspect of C++ that periodically frustrates me is deciding where templates fit between header files (traditionally describing the interface) and implemention (.cpp) files. Templates often need to go in the header, exposing the implementation and sometimes pulling in extra headers which previously only needed to be included in the .cpp file. I encountered this problem yet again recently, and a simplified example of it is shown below.

#include <iostream> // for ~Counter() and countAndPrint()

class Counter
{
  unsigned int count_;
public:
  Counter() : count_(0) {}
  virtual ~Counter();

  template<class T>
  void
  countAndPrint(const T&a);
};

Counter::~Counter() {
    std::cout << "total count=" << count_ << "\n";
}

template<class T>
void
Counter::countAndPrint(const T&a) {
  ++count_;
  std::cout << "counted: "<< a << "\n";
}

// Simple example class to use with Counter::countAndPrint
class IntPair {
  int a_;
  int b_;
public:
  IntPair(int a, int b) : a_(a), b_(b) {}
  friend std::ostream &
  operator<<(std::ostream &o, const IntPair &ip) {
    return o << "(" << ip.a_ << "," << ip.b_ << ")";
  }
};

int main() {
  Counter ex;
  int i = 5;
  ex.countAndPrint(i);
  double d=3.2;
  ex.countAndPrint(d);
  IntPair ip(2,4);
  ex.countAndPrint(ip);
}

Note that I intend to use my actual class as a base class, hence the virtual destructor; I doubt it matters, but I've left it in Counter just in case. The resulting output from the above is

counted: 5
counted: 3.2
counted: (2,4)
total count=3

Now Counter's class declaration could all go in a header file (e.g., counter.h). I can put the implementation of the dtor, which requires iostream, into counter.cpp. But what to do for the member function template countAndPrint(), which also uses iostream? It's no use in counter.cpp since it needs to be instantiated outside of the compiled counter.o. But putting it in counter.h means that anything including counter.h also in turn includes iostream, which just seems wrong (and I accept that I may just have to get over this aversion). I could also put the template code into a separate file (counter.t?), but that would be a bit surprising to other users of the code. Lakos doesn't really go into this as much as I'd like, and the C++ FAQ doesn't go into best practice. So what I'm after is:

1. are there any alternatives for dividing the code to those I've suggested?
2. in practice, what works best?

Answer 1

A rule of thumb (the reason of which should be clear).

• Private member templates should be defined in the .cpp file (unless they need to be callable by friends of your class template).
• Non-private member templates should be defined in headers, unless they are explicitly instantiated.

You can often avoid having to include lots of headers by making names be dependent, thus delaying lookup and/or determination of their meaning. This way, you need the complete set of headers only at the point of instantiation. As an example

#include <iosfwd> // suffices

class Counter
{
  unsigned int count_;
public:
  Counter() : count_(0) {}
  virtual ~Counter();

  // in the .cpp file, this returns std::cout
  std::ostream &getcout();

  // makes a type artificially dependent
  template<typename T, typename> struct ignore { typedef T type; };

  template<class T>
  void countAndPrint(const T&a) {
    typename ignore<std::ostream, T>::type &cout = getcout();
    cout << count_;
  }
};

This is what I used for implementing a visitor pattern that uses CRTP. It looked like this initially

template<typename Derived>
struct Visitor {
  Derived *getd() { return static_cast<Derived*>(this); }
  void visit(Stmt *s) {
    switch(s->getKind()) {
      case IfStmtKind: {
        getd()->visitStmt(static_cast<IfStmt*>(s));
        break;
      }
      case WhileStmtKind: {
        getd()->visitStmt(static_cast<WhileStmt*>(s));
        break;
      }
      // ...
    }
  }
};

This will need the headers of all statement classes because of those static casts. So I have made the types be dependent, and then I only need forward declarations

template<typename T, typename> struct ignore { typedef T type; };

template<typename Derived>
struct Visitor {
  Derived *getd() { return static_cast<Derived*>(this); }
  void visit(Stmt *s) {
    typename ignore<Stmt, Derived>::type *sd = s;
    switch(s->getKind()) {
      case IfStmtKind: {
        getd()->visitStmt(static_cast<IfStmt*>(sd));
        break;
      }
      case WhileStmtKind: {
        getd()->visitStmt(static_cast<WhileStmt*>(sd));
        break;
      }
      // ...
    }
  }
};

Answer 2

The Google Style Guide suggests putting the template code in a "counter-inl.h" file. If you want to be very careful about your includes, that might be the best way.

However, clients getting an included iostream header by "accident" is probably a small price to pay for having all your class's code in a single logical place—at least if you only have a single member function template.

Answer 3

Practically your only options are to place all template code in a header, or to place template code in a .tcc file and include that file at the end of your header.

Also, if possible you should try to avoid #includeing <iostream> in headers, because this has a significant toll on compile-time. Headers are often #included by multiple implementation files, after all. The only code you need in your header is template and inline code. The destructor doesn't need to be in the header.