c++ template class; function with arbitrary container type, how to define it?

Traits solution.

Generalize not more than needed, and not less.

In some cases that solution might not be enough as it will match any template with such signature (e.g. shared_ptr), in which case you could make use of type_traits, very much like duck-typing (templates are duck typed in general).

#include <type_traits>

// Helper to determine whether there's a const_iterator for T.
template<typename T>
struct has_const_iterator
{
private:
    template<typename C> static char test(typename C::const_iterator*);
    template<typename C> static int  test(...);
public:
    enum { value = sizeof(test<T>(0)) == sizeof(char) };
};


// bar() is defined for Containers that define const_iterator as well
// as value_type.
template <typename Container>
typename std::enable_if<has_const_iterator<Container>::value,
                        void>::type
bar(const Container &c, typename Container::value_type const & t)
{
  // Note: no extra check needed for value_type, the check comes for
  //       free in the function signature already.
}


template <typename T>
class DoesNotHaveConstIterator {};

#include <vector>
int main () {
    std::vector<float> c;
    bar (c, 1.2f);

    DoesNotHaveConstIterator<float> b;
    bar (b, 1.2f); // correctly fails to compile
}

A good template usually does not artificially restrict the kind of types for which they are valid (why should they?). But imagine in the example above you need to have access to an objects const_iterator, then you can use SFINAE and type_traits to put those constraints on your function.

Or just to as the standard library does

Generalize not more than needed, and not less.

template <typename Iter>
void bar (Iter it, Iter end) {
    for (; it!=end; ++it) { /*...*/ }
}

#include <vector>
int main () {
    std::vector<float> c;
    bar (c.begin(), c.end());
}

For more such examples, look into <algorithm>.

This approach’s strength is its simplicity and is based on concepts like ForwardIterator. It will even work for arrays. If you want to report errors right in the signature, you can combine it with traits.

std containers with signature like std::vector (not recommended)

The simplest solution is approximated by Kerrek SB already, though it is invalid C++. The corrected variant goes like so:

#include <memory> // for std::allocator
template <template <typename, typename> class Container, 
          typename Value,
          typename Allocator=std::allocator<Value> >
void bar(const Container<Value, Allocator> & c, const Value & t)
{
  //
}

However: this will only work for containers that have exactly two template type arguments, so will fail miserably for std::map (thanks Luc Danton).

Any kind of secondary template arguments (not recommended)

The corrected version for any secondary parameter count is as follows:

#include <memory> // for std::allocator<>

template <template <typename, typename...> class Container, 
          typename Value,
          typename... AddParams >
void bar(const Container<Value, AddParams...> & c, const Value & t)
{
  //
}

template <typename T>
class OneParameterVector {};

#include <vector>
int main () {
    OneParameterVector<float> b;
    bar (b, 1.2f);
    std::vector<float> c;
    bar (c, 1.2f);
}

However: this will still fail for non-template containers (thanks Luc Danton).