Default values in templates with template arguments ( C++ )

Question

Assume I have a template (called ExampleTemplate) that takes two arguments: a container type (e.g. list, vector) and a contained type (e.g. float, bool, etc). Since containe

Answer 1

Perhaps you'd prefer this:

#include 
#include 

using namespace std;

template 
class ForExamplePurposes {
    typedef typename Container::value_type T;
    Container items;
public:
};

int main()
{
    ForExamplePurposes< list > a;
    ForExamplePurposes< vector > b;
}

This uses "static duck typing". It is also a bit more flexible as it doesn't force the Container type to support STL's Allocator concept.

---

Perhaps using the type traits idiom can give you a way out:

#include 
#include 

using namespace std;

struct MyFunkyContainer
{
    typedef int funky_type;
    // ... rest of custom container declaration
};

// General case assumes STL-compatible container
template 
struct ValueTypeOf
{
    typedef typename Container::value_type type;
};

// Specialization for MyFunkyContainer
template <>
struct ValueTypeOf
{
    typedef MyFunkyContainer::funky_type type;
};


template 
class ForExamplePurposes {
    typedef typename ValueTypeOf::type T;
    Container items;
public:
};

int main()
{
    ForExamplePurposes< list > a;
    ForExamplePurposes< vector > b;
    ForExamplePurposes< MyFunkyContainer > c;
}

Someone who wants to use ForExamplePurposes with a non-STL-compliant container would need to specialize the ValueTypeOf traits class.