Function specialized with template

问题

I need to make a specialization of my function with template class and have problem with "illegal use of explicit template arguments".

template <typename T>
class MyClass { /* ... */ }; // it can be any template class, eg std::vector

template <typename T>
void foo() { /* ... */ } // my template function which need a specialization

template<> 
void foo<int>() /* sth special for integers - it works */ }

template<template T> 
void foo<MyClass<T> >() /* sth special for template class with any parameter - it doesnt work :( */ }

Of course i can type a few specialization for all MyClass'es which i need to, but maybe it can be replaced with one?

回答1:

Template specialization of function is not as flexible as specialization of struct: only full specialization is allowed. If you want to do partial specialization you need to wrap your foo function inside a struct:

template <typename T> class MyClass { };

template <typename T> struct Foo;

template <typename T> struct Foo { void foo() {}};

template<> struct Foo<int> { void foo() { } };

template<typename T> struct Foo< MyClass<T> > { void foo() {} };

And then instead of calling

foo<MyClass<...>>()

you call

Foo< MyClass<...> >::foo()

回答2:

You cannot partially speciallise a template function. There are discussions about removing that restriction though.

The advocated workarounds are:

Use a class template from the template function.
Wrap your function in a template class.

template <typename T>
struct foo_impl {
};
template <typename T>
void foo() {
    foo_impl<T>();
}

// And now specialize foo_impl as you want:

template<>
struct foo_impl<int> {
    foo_impl(){/* sth special for integers - it works */}
};
template<typename T>
struct foo_impl<myclass<T>> {
   foo_impl() {/* ... */}
};

If you wanted a return-value, you should use a member-function - probably operator() - instead of the ctor.

回答3:

This is a lot of extra typing, but how about:

template <typename T>
class MyClass { /* ... */ }; // it can be any template class, eg std::vector

template<typename T>
struct FooWrapper
{
    static void foo()
    {
       // default implementation
    }
};

template<typename T>
struct FooWrapper<MyClass<T>>
{
    static void foo()
    {
        // MyClass<T> implementation
    }
};

template<typename T>
void foo()
{
    FooWrapper<T>::foo();
}

回答4:

A possible solution could be using a base class

template<typename T> class MyClass;

class base {
private:
    template<typename T> friend class MyClass;
    base(); // Can't build a base object directly
};

template <typename T>
class MyClass : public base { 
public:
}; // it can be any template class, eg std::vector

template <typename T>
void foo() {
} // my template function which need a specialization

template<> 
void foo<int>() { /* sth special for integers - it works */ }

template<>
void foo<base>() { /* sth special for template class with any parameter - it doesnt work :( */ }

The above might also work in case you want a template parameter to your function. If you can wrap your function up I'd go with hivert's solution.

来源：https://stackoverflow.com/questions/23550281/function-specialized-with-template

标签

c++

templates

template-specialization