Should I specialize or overload?

Suppose I have a function template:

template <typename T>
std::string foo(const T& x)
{
    return some_computation_involving(x);
}

If x is already a string, I just want to pass it back verbatim. Should I specialize the function template?

template <>
std::string foo(const std::string& x)
{
    return x;
}

Or should I provide a non-template function?

std::string foo(const std::string& x)
{
    return x;
}

Under what circumstances should I choose which option, and what are the pitfalls I need to be aware o开发者_开发技巧f?

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Prefer overload over function specialization, says Herb Sutter. He explains this in his articles:

• Why Not Specialize Function Templates?
• Template Specialization and Overloading

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Always choose an overload if you can. Function template specialization is a fickle bitch. For example, no implicit conversions are considered, including covariance, when deciding about specializations.

Edit: For example, if you do something like,

template<typename T> void func(T t);
template<> void func(base*);

int main() {
    func(new derived);
}

The specialization will not be called. If it were an overload, it would be called.