Errors with template function within a class (C++)

Question

I'm having some difficulties with a template function within a class I'm writing. I've boiled it down to a concise set of examples to make it easy to demonstrate (and this proves that I'm missing or screwing up a fundamental concept here).

Basically, if I define this function outside of the class, or if I set it up as a member function, it works perfectly. These two examples work:

As a member function...

// testing.hpp:

class testing {
public:
    template <typename T>
    T getVal(std::string szKeyName, T TDefaultVal) {
        return TDefaultVal;
    }
};

// main.cpp:

#include "testing.hpp"
int main(int argc, const char * argv[]) {
    testing x;
    std::string st = x.getVal("ConfigKey", "ConfigVal");

    std::cout << "Returned value is: " << st << std::endl;

    return 0;
}

Stand-alone...

template <typename T>
T getVal(std::string szKeyName, T TDefaultVal) {
    return TDefaultVal;
}

int main(int argc, const char * argv[]) {
    std::string st = getVal("ConfigKey", "ConfigVal");

    std::cout << "Returned value is: " << st << std::endl;

    return 0;
}

However, if I separate declaration and implementation of the testing class, it does not work:

// testing.hpp

class testing {
public:
    template <typename T>
    T getVal(std::string szKeyName, T TDefaultVal);
};

// testing.cpp

#include "testing.hpp"

template <typename T>
T testing::getVal(std::string szKeyName, T TDefaultVal) {
    return TDefaultVal;
}

// main.cpp

#include "testing.hpp"

int main(int argc, const char * argv[]) {
    testing x;
    std::string st = x.getVal("ConfigKey", "ConfigVal");

    std::cout << "Returned value is: " << st << std::endl;

    return 0;
}

The above fails to compile with:

Undefined symbols for architecture x86_64: "char const* testing::getVal(std::__1::basic_string, std::__1::allocator >, char const*)", referenced from: _main in main.o ld: symbol(s) not found for architecture x86_64 clang: error: linker command failed with exit code 1 (use -v to see invocation)

It's really frustrating because I feel like I really should be able to identify the problem, but after a fair bit of tinkering I can't, so I'm hoping someone can square me away here. It's worth noting that regardless of what type I use -- std::string, int, etc. -- I get the linker error. I've also tried calling the function with

std::string st = x.getVal<std::string>("ConfigKey", "ConfigVal");

but this did not help, either. Any help or clarification would be greatly appreciated.

Thank you.