Letâ€™s have a look at this small function:

template<typename T> T mix(T a, T b, T r) { return r * a + (1 - r) * b; }

This is a quite basic function that just
combine two values with a weighted sum. Its implementation tells us
that r should probably be in
[0,1], but most
importantly, the computation makes sense only if T is a float-like
type. If we want to prevent the compiler or the programmer to call
this function with an incompatible type, we can add some
SFINAE^{15}, for example by introducing a type deduced
from T that would not be defined if T is not a float-like type. In
C++98 the implementation could have been similar to:

// This struct declares a type identical to T if and only if T is // a float-like type. We are going to specialize it only for the // valid types. For the other types, the missing type declaration // in the struct will trigger a substitution failure. template<typename T> struct only_if_float_like; template<> struct only_if_float_like<float> { typedef float type; }; template<> struct only_if_float_like<double> { typedef double type; }; template<typename T> typename only_if_float_like<T>::type mix(T a, T b, T r) { return r * a + (1 - r) * b; }

Aside from the fact that long double is not handled, does it work as expected?

void test() { // float and double are handled as expected. printf("%f\n", mix(1.f, 3.f, 0.5f)); printf("%f\n", mix(1.d, 3.d, 0.5d)); // Using integers triggers an error, we can say that it fails successfully! // printf("%d\n", mix(1, 3, 2)); }

Typing custom type like only_if_float_like for every use case is repetitive, so we would certainly end up splitting it into two types, that could be used like only_if<is_float_like<T>::value, T>::type.

<type_traits>

Lucky us, C++11 greatly simplify this work by introducing the required types, in the form of std::enable_if and std::is_floating_point_type. Using these types the whole implementation is reduced to the following:

#include <type_traits> template<typename T> typename std::enable_if<std::is_floating_point<T>::value, T>::type mix(T a, T b, T r) { return r * a + (1 - r) * b; }

There are many other predicates and operations added in <type_traits> in C++11, that can greatly help for metaprogramming. I can only suggest to have a look at them on a nice online reference.

^{15}Substitution Failure Is Not An Error, is a
principle in template instantiations according to which the
compiler must not emit an error if it fails to instantiate a
template. Instead it should try another template candidate. This
feature is often used and abused to provide multiple implementation
behind the same function signature.