Unlocking the Power of Templates in C++
Class Templates: A World of Possibilities
When working with class templates, we have the flexibility to explicitly specify the types that the template should generate code for. For instance, we can create a Rectangle object with float dimensions like this: auto r1 = Rectangle<float>{2.0f, 2.0f, 4.0f, 4.0f};. However, we can also leverage class template argument deduction (CTAD) to let the compiler deduce the argument type for us. This means we can simply write auto r2 = Rectangle{-2, -2, 4, 4}; and the compiler will instantiate a Rectangle<int>.
Function Templates: Accepting Arbitrary Types
Function templates take this flexibility a step further by allowing us to accept Rectangle objects with dimensions defined using an arbitrary type T. We can write a function template like this: template <typename T> auto is_square(const Rectangle<T>& r) { return r.width() == r.height(); }. This function can work with Rectangle objects of any type, making it incredibly versatile.
Numeric Template Parameters: A New Dimension
Beyond general types, templates can also be defined using numeric parameters. This allows us to create functions that can work with specific integer values. For example, we can define a function template like this: template <int N, typename T> auto const_pow_n(const T& v) {... }. This function will generate a new implementation for every unique integer value passed as a template argument. We can then use this function to square or cube a value, like this: auto x2 = const_pow_n<2>(4.0f); and auto x3 = const_pow_n<3>(4.0f);.
Compile-Time Programming: The Power of Specialization
One of the most powerful features of templates is the ability to provide custom implementations for specific values of the template parameters. This is known as template specialization. We can write a specialized implementation for our const_pow_n function when it’s used with integers and the value of N is 2, like this: template<> auto const_pow_n<2, int>(const int& v) { return v * v; }. This allows us to optimize our code for specific use cases.
How the Compiler Handles Templates
So, how does the compiler handle all these template functions and specializations? When the compiler encounters a template function, it generates a regular function with the template parameters expanded. This means that each time we use a template function with different parameters, the compiler generates a new function implementation. For example, the following code will generate six distinct functions: auto a = pow_n(42, 3);, auto b = pow_n(42.f, 2);, auto c = pow_n(17.f, 5);, auto d = const_pow_n<2>(42.f);, auto e = const_pow_n<2>(99.f);, and auto f = const_pow_n<3>(42.f);.