05_Templates.md

5. Templates

5.1 Introduction

• Vector doesn't care what types of elements are stored in it
• It should be parameterized by a type, so that anything can be stored in it
• A template is a class or a function parameterized with a set of types or values

5.2 Parameterized Types

• Prefix class with template<typename T> to parameterize it with an arbitrary type T

• It is C++'s version of the mathematical [...] "for all types T"

• We can then parameterize the class with the type: Vector<int>
• To implement a range-for loop, begin and end need to be implemented: for(auto& x : vec) { … }. The iterator keeps yielding the return value of begin until its value equals the return value of end
• Templates are a compile-time mechanism, so there is no run-time overhead
• Templates can take value arguments: template<typename T, int N>
• Template value arguments must be constant expressions

5.3 Function Templates

• Functions can be parameterized in the same way
• std::accumulate can be used for summing up elements of containers

5.4 Concepts and Generic Programming

• The most common use case for templates is generic programming as they provide parametric polymorphism
• Concept: First template argument is a container, the second is some kind of number

5.5 Function Objects

• Function objects (sometimes called functors) are templates that define objects that can be called as functions
• operator() can be used to overwrite the function application operator ()
• Useful for passing around functions that hold state
• The compiler can still inline functions
• [&](int a) { return a < x } is a lambda expression
• Capture nothing: [], capture everything: [&], capture only x: [&x], capture copy of x: [=x]

5.6 Variadic Templates

• Templates and functions can be defined to take an arbitrary number of argument sof arbitrary types. In that case, they are called variadic
• template <typename T, typename... Tail>
• void f(T head, Tail... tail) { … }
• tail... can then be used to refer to the remaining arguments

5.7 Aliases

• Alias: using a = b;
• E.g. using size_t = unsigned int;