Using string_view instead of constant string references

When working with strings, temporary objects are created all the time, even if you might not be really aware of it. Many times, these temporary objects are irrelevant and only serve the purpose of copying data from one place to another (for example, from a function to its caller). This represents a performance issue because they require memory allocation and data copying, which should be avoided. For this purpose, the C++17 standard provides a new string class template called std::basic_string_view that represents a non-owning constant reference to a string (that is, a sequence of characters). In this recipe, you will learn when and how you should use this class.

Getting ready

The string_view class is available in the namespace std in the string_view header.

How to do it...

You should use std::string_view to pass a parameter to a function (or return a value from a function), instead of std::string const &, unless your code...

Formatting and printing text with std::format and std::print

The C++ language has two ways of formatting text: the printf family of functions and the I/O streams library. The printf functions are inherited from C and provide a separation of the formatting text and the arguments. The streams library provides safety and extensibility and is usually recommended over printf functions, but is, in general, slower. The C++20 standard proposes a new formatting library alternative for output formatting, which is similar in form to printf but safe and extensible and is intended to complement the existing streams library. In this recipe, we will learn how to use the new functionalities instead of the printf functions or the streams library.

Getting ready

The new formatting library is available in the header <format>. You must include this header for the following samples to work.

How to do it...

The std::format() function formats its arguments according to the provided formatting string...

Using std::format with user-defined types

The C++20 formatting library is a modern alternative to using printf-like functions or the I/O streams library, which it actually complements. Although the standard provides default formatting for basic types, such as integral and floating-point types, bool, character types, strings, and chrono types, the user can create custom specialization for user-defined types. In this recipe, we will learn how to do that.

Getting ready

You should read the previous recipe, Formatting text with std::format, to familiarize yourself with the formatting library.In the examples that we'll be showing here, we will use the following class:

struct employee
{
   int         id;
   std::string firstName;
   std::string lastName;
};

In the next section, we'll introduce the necessary steps to implement to enable text formatting using std::format() for user-defined types.

How to do it...

To enable formatting using the new formatting library for user-defined...

Using generic and template lambdas

In the preceding recipe, we saw how to write lambda expressions and use them with standard algorithms. In C++, lambdas are basically syntactic sugar for unnamed function objects, which are classes that implement the call operator. However, just like any other function, this can be implemented generically with templates. C++14 takes advantage of this and introduces generic lambdas that do not need to specify actual types for their parameters and use the auto specifier instead. Though not referred to by this name, generic lambdas are basically lambda templates. They are useful in cases where we want to use the same lambda but with different types of parameters. Moreover, the C++20 standard takes this a step further and supports explicitly defining template lambdas. This helps with some scenarios where generic lambdas are cumbersome.

Getting started

It is recommended that you read the preceding recipe, Using lambdas with standard algorithms...

Writing a recursive lambda

Lambdas are basically unnamed function objects, which means that it should be possible to call them recursively. Indeed, they can be called recursively; however, the mechanism for doing so is not obvious as it requires assigning the lambda to a function wrapper and capturing the wrapper by reference. Though it can be argued that a recursive lambda does not really make sense and that a function is probably a better design choice, in this recipe, we will look at how to write a recursive lambda.

Getting ready

To demonstrate how to write a recursive lambda, we will consider the well-known example of the Fibonacci function. This is usually implemented recursively in C++, as follows:

constexpr int fib(int const n)
{
  return n <= 2 ? 1 : fib(n - 1) + fib(n - 2);
}

Having this implementation as a starting point, let’s see how we can rewrite it using a recursive lambda.

How to do it...

In C++11, in order to write a recursive lambda...

Writing function templates

Generic code is key to avoid writing repetitive code. In C++, this is achieved with the help of templates. Classes, functions, and variables can be templated. Although templates are often seen as complex and cumbersome, they enable the creation of general-purpose libraries, such as the standard library, and help us write less and better code.

Templates are first-class citizens of the C++ language and could take an entire book to cover in detail. In fact, multiple recipes in this book deal with various aspects of templates. In this recipe, we will discuss the basics of writing function templates.

How to do it…

Do the following to create function templates:

• To create a function template, precede the function declaration with the template keyword followed by the list of template parameters in angle brackets:

  template <typename T>
  T minimum(T a, T b)
  {
     return a <= b ? a : b;
  }
  minimum(3, 4);
  minimum(3.99, 4...

Writing a function template with a variable number of arguments

It is sometimes useful to write functions with a variable number of arguments or classes with a variable number of members. Typical examples include functions such as printf, which takes a format and a variable number of arguments, or classes such as tuple. Before C++11, the former was possible only with the use of variadic macros (which enable writing only type-unsafe functions) and the latter was not possible at all. C++11 introduced variadic templates, which are templates with a variable number of arguments that make it possible to write both type-safe function templates with a variable number of arguments, and also class templates with a variable number of members. In this recipe, we will look at writing function templates.

Getting ready

Functions with a variable number of arguments are called variadic functions. Function templates with a variable number of arguments are called variadic function templates...

Using fold expressions to simplify variadic function templates

In this chapter, we have already discussed folding several times; this is an operation that applies a binary function to a range of values to produce a single value. We saw this when we discussed variadic function templates and we will see it again with higher-order functions. It turns out there is a significant number of cases where the expansion of a parameter pack in variadic function templates is basically a folding operation. To simplify writing such variadic function templates, C++17 introduced fold expressions, which fold an expansion of a parameter pack over a binary operator. In this recipe, we will learn how to use fold expressions to simplify writing variadic function templates.

Getting ready

The examples in this recipe are based on the variadic function template add (), which we wrote in the previous recipe, Writing a function template with a variable number of arguments. That implementation is a left...

Implementing the higher-order functions map and fold

Throughout the preceding recipes in this book, we have used the general-purpose algorithms std::transform() and std::accumulate() in several examples, such as for implementing string utilities to create uppercase or lowercase copies of a string or for summing the values of a range.

These are basically implementations of the higher-order functions, map and fold. A higher-order function is a function that takes one or more other functions as arguments and applies them to a range (a list, vector, map, tree, and so on), thus producing either a new range or a value. In this recipe, we will learn how to implement the map and fold functions so that they work with C++ standard containers.

Getting ready

map is a higher-order function that applies a function to the elements of a range and returns a new range in the same order.

fold is a higher-order function that applies a combining function to the elements of the range to...

Composing functions into a higher-order function

In the previous recipe, we implemented two higher-order functions, map and fold, and saw various examples of using them. At the end of the recipe, we saw how they can be pipelined to produce a final value after several transformations of the original data. Pipelining is a form of composition, which means creating one new function from two or more given functions. In the mentioned example, we didn’t actually compose functions; we only called a function with the result produced by another, but in this recipe, we will learn how to actually compose functions together into a new function. For simplicity, we will only consider unary functions (functions that take only one argument).

Getting ready

Before you go forward, it is recommended that you read the previous recipe, Implementing the higher-order functions map and fold. It is not mandatory for understanding this recipe, but we will refer to the map and fold functions we...

Uniformly invoking anything callable

Developers, especially those who implement libraries, sometimes need to invoke a callable object in a uniform manner. This can be a function, a pointer to a function, a pointer to a member function, or a function object. Examples of such cases include std::bind, std::function, std::mem_fn, and std::thread::thread. C++17 defines a standard function called std::invoke() that can invoke any callable object with the provided arguments. This is not intended to replace direct calls to functions or function objects, but it is useful in template metaprogramming for implementing various library functions.

Getting ready

For this recipe, you should be familiar with how to define and use function pointers.

To exemplify how std::invoke() can be used in different contexts, we will use the following function and class:

int add(int const a, int const b)
{
  return a + b;
}
struct foo
{
  int x = 0;
  void increment_by(int const n) { x += n; }
...