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Topos Uranos
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Characters, Booleans, conversions and constants

Summary

A type fixes not only how many bytes a datum occupies, but also how its bits are read: the same byte is the letter “A” for a char and the number 65 for an int. This lesson completes the basic types with characters and Booleans, and studies the passage from one type to another: implicit conversions, which the compiler performs without being asked and which can lose information; explicit conversion with static_cast; and brace initialization, which rejects losses. It ends with constants, const and constexpr, which turn an intention (“this value does not change”) into a guarantee that the compiler verifies, and with literals and auto, which decide the type of a value written in the source code.

Learning objectives

  1. Use the numeric code of a character to compute with it, for example to obtain the value of a digit read as a character.
  2. Predict the result of an implicit conversion between integers, reals and Booleans, and write the explicit conversion with static_cast.
  3. Explain why brace initialization rejects the conversions that lose information, and what GCC does in each case.
  4. Declare with const or constexpr every value that must not change, and distinguish one keyword from the other.
  5. Deduce the type of a literal (5, 5u, 5.0, 5.0f, '5', "5") and the type that auto assigns to it.

The type as an interpretation

The lesson Variables and memory presented the variable as a cell with a name, a type and a value, and the lessons Integers and their representation and Floating-point numbers showed that inside the cell there are no numbers but bits, and that the type says how to read them. This lesson carries that idea to its consequence: if the type is the rule for reading, then the same bits mean different things depending on the type with which they are read, and passing a value from one type to another, which in C++ is called a conversion, is changing the rule. Some conversions preserve the value (the integer 7 becomes the real 7.0 without losing anything); others alter it (the real 2.7 becomes the integer 2). From this follows the question that runs through the lesson: in each conversion, is the value preserved, and who checks it?