5.x — Chapter 5 summary and quiz

Chapter Review

A constant is a value that may not be changed during the program’s execution. C++ supports two types of constants: named constants, and literals.

A named constant is a constant value that is associated with an identifier. A Literal constant is a constant value not associated with an identifier.

A variable whose value can not be changed is called a constant variable. The const keyword can be used to make a variable constant. Constant variables must be initialized. Avoid using const when passing by value or returning by value.

A type qualifier is a keyword that is applied to a type that modifies how that type behaves. As of C++23, C++ only supports const and volatile as type qualifiers.

A constant expression is an expression that can be evaluated at compile-time. An expression that is not a constant expression is sometimes called a runtime expression.

A compile-time constant is a constant whose value is known at compile-time. A runtime constant is a constant whose initialization value isn’t known until runtime.

A constexpr variable must be a compile-time constant, and initialized with a constant expression. Function parameters cannot be constexpr.

Literals are values inserted directly into the code. Literals have types, and literal suffixes can be used to change the type of a literal from the default type.

A magic number is a literal (usually a number) that either has an unclear meaning or may need to be changed later. Don’t use magic numbers in your code. Instead, use symbolic constants.

In everyday life, we count using decimal numbers, which have 10 digits. Computers use binary, which only has 2 digits. C++ also supports octal (base 8) and hexadecimal (base 16). These are all examples of numeral systems, which are collections of symbols (digits) used to represent numbers.

A string is a collection of sequential characters that is used to represent text (such as names, words, and sentences). String literals are always placed between double quotes. String literals in C++ are C-style strings, which have a strange type that is hard to work with.

std::string offers an easy and safe way to deal with text strings. std::string lives in the <string> header. std::string is expensive to initialize (or assign to) and copy.

std::string_view provides read-only access to an existing string (a C-style string literal, a std::string, or a char array) without making a copy. A std::string_view that is viewing a string that has been destroyed is sometimes called a dangling view. When a std::string is modified, all views into that std::string are invalidated, meaning those views are now invalid. Using an invalidated view (other than to revalidate it) will produce undefined behavior.

Because C-style string literals exist for the entire program, it is okay to set a std::string_view to a C-style string literal, and even return such a std::string_view from a function.

A substring is a contiguous sequence of characters within an existing string.

Quiz time

Question #1

Why are named constants often a better choice than literal constants?

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Why are const/constexpr variables usually a better choice than #defined symbolic constants?

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Question #2

Find 3 issues in the following code:

#include <cstdint> // for std::uint8_t
#include <iostream>

int main()
{
  std::cout << "How old are you?\n";

  std::uint8_t age{};
  std::cin >> age;

  std::cout << "Allowed to drive a car in Texas: ";

  if (age >= 16)
      std::cout << "Yes";
  else
      std::cout << "No";

  std::cout << '.\n';

  return 0;
}

Sample desired output:

How old are you?
6
Allowed to drive a car in Texas: No

How old are you?
19
Allowed to drive a car in Texas: Yes

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Question #3

What are the primary differences between std::string and std::string_view?

What can go wrong when using a std::string_view?

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Question #4

Write a program that asks for the name and age of two people, then prints which person is older.

Here is the sample output from one run of the program:

Enter the name of person #1: John Bacon
Enter the age of John Bacon: 37
Enter the name of person #2: David Jenkins
Enter the age of David Jenkins: 44
David Jenkins (age 44) is older than John Bacon (age 37).

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#include <iostream>
#include <string>
#include <string_view>

std::string getName(int num)
{
    std::cout << "Enter the name of person #" << num << ": ";
    std::string name{};
    std::getline(std::cin >> std::ws, name); // read a full line of text into name

    return name;
}

int getAge(std::string_view sv)
{
    std::cout << "Enter the age of " << sv << ": ";
    int age{};
    std::cin >> age;

    return age;
}

void printOlder(std::string_view name1, int age1, std::string_view name2, int age2)
{
    if (age1 > age2)
        std::cout << name1 << " (age " << age1 <<") is older than " << name2 << " (age " << age2 <<").\n";
    else
        std::cout << name2 << " (age " << age2 <<") is older than " << name1 << " (age " << age1 <<").\n";
}

int main()
{
    const std::string name1{ getName(1) };
    const int age1 { getAge(name1) };
    
    const std::string name2{ getName(2) };
    const int age2 { getAge(name2) };

    printOlder(name1, age1, name2, age2);

    return 0;
}

Question #5

In the solution to the above quiz, why can’t variable age1 in main be constexpr?

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