4.14 — Literal constants

In programming, a constant is a fixed value that may not be changed. C++ has two kinds of constants: literal constants, and symbolic constants. We’ll cover literal constants in this lesson, and symbolic constants in the next lesson (4.15 -- Symbolic constants: const and constexpr variables).

Literal constants (usually just called literals) are unnamed values inserted directly into the code. For example:

return 5; // 5 is an integer literal
bool myNameIsAlex { true }; // true is a boolean literal
std::cout << 3.4; // 3.4 is a double literal

They are constants because their values can not be changed dynamically (you have to change them, and then recompile for the change to take effect).

Just like objects have a type, all literals have a type. The type of a literal is assumed from the value and format of the literal itself.

By default:

Literal suffixes

If the default type of a literal is not as desired, you can change the type of a literal by adding a suffix:

You generally won’t need to use suffixes for integer types, but here are examples:

std::cout << 5; // 5 (no suffix) is type int (by default)
std::cout << 5u; // 5u is type unsigned int
std::cout << 5L; // 5L is type long

By default, floating point literal constants have a type of double. To make them float literals instead, the f (or F) suffix should be used:

std::cout << 5.0; // 5.0 (no suffix) is type double (by default)
std::cout << 5.0f; // 5.0f is type float

New programmers are often confused about why the following causes a compile warning:

float f { 4.1 }; // warning: 4.1 is a double literal, not a float literal

Because 4.1 has no suffix, it’s treated as a double literal, not a float literal. When C++ defines the type of a literal, it does not care what you’re doing with the literal (e.g. in this case, using it to initialize a float variable). Therefore, the 4.1 must be converted from a double to a float before it can be assigned to variable f, and this could result in a loss of precision.

Literals are fine to use in C++ code so long as their meanings are clear. This is most often the case when used to initialize or assign a value to a variable, do math, or print some text to the screen.

String literals

In lesson 4.11 -- Chars, we defined a string as a collection of sequential characters. C++ supports string literals:

std::cout << "Hello, world!"; // "Hello, world!" is a C-style string literal
std::cout << "Hello," " world!"; // C++ will concatenate sequential string literals

String literals are handled strangely in C++ for historical reasons. For now, it’s fine to use string literals to print text with std::cout, or as initializers for std::string.

#include <iostream>
#include <string>      // for std::string
#include <string_view> // for std::string_view

int main()
{
    using namespace std::literals; // easiest way to access the s and sv suffixes

    std::cout << "foo\n";   // no suffix is a C-style string literal
    std::cout << "goo\n"s;  // s suffix is a std::string literal
    std::cout << "moo\n"sv; // sv suffix is a std::string_view literal

    return 0;
};

We’ll talk more about string literals in future lessons.

Scientific notation for floating point literals

There are two different ways to declare floating-point literals:

double pi { 3.14159 }; // 3.14159 is a double literal in standard notation
double avogadro { 6.02e23 }; // 6.02 x 10^23 is a double literal in scientific notation

In the second form, the number after the exponent can be negative:

double electron { 1.6e-19 }; // charge on an electron is 1.6 x 10^-19

Octal and hexadecimal literals

In everyday life, we count using decimal numbers, where each numerical digit can be 0, 1, 2, 3, 4, 5, 6, 7, 8, or 9. Decimal is also called “base 10”, because there are 10 possible digits (0 through 9). In this system, we count like this: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, … By default, numbers in C++ programs are assumed to be decimal.

int x { 12 }; // 12 is assumed to be a decimal number

In binary, there are only 2 digits: 0 and 1, so it is called “base 2”. In binary, we count like this: 0, 1, 10, 11, 100, 101, 110, 111, …

There are two other “bases” that are sometimes used in computing: octal, and hexadecimal.

Octal is base 8 -- that is, the only digits available are: 0, 1, 2, 3, 4, 5, 6, and 7. In Octal, we count like this: 0, 1, 2, 3, 4, 5, 6, 7, 10, 11, 12, … (note: no 8 and 9, so we skip from 7 to 10).

To use an octal literal, prefix your literal with a 0 (zero):

#include <iostream>

int main()
{
    int x{ 012 }; // 0 before the number means this is octal
    std::cout << x;
    return 0;
}

This program prints:

10

Why 10 instead of 12? Because numbers are printed in decimal, and 12 octal = 10 decimal.

Octal is hardly ever used, and we recommend you avoid it.

Hexadecimal is base 16. In hexadecimal, we count like this: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F, 10, 11, 12, …

To use a hexadecimal literal, prefix your literal with 0x.

#include <iostream>

int main()
{
    int x{ 0xF }; // 0x before the number means this is hexadecimal
    std::cout << x;
    return 0;
}

This program prints:

15

Because there are 16 different values for a hexadecimal digit, we can say that a single hexadecimal digit encompasses 4 bits. Consequently, a pair of hexadecimal digits can be used to exactly represent a full byte.

Consider a 32-bit integer with value 0011 1010 0111 1111 1001 1000 0010 0110. Because of the length and repetition of digits, that’s not easy to read. In hexadecimal, this same value would be: 3A7F 9826. This makes hexadecimal values useful as a concise way to represent a value in memory. For this reason, hexadecimal values are often used to represent memory addresses or raw values in memory.

Prior to C++14, there is no way to assign a binary literal. However, hexadecimal values provide us with a useful workaround:

#include <iostream>

int main()
{
    int bin{};    // assume 16-bit ints
    bin = 0x0001; // assign binary 0000 0000 0000 0001 to the variable
    bin = 0x0002; // assign binary 0000 0000 0000 0010 to the variable
    bin = 0x0004; // assign binary 0000 0000 0000 0100 to the variable
    bin = 0x0008; // assign binary 0000 0000 0000 1000 to the variable
    bin = 0x0010; // assign binary 0000 0000 0001 0000 to the variable
    bin = 0x0020; // assign binary 0000 0000 0010 0000 to the variable
    bin = 0x0040; // assign binary 0000 0000 0100 0000 to the variable
    bin = 0x0080; // assign binary 0000 0000 1000 0000 to the variable
    bin = 0x00FF; // assign binary 0000 0000 1111 1111 to the variable
    bin = 0x00B3; // assign binary 0000 0000 1011 0011 to the variable
    bin = 0xF770; // assign binary 1111 0111 0111 0000 to the variable

    return 0;
}

C++14 binary literals and digit separators

In C++14, we can assign binary literals by using the 0b prefix:

#include <iostream>

int main()
{
    int bin{};        // assume 16-bit ints
    bin = 0b1;        // assign binary 0000 0000 0000 0001 to the variable
    bin = 0b11;       // assign binary 0000 0000 0000 0011 to the variable
    bin = 0b1010;     // assign binary 0000 0000 0000 1010 to the variable
    bin = 0b11110000; // assign binary 0000 0000 1111 0000 to the variable

    return 0;
}

Because long literals can be hard to read, C++14 also adds the ability to use a quotation mark (‘) as a digit separator.

#include <iostream>

int main()
{
    int bin { 0b1011'0010 };  // assign binary 1011 0010 to the variable
    long value { 2'132'673'462 }; // much easier to read than 2132673462

    return 0;
}

If your compiler isn’t C++14 compatible, your compiler will complain if you try to use either of these.

Also note that the separator can not occur before the first digit of the value:

    int bin { 0b'1011'0010 };  // error: ' used before first digit of value

Printing decimal, octal, hexadecimal, and binary numbers

By default, C++ prints values in decimal. However, you can tell it to print in other formats. Printing in decimal, octal, or hex is easy via use of std::dec, std::oct, and std::hex:

#include <iostream>

int main()
{
    int x { 12 };
    std::cout << x << '\n'; // decimal (by default)
    std::cout << std::hex << x << '\n'; // hexadecimal
    std::cout << x << '\n'; // now hexadecimal
    std::cout << std::oct << x << '\n'; // octal
    std::cout << std::dec << x << '\n'; // return to decimal
    std::cout << x << '\n'; // decimal

    return 0;
}

This prints:

12
c
c
14
12
12

Printing in binary is a little harder, as std::cout doesn’t come with this capability built-in. Fortunately, the C++ standard library includes a type called std::bitset that will do this for us (in the <bitset> header). To use std::bitset, we can define a std::bitset variable and tell std::bitset how many bits we want to store. The number of bits must be a compile time constant. std::bitset can be initialized with an unsigned integral value (in any format, including decimal, octal, hex, or binary).

#include <bitset> // for std::bitset
#include <iostream>

int main()
{
	// std::bitset<8> means we want to store 8 bits
	std::bitset<8> bin1{ 0b1100'0101 }; // binary literal for binary 1100 0101
	std::bitset<8> bin2{ 0xC5 }; // hexadecimal literal for binary 1100 0101

	std::cout << bin1 << ' ' << bin2 << '\n';
	std::cout << std::bitset<4>{ 0b1010 } << '\n'; // create a temporary std::bitset and print it

	return 0;
}

This prints:

11000101 11000101
1010

In the above code, this line:

std::cout << std::bitset<4>{ 0b1010 } << '\n'; // create a temporary std::bitset and print it

creates a temporary (unnamed) std::bitset object with 4 bits, initializes it with 0b1010, prints the value in binary, and then discards the temporary std::bitset.