1.9 — Introduction to literals and operators

Literals

Consider the following two statements:

std::cout << "Hello world!";
int x{ 5 };

What are “Hello world!” and 5? They are literals. A literal (also known as a literal constant) is a fixed value that has been inserted directly into the source code.

Literals and variables both have a value (and a type). However, the value of a literal is fixed and can’t be changed (hence it being called a constant), whereas the value of a variable can be changed through initialization and assignment.

Operators

In mathematics, an operation is a mathematical calculation involving zero or more input values (called operands) that produces a new value (called an output value). The specific operation to be performed is denoted by a construct (typically a symbol or pair of symbols) called an operator.

For example, as children we all learn that 2 + 3 equals 5. In this case, the literals 2 and 3 are the operands, and the symbol + is the operator that tells us to apply mathematical addition on the operands to produce the new value 5.

You are likely already quite familiar with standard arithmetic operators from common usage in mathematics, including addition (+), subtraction (-), multiplication (*), and division (/). In C++, assignment (=) is an operator as well, as are <<; (insertion), >> (extraction), and == (equality). While most operators have symbols for names (e.g. +, or ==), there are also a number of operators that are words (e.g. new, delete, and throw).

The number of operands that an operator takes as input is called the operator’s arity (almost nobody knows what this word means, so don’t drop it in a conversation and expect anybody to have any idea what you’re talking about). Operators in C++ come in four different arities:

Unary operators act on one operand. An example of a unary operator is the - operator. For example, given -5, operator- takes literal operand 5 and flips its sign to produce new output value -5.

Binary operators act on two operands (known as left and right). An example of a binary operator is the + operator. For example, given 3 + 4, operator+ takes the left operand (3) and the right operand (4) and applies mathematical addition to produce new output value 7. The insertion (<<) and extraction (>>) operators are binary operators, taking std::cout or std::cin on the left side, and the value to output or variable to input to on the right side.

Ternary operators act on three operands. There is only one of these in C++ (the conditional operator), which we’ll cover later.

Nullary operators act on zero operands. There is also only one of these in C++ (the throw operator), which we’ll also cover later.

Note that some operators have more than one meaning depending on how they are used. For example, operator- has two contexts. It can be used in unary form to invert a number’s sign (e.g. to convert 5 to -5, or vice versa), or it can be used in binary form to do subtraction (e.g. 4 - 3).

Chaining operators

Operators can be chained together such that the output of one operator can be used as the input for another operator. For example, given the following: 2 * 3 + 4, the multiplication operator goes first, and converts left operand 2 and right operand 3 into new value 6 (which becomes the left operand for the plus operator). Next, the plus operator executes, and converts left operand 6 and right operand 4 into new value 10.

We’ll talk more about the order in which operators execute when we do a deep dive into the topic of operators. For now, it’s enough to know that the arithmetic operators execute in the same order as they do in standard mathematics: Parenthesis first, then Exponents, then Multiplication & Division, then Addition & Subtraction. This ordering is sometimes abbreviated PEMDAS, or expanded to the mnemonic “Please Excuse My Dear Aunt Sally”.

Quiz time

std::cout << 3 + 4;

std::cout << 3 + 4 - 5;

std::cout << 2 + 3 * 4;