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 set and changed through initialization and assignment respectively.

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 symbol 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*.

In C++, operations work as you’d expect. For example:

```
#include <iostream>
int main()
{
std::cout << 1 + 2 << '\n';
return 0;
}
```

In this program, the literals `1`

and `2`

are operands to the plus (`+`

) operator, which produces the output value `3`

. This output value is then printed to the console. In C++, the output value of an operation is often called a return value.

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 keywords (e.g. `new`

, `delete`

, and `throw`

).

Author’s note

For reasons that will become clear when we discuss operators in more detail, for operators that are symbols, it is common to append the operator’s symbol to the word *operator*.

For example, the plus operator would be written `operator+`

, and the extraction operator would be written `operator>>`

.

The number of operands that an operator takes as input is called the operator’s arity. Few people know 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 (often called *left* and *right*, as the left operand appears on the left side of the operator, and the right operand appears on the right side of the operator). 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 return 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”.

Author’s note

In some countries, PEMDAS is taught as PEDMAS, BEDMAS, BODMAS, or BIDMAS instead.

Return values and side effects

Most operators in C++ just use their operands to calculate a return value. For example, `-5`

produces return value `-5`

and `2 + 3`

produces return value `5`

. There are a few operators that do not produce return values (such as `delete`

and `throw`

). We’ll cover what these do later.

Some operators have additional behaviors. An operator that has some observable effect beyond producing a return value is said to have a side effect. For example, when `x = 5`

is evaluated, the assignment operator has the side effect of assigning the value `5`

to variable `x`

. The changed value of `x`

is observable (e.g. by printing the value of `x`

) even after the operator has finished executing. `std::cout << 5`

has the side effect of printing `5`

to the console. We can observe the fact that `5`

has been printed to the console even after `std::cout << 5`

has finished executing.

Operators with side effects are usually called for the behavior of the side effect rather than for the return value (if any) those operators produce.

For advanced readers

For the operators that we call primarily for their return values (e.g. `operator+`

or `operator*`

), it’s usually obvious what their return values will be (e.g. the sum or product of the operands).

For the operators we call primarily for their side effects (e.g. `operator=`

or `operator<<`

), it’s not always obvious what return values they produce (if any). For example, what return value would you expect `x = 5`

to have?

Both `operator=`

and `operator<<`

(when used to output values to the console) return their left operand. Thus, `x = 5`

returns `x`

, and `std::cout << 5`

returns `std::cout`

. This is done so that these operators can be chained.

For example, `x = y = 5`

evaluates as `x = (y = 5)`

. First `y = 5`

assigns `5`

to `y`

. This operation then returns `y`

, which can then be assigned to `x`

.

`std::cout << "Hello " << "world"`

evaluates as `(std::cout << "Hello ") << "world!"`

. This first prints `"Hello "`

to the console. This operation returns `std::cout`

, which can then be used to print `"world!"`

to the console as well.

Quiz time

Question #1

For each of the following, indicate what output they produce:

a)

`std::cout << 3 + 4 << '\n';`

b)

`std::cout << 3 + 4 - 5 << '\n';`

c)

`std::cout << 2 + 3 * 4 << '\n';`

d) Extra credit:

```
int x { 2 };
std::cout << (x = 5) << '\n';
```