Built-in Functions

Introduction

The predefined functions, which can be used without having to import a package to get access to them, are called built-in functions. They sometimes apply to different types, e.g. len, cap, and append, or they have to operate near system level like panic. That’s why they need support from the compiler.

Explanation

Note: We’ll only list the name of the functions and their functionalities in this lesson. Some of the functions are covered before in detail and used multiple times, so we won’t run them from scratch. Functions that were not discussed previously, their running examples will be provided later in the course in detail, section by section. So don’t panic if you don’t have a clear idea.

The following are some common and important built-in functions provided by Go:

close

It is used in channel communication.

len and cap

The function len gives the length of a number of types (strings, arrays, slices, maps, channels). Whereas, cap is the capacity, the maximum storage (only applicable to slices and maps).

new and make

Both new and make are used for allocating memory. The function new is used for value types and user-defined types like structs. Whereas, make is used for built-in reference types (slices, maps, channels). They are used like functions with the type as its argument:

new(type) 
make(type)

new(T) allocates zeroed storage for a new item of type T and returns its address. It returns a pointer to the type T (details are in Chapter 8), and it can be used with primitive types as well:

v := new(int) // v has type *int

The function make(T) returns an initialized variable of type T, so it does more work than new.

Remark: new() is a function; don’t forget its parentheses.

copy and append

These are used for copying and concatenating slices (see Chapter 5).

panic and recover

These both are used in a mechanism for handling errors (see Chapter 11).

print and println

These are low-level printing functions. Use the fmt package in production programs.

complex, real, and imag

These are used for making and manipulating complex numbers.

Note: We won’t be covering complex(), real(), and imag() anywhere because they don’t fall within the scope of this course. If you want to study them in detail, you can follow their documentation. For complex() click here. For real() click here. For imag() click here.

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Built-in functions are never hard to understand. We can always learn them when we feel a need to use them. You just need to call them and see what they do. The next lesson brings an important function type known as recursive function.