请教《Ray Tracing in One Weekend》中vec3类构造函数的语法及相关问题

Hey there! Let's clear up your confusion right away—this is not the singleton pattern. What you're looking at is standard C++ constructor syntax, specifically combining a member initialization list with an empty function body. Let's break it down step by step:

1. The Default Constructor & Member Initialization List #

vec3() is the default constructor of the vec3 class—this is the constructor that gets called when you create a vec3 object without passing any arguments (like vec3 my_vector;).

The : e{0,0,0} part is the member initialization list. Here, e is almost certainly a member variable of the vec3 class (likely an array or struct holding the three components of the vector). The syntax e{0,0,0} uses C++11's list initialization to set the three elements of e to 0.0 each. This is a clean, efficient way to initialize class members before the constructor's main body runs.

2. Why the Trailing {} is Required #

The {} at the end is the body of the constructor. In this case, it's empty because all the work we need to do—initializing the e member—has already been handled by the initialization list.

So why can't we just omit it? Because in C++, when you define a constructor (instead of just declaring it), the syntax requires you to provide a function body. Even if there's no code to run inside the constructor, you still need to include the empty {} to satisfy the compiler's syntax rules.

For comparison, look at the other constructor in your code snippet:

vec3(double e0, double e1, double e2) : e{e0, e1, e2} {}

This follows exactly the same pattern: it uses the initialization list to set e with the passed-in values, and has an empty body since no additional logic is needed.

Quick Note on the Singleton Misconception #

Just to be clear, the singleton pattern is a design pattern used to ensure a class has only one instance in a program. This syntax has nothing to do with that—this is just a standard way to write a constructor that initializes class members cleanly.

内容的提问来源于stack exchange，提问作者Spagetti