nodiscard in C++

The [[nodiscard]] attribute is a part of C++17 and onwards, which signals the compiler to issue a warning if the return value of a function is ignored. This attribute can be applied to functions, enumerations, and classes.

Usage

1. To annotate a function to ensure its return value is not ignored:

[[nodiscard]] int compute();

2. To annotate an enumeration or class so that when its object is returned by a function, it should not be ignored:

class [[nodiscard]] ImportantResult {...};

3. Since C++20, a string literal can be provided to explain why the return should not be ignored:

[[nodiscard("Reason for not discarding")]] int performAction();

When Is a Warning Issued?

A warning is encouraged to be issued by the compiler in the following cases:

• A function declared with [[nodiscard]] is called and its value is not used.
• An object of a class or enumeration declared with [[nodiscard]] is initialized and not used.
• In C++20, if a string literal is provided with the [[nodiscard]] attribute, it is usually included in the warning message issued by the compiler.

Purpose

The purpose of the [[nodiscard]] attribute is to catch potential bugs where a function’s return value, which might be significant, is accidentally ignored.