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Last updated：2026-10-12

C++17 [[nodiscard]] Attribute

Since the [[nodiscard]] attribute was introduced in C++17, we have had a more powerful tool for marking function return values that should not be ignored at the call site. In this article we will take a deep dive into how to use [[nodiscard]] and watch out for some potential pitfalls.

What is [[nodiscard]]?

[[nodiscard]] is a function attribute that tells the compiler that the return value of a function should not be ignored. This is especially useful for functions that rely on their return value, such as predicate functions or factory functions.

How to use [[nodiscard]]?

It is possible to use [[nodiscard]] on classes, constructors, and enums. Here are some examples:

class [[nodiscard]] A {};

class B {
public:
    [[nodiscard]] B() = default;
};

enum class [[nodiscard]] State {
    Default,
    Foo,
    Bar
};

In the examples above, the compiler will issue a warning whenever the return value is ignored, which helps us catch potential bugs early.

Watch out: pitfalls of [[nodiscard]]

Although [[nodiscard]] can effectively prevent return values from being ignored unintentionally, we need to be aware of some potential pitfalls. A common pitfall is a [[nodiscard]] function that returns a scope-guard-like object in order to perform cleanup at the end of the scope (for example, committing a transaction).

[[nodiscard]] ScopeGuard createScopeGuard() {
    // Implementation of a scope guard
}

// 在这里，用户可以选择忽略返回值，导致作用域立即销毁，从而失去清理效果
std::ignore = createScopeGuard();

Although [[nodiscard]] indicates that the return value of the function should not be ignored, the user can still explicitly discard the return value by means such as std::ignore, thereby bypassing the cleanup.

Complete example

#include <tuple> // std::ignore

class [[nodiscard]] A {};
class B {
public:
    [[nodiscard]] bool isOk() const { return true; }
};

// no need for nodiscard, A is nodiscard
A foo() {
    return A();
}

// B is not nodiscard, mark it explicitly
[[nodiscard]] B bar() {
    return B();
}

int main()
{    
    foo(); // warning: ignoring temporary created by a constructor declared with 'nodiscard' attribute
    bar(); // warning

    std::ignore = foo(); // ok
    auto b = bar(); // ok
    b.isOk(); // warning
    if (b.isOk()) {} // ok
}

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