nodiscard.md

[[nodiscard]]

Some functions have result values that must be stored or used, or else risk incorrect behavior (sometimes silently!):

template 
class MyOwningPointer {
    T* data = nullptr;

public:
    T* release() { return std::exchange(data, nullptr); }
};

void steal_ownership(MyOwningPointer& ptr) {
    /* delete */ ptr.release(); // Silent memory leak!
}

C++17 provides the [[nodiscard]] attribute to encourage compilers to emit a warning when the result of a function is potentially discarded:

Code

template 
class MyOwningPointer {
    T* data = nullptr;

public:
    [[nodiscard]]
    T* release() { return std::exchange(data, nullptr); }
};

void steal_ownership(MyOwningPointer& ptr) {
    /* delete */ ptr.release(); // Warning emitted!
}

Warning

GCC

warning: ignoring return value of ‘MyOwningPointer<int>::release()’, declared with attribute nodiscard [-Wunused-result]
     ptr.release();
                 ^

Clang

warning: ignoring return value of function declared with 'nodiscard' attribute [-Wunused-result]
    ptr.release();
    ^~~~~~~~~~~~~