The All-in-One Guide to C++20/

...

/

The [[nodiscard("reason")]] Attribute

The [[nodiscard("reason")]] Attribute

Understand the use of the '[[nodiscard("reason")]]' attribute in C++20.

We'll cover the following...

C++17 introduced the new attribute [[nodiscard]] without a reason. C++20 added the possibility to add a message to the attribute.

Discarding objects and error codes

Press + to interact
#include <utility> 
struct MyType {
    MyType(int, bool) {} 
};
template<typename T,typename ... Args> 
T* create(Args&& ... args) {
    return new T(std::forward<Args>(args)...); 
}
enum class ErrorCode{ 
    Okay,
    Warning,
    Critical,
    Fatal
}; 
ErrorCode errorProneFunction() {
    return ErrorCode::Fatal;
} 
int main() {
    int* val = create<int>(5); 
    delete val;
    create<int>(5);
    errorProneFunction();
    
    MyType(5, true);
}

Thanks to perfect forwarding and parameter packs, the factory function create (at line 7) can call any constructor and return a heap-allocated object. The program has many issues. First, line 27 has a memory leak, because the int created on the heap is never deleted. Second, the error code of the function errorProneFunction (at line 29) is not checked.

Lastly, the constructor call MyType(5, true) (at line 31) creates a temporary, which is created and immediately destroyed. This is at ...