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

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

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

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

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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

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#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 ...