C++17_Attributes.txt

********* C++17 Attributes ********** 

1. [[fallthrough]] -  In C++17 and on, a standard attribute was introduced to indicate that the warning is not needed when the code is meant to fall through.
With this attribute, you can now merge the bodies of two adjacent case branches in a switch without getting any warnings from the compiler. 
By using it, you tell the compiler that the prior case body is non-terminated intentionally. 
[[fallthrough]] silences an implicit fallthrough warning in a switch

Example -  switch(input) {
    case 1:
    case 2:
         std::cout << "Hello" << std::endl // // WARNING: implicit fallthrough
    case 2:
        std::cout << " C++" << std::endl;
        [[fallthrough]]; // > No warning
    case 3:
    
}

2. [[nodiscard]] -   forgot to check the return value of your functions ? with this attribute, discarding the return values will become a reason for compiler warnings.
It creates a warning if the return value of a function call is discarded.It's used to indicate that the return value of a function shouldn't be ignored.

Example - [[nodiscard]] int fun();
void callingfun() {
f(); // WARNING: return value of nodiscard function discarded.
}

3.[[unused]]  - This attribute silences an unused entity warning. An entity marked [[unused]] means that the entity may appear to be unused for some reason. If an
implementation would have otherwise warned that the entity isn’t used, the [[unused]] attribute suppresses the warning:

Example - int x; // WARNING: x is unused
[[unused]] int x; // OK