[[deprecated]] is another attribute that is useful to mark something (a function, a method, a variable, a class, etc.) as still valid, but that has been superseded by other newer stuff and that probably will be removed in the future.

In a similar vein to [[nodiscard]], [[deprecated]] can return a message explaining why this entity has been marked as such.

The compiler will show a warning when a deprecated entity is being actually used in our code.

For example, I have this code:

#include <iostream>

void print(const std::string& msg)
{
    std::cout << msg << "\n";
}

int main()
{
    print("Hello world");
}

After that a lot of functions and code started to use my print() function, I realize that a newer version with std::string_view instead of std::string could have better performance and, since I do not want to break any code, I consider having both functions in my system.

So, to discourage the usage of my old function, I mark it as deprecated:

#include <iostream>


void println(std::string_view msg)
{
    std::cout << msg << "\n";
}

[[deprecated("Use println instead")]]
void print(const std::string& msg)
{
    std::cout << msg << "\n";
}

int main()
{
    print("Hello world");
}

But, since I am still using the old version in my main() function, the compiler will return a warning like this one:

main.cpp:17:24: warning: ‘void print(const string&)’ is deprecated: Use println instead [-Wdeprecated-declarations]

That will dissappear when I will replace all the invocation to print() with println().