cppfastio
mark gnu::pure to some const method
are you sure this works? list root can change, it would just produce wrong results
apologizing for misunderstanding, gnu::pure should be the right one.
pure function means:
y=f(x)
Any x must produce the same y. Here it is clearly not the case.
Cause C++ is not a functional programming language, gnu::pure should mean
- the output only rely on the input parameter (✓ this pointer)
- no side effect occurred.
Why empty() and size() contains side effects? They did not edit anything except generating the output.
Why you consider bool empty(Class const*) and size_t size(Container const*) aren't pure function?
Basically, C++ is not a functional programming language and one key reason is that variables can be mutable, which can lead to things like:
int plus(int a) {
return a+1;
}
int main() {
int a{};
a = plus(a);
plus(a); // same variable but different result
}
Anyway, pure function for C++ can be explained as given the same STATUS (not the same variable) always produces same result.
With above concept, size and empty are definitely pure, without side effect.
int main() {
fast_io::string str{};
str.size(); // no side effect
str.append("text"); // status changed
str.size(); // like above example, same variable but not same status, is actually different input for pure function
}
Can you go LLVM and ask LLVM folks on whether it is correct to add pure here first? thanks
Hi, I have investigated lots of stuff and I'm pretty ensure it's correct.
See GCC Document: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-pure-function-attribute
However, the caller may safely change the contents of the array between successive calls to the function (doing so disables the optimization). The restriction also applies to member objects referenced by the this pointer in C++ non-static member functions.
See also: https://clang.llvm.org/docs/AttributeReference.html#pure Bad news! Clang does not document anything other than the title ) :
To avoid (potential) off-topic discuss on llvm discord/issue, I asked this question to ykiko&Kimiv2&Another C++ community on discord, and the answer is yes.
My final prove is https://github.com/llvm/llvm-project/blob/3a8f6979cef26ceb5ef5c6b8dba1fc1fd770c44c/clang/lib/AST/Expr.cpp#L3750 of its influence on side effect.
Is above enough?