CODEWITHSUNDEEP
c++floating-pointclangrounding
Koosha
Koosha

Reputation: 1584

Is this Clang's bug or my bug?

Run the the following C++ program twice. Once with the given destructor and once with std::fesetround(value); removed from the destructor. Why do I receive different outputs? Shouldn't destructor be called after function add? I ran both versions on http://cpp.sh/ and Clang++ 6.0, and g++ 7.2.0. For g++, I also included #pragma STDC FENV_ACCESS on in the source code, nothing changed.

#include <iostream>
#include <string>
#include <cfenv>

struct raii_feround {
  raii_feround() : value(std::fegetround()) {    }
 ~raii_feround() { std::fesetround(value); }
  inline void round_up  () const noexcept { std::fesetround(FE_UPWARD  ); }
  inline void round_down() const noexcept { std::fesetround(FE_DOWNWARD); } 
  template<typename T>
  T add(T fst, T snd) const noexcept { return fst + snd; }
private: 
  int value; };

float a = 1.1;
float b = 1.2;
float c = 0;
float d = 0;

int main() {   
    {
        raii_feround raii;
        raii.round_up();
        c = raii.add(a, b);
    }
    {
        raii_feround raii;
        raii.round_down();
        d = raii.add(a, b);
    }
    std::cout << c << "\n"; // Output is: 2.3
    std::cout << d << "\n"; // Output is: 2.3 or 2.29999
}

Upvotes: 0

Views: 367

Answers (2)

Koosha
Koosha

Reputation: 1584

All I needed to do was to do std::cout << std::setprecision(30); before calling std::cout in the code (iomanip should be included as well).

Upvotes: 0

Eric Postpischil
Eric Postpischil

Reputation: 222900

Using the floating-point environment facilities requires inserting #pragma STDC FENV_ACCESS on into the source (or ensure that they default to on for the implementation you are using. (Although STDC is a C feature, the C++ standard says that these facilities are imported into C++ by the <cfenv> header.)

Doing so at cpp.sh results in “warning: ignoring #pragma STDC FENV_ACCESS [-Wunknown-pragmas]”.

Therefore, accessing and modifying the floating-point environment is not supported by the compiler at cpp.sh.

Upvotes: 1

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