Division in Variable Template Returns Zero in Visual Studio 2017

Question

This is probably a visual-studio-2017 bug related to this question: Templated Variables Bug With Lambdas in Visual Studio? And as mentioned in the comments there seems to be optimizer related.

Division in the definition of a variable template seems to have a bug in Visual Studio 2017. So this code for example:

template <typename T>
const T PI = std::acos(static_cast<T>(-1));

template <typename T>
const T ONE_EIGHTY = 180;

template <typename T>
const T DEG_TO_RAD = PI<T> / ONE_EIGHTY<T>;

int main() {
    cout << DEG_TO_RAD<float> << endl;
}

On gcc 6.3 this outputs:

0.0174533

On Visual Studio 2017 this outputs:

0.0

I'm assuming this is another Visual Studio bug? Is there a workaround here?

Interesting.. If you try to `cout << PI << endl` prior to `DEG_TO_RAD` - you get the correct result. — Algirdas Preidžius, Mar 05 '18 at 20:15
I was also able to get the correct result by changing `PI` to a hard-coded `3.14159265` and changing the variable templates to be `constexpr`. — 0x5453, Mar 05 '18 at 20:18
@AlgirdasPreidžius Ugh there needs to be a good workaround for this... and there just isn't :( Nonetheless, good find. — Jonathan Mee, Mar 05 '18 at 20:20
`auto test2= DEG_TO_RAD;` gets the correct result so this is a std::cout thing. Try setting std::cout for more resolution. — lakeweb, Mar 05 '18 at 20:21
This is funny, changing `const T PI` to `const auto PI` works — Killzone Kid, Mar 05 '18 at 20:23
@lakeweb I am not seeing that locally when running in debug. — Jonathan Mee, Mar 05 '18 at 20:23
And it looks like a casting problem as `std::cout << (float)DEG_TO_RAD` works. — lakeweb, Mar 05 '18 at 20:24
@KillzoneKid Oh goodness! I changed that to get the linked question to work! What the what? — Jonathan Mee, Mar 05 '18 at 20:24
When I hover over `test2` I see it. But then I'm using vs140 sp1 — lakeweb, Mar 05 '18 at 20:25
@lakeweb No again, neither `static_cast` nor C-style cast gives me anything but 0.0F. — Jonathan Mee, Mar 05 '18 at 20:27
Well then, 2017 vsxxx may have a new bug. Sorry I only have 2015. — lakeweb, Mar 05 '18 at 20:29
@lakeweb That's also interesting, I didn't even thing VS2015 had Variable Templates? — Jonathan Mee, Mar 05 '18 at 20:30
But `constexpr` is a little rough. I can't run the full testing on boost hana. — lakeweb, Mar 05 '18 at 20:34
@JonathanMee Check this out! If you add `PI;` before `cout` it works. So maybe template is not instantiated properly? — Killzone Kid, Mar 05 '18 at 20:50
@KillzoneKid That's an excellent workaround again... however, nothing that has been said here has instilled confidence in Visual Studio's Variable Templates :( — Jonathan Mee, Mar 05 '18 at 20:52
No need to cast the value as it is const template variable; wrap the RHS expression in parenthesis and that will fix the ambiguity of the operator precedence order. — Francis Cugler, Mar 05 '18 at 21:42
I mentioned no need to cast but this might also vary from version to version, machine to machine, platform to platform, os to os, compiler to compiler... If the parenthesis doesn't work, then go ahead and use the cast that does. — Francis Cugler, Mar 05 '18 at 22:01

score 2 · Accepted Answer · answered Mar 06 '18 at 13:40

Posting a workaround here at the request of @JonathanMee as it also works for the similar problem reported by him earlier. Seems to be connected with some sort of bug in latest VS2017 that prevents template from activating automatically and needs a force activation:

template <typename T>
const T PI = std::acos(static_cast<T>(-1));

template <typename T>
const T ONE_EIGHTY = 180;

template <typename T>
const T DEG_TO_RAD = PI<T> / ONE_EIGHTY<T>;

int main() 
{
    PI<float>; // <---- workaround
    std::cout << DEG_TO_RAD<float> << std::endl;
}

Here is a bug ticket filed with Microsoft: https://developercommunity.visualstudio.com/content/problem/207741/template-needs-to-be-force-instantiated-vs2017-bug.html

Francis Cugler · Answer 2 · 2018-03-05T22:00:45.340

I tested your code as is and got the same results. However I also tested PI & ONE_EIGHTY individually and they were giving me the correct results in the output. So I had thought about it, and for some reason as I don't know why Visual Studio is doing this which leads me to think that it could be a bug or that it could be left to the compiler-implementation-design but it appears that it is not setting the const T DEG_RAD from the division of the two predefined constants.

To fix this problem in visual studio it is quite simple and easily over looked; all you have to do is wrap the RHS expression with parenthesis.

template<typename T>
const T DEG_TO_RAD = (PI<T> / ONE_EIGHT<T>);

And this will not print out the correct values as it will do the division before assigning the value to the LHS const T variable declared as DEG_TO_RAD.

I'm not 100% certain and don't quote me on this, but from my own understanding I think this could be an issue of operator precedence order. I think in visual studio without wrapping the RHS in parenthesis it is not performing the operation of division properly thus DEG_TO_RAD is being set to 0.

EDIT

Here is my full code: I am using about 80% of the standard library for my particular solution for when I try to work out problems to answer here on stack so I will not list all of the includes but I do have most all containers, I/O, numeric, algorithms, memory, functional, chrono, cmath, GLM etc.

Using Visual Studio 2017 v15.4.4 on Win 7 - 64bit Home Premium on a Intel Quad Core Extreme.

template <typename T>
const T PI = std::acos( static_cast<T>(-1) );

template <typename T>
const T ONE_EIGHTY = 180;

template <typename T>

const T DEG_TO_RAD = PI<T> / ONE_EIGHTY<T>; 

int main() {
    std::cout << PI<float> << std::endl;
    std::cout << ONE_EIGHTY<float> std::endl;
    std::cout << DEG_TO_RAD<float> std::endl;

    _getch(); // include <conio.h> // just to stop the debugger from auto close
    return 0;
}

Output:

3.14159
180
0

Then when I do this:

template <typename T>
const T PI = std::acos( static_cast<T>(-1) );

template <typename T>
const T ONE_EIGHTY = 180;

template <typename T>

const T DEG_TO_RAD = (PI<T> / ONE_EIGHTY<T>); 

int main() {
    std::cout << PI<float> << std::endl;
    std::cout << ONE_EIGHTY<float> std::endl;
    std::cout << DEG_TO_RAD<float> std::endl;

    _getch(); // include <conio.h> // just to stop the debugger from auto close
    return 0;
}

Output:

3.14159
180
0.0174533

I tried this in all 4 modes: Debug/Release x86 & x64 and I get the same results. This may be different on your machine - platform.

hmm; I'm using VS 2017 and it works for me in Win7 64bit home premium. — Francis Cugler, Mar 05 '18 at 21:46
Are you compiling in 32bit or 64bit mode, debug or release, any special optimizations? — Francis Cugler, Mar 05 '18 at 21:46
Debug/release/32/64 all of them same result, also Win7 64bit home premium — Killzone Kid, Mar 05 '18 at 21:48
I checked all 4 states too and I am getting the correct values. Do you have any includes such as ``, ``, `` etc? — Francis Cugler, Mar 05 '18 at 21:49
No I have solution that is specifically setup to work out problems when answering questions here, therefore I am using `stdafx.h` & `stdafx.cpp` where I am including about 80% of the standard library, as well as a few other general includes and windows... — Francis Cugler, Mar 05 '18 at 21:52
Let us [continue this discussion in chat](https://chat.stackoverflow.com/rooms/166276/discussion-between-francis-cugler-and-killzone-kid). — Francis Cugler, Mar 05 '18 at 21:52
Okay if the parenthesis doesn't resolve your issue but casting it does, then use the cast. — Francis Cugler, Mar 05 '18 at 21:53

Division in Variable Template Returns Zero in Visual Studio 2017

2 Answers2

Linked