M PI works with math h but not with cmath in Visual Studio

Question

I am using Visual Studio 2010  I have read that in C   it is better to use  lt cmath gt  rather than  lt math h gt     But in the program I am trying to write  Win32 console application  empty project  if I write    define  USE MATH DEFINES  include  lt math h gt    it compiles  while if I write   define  USE MATH DEFINES  include  lt cmath gt    it fails with      error C2065   M PI    undeclared identifier   Is it normal  Does it matter if I use cmath or math h  If yes  how can I make it work with cmath   UPDATE  if I define  USE MATH DEFINES in the GUI  it works  Any clue why this is happening

User · Accepted Answer

Interestingly I checked this on an app of mine and I got the same error.

I spent a while checking through headers to see if there was anything undef'ing the _USE_MATH_DEFINES and found nothing.

So I moved the

#define _USE_MATH_DEFINES
#include <cmath>

to be the first thing in my file (I don't use PCHs so if you are you will have to have it after the #include "stdafx.h") and suddenly it compile perfectly.

Try moving it higher up the page. Totally unsure as to why this would cause issues though.

Edit: Figured it out. The #include <math.h> occurs within cmath's header guards. This means that something higher up the list of #includes is including cmath without the #define specified. math.h is specifically designed so that you can include it again with that define now changed to add M_PI etc. This is NOT the case with cmath. So you need to make sure you #define _USE_MATH_DEFINES before you include anything else. Hope that clears it up for you :)

Failing that just include math.h you are using non-standard C/C++ as already pointed out :)

Edit 2: Or as David points out in the comments just make yourself a constant that defines the value and you have something more portable anyway :)

User · Answer

According to Microsoft documentation about Math Constants      The file ATLComTime h includes math h when your project is built in Release mode  If you use one or more of the math constants in a project that also includes ATLComTime h  you must define  USE MATH DEFINES before you include ATLComTime h    File ATLComTime h may be included indirectly in your project  In my case one possible order of including was the following       project s  stdafx h     lt afxdtctl h gt     lt afxdisp h gt     lt ATLComTime h gt     lt math h gt

User · Answer

This is still an issue in VS Community 2015 and 2017 when building either console or windows apps  If the project is created with precompiled headers  the precompiled headers are apparently loaded before any of the  includes  so even if the  define  USE MATH DEFINES is the first line  it won t compile    including math h instead of cmath does not make a difference   The only solutions I can find are either to start from an empty project  for simple console or embedded system apps  or to add  Y- to the command line arguments  which turns off the loading of precompiled headers   For information on disabling precompiled headers  see for example  https   msdn microsoft com en-us library 1hy7a92h aspx   It would be nice if MS would change fix this   I teach introductory programming courses at a large university  and explaining this to newbies never sinks in until they ve made the mistake and struggled with it for an afternoon or so

User · Answer

This works for me    define  USE MATH DEFINES  include  lt cmath gt   include  lt iostream gt   using namespace std   int main         cout  lt  lt  M PI  lt  lt  endl       return 0      Compiles and prints pi like is should  cl  O2 main cpp  link  out test exe   There must be a mismatch in the code you have posted and the one you re trying to compile   Be sure there are no precompiled headers being pulled in before your  define

User · Answer

With CMake it would just be add compile definitions  USE MATH DEFINES   in CMakeLists txt

User · Answer

Consider adding the switch  D USE MATH DEFINES to your compilation command line  or to define the macro in the project settings  This will drag the symbol to all reachable dark corners of include and source files leaving your source clean for multiple platforms  If you set it globally for the whole project  you will not forget it later in a new file s

User · Answer

As suggested by user7860670  right-click on the project  select properties  navigate to C C   -  Preprocessor and add  USE MATH DEFINES to the Preprocessor Definitions   That s what worked for me

[c++] M_PI works with math.h but not with cmath in Visual Studio

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