M_PI works with math.h but not with cmath in Visual Studio

user_518169 0
Viewed 166224

I am using Visual Studio 2010. I have read that in C++ it is better to use <cmath> rather than <math.h>.

But in the program I am trying to write (Win32 console application, empty project) if I write:

#define _USE_MATH_DEFINES
#include <math.h>

it compiles, while if I write

#define _USE_MATH_DEFINES
#include <cmath>

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?

visual-studio-2010 visual-studio c++
7 Answers

With CMake it would just be

add_compile_definitions(_USE_MATH_DEFINES)

in CMakeLists.txt.

user_32437510

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" → <afxdtctl.h> → <afxdisp.h> → <ATLComTime.h> → <math.h>

user_32406810

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.

user_16079370
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