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

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?

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.

This works for me:

#define _USE_MATH_DEFINES
#include <cmath>
#include <iostream>

using namespace std;

int main()
{
    cout << M_PI << 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.

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).

With CMake it would just be

add_compile_definitions(_USE_MATH_DEFINES)

in CMakeLists.txt.

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>

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.