invalid type argument of unary '*' (have double)

Question

Just started learning c++ got this error:

C:\Users\KC\Documents\Math.cpp|9|error: invalid type argument of unary '*' (have 'double')|

This is the code:

#include <iostream>
#include <cmath>
#define M_PI
using namespace std;

int main()
{
   double area, radius = 1.5;
      area = M_PI * radius * radius;
   cout << area << "\n";
}

can someone explain to me what did I do wrong. Thanks

You haven’t put a value for M_PI. See also https://stackoverflow.com/questions/6563810/m-pi-works-with-math-h-but-not-with-cmath-in-visual-studio . Without a value for M_PI, your area assignment simply becomes ‘area = * radius * radius’. Hence the error. — racraman, Apr 08 '19 at 07:41
Some standard libraries define their own `M_PI` when is included. I'd highly recommend picking a different name or else you're asking for a collision and undefined behaviour. — chris, Apr 08 '19 at 07:45
@chris how is a colliding `M_PI` preprocessor symbol supposed to cause undefined behviour? — 463035818_is_not_an_ai, Apr 08 '19 at 07:49
@JonathanGoh: Code::Blocks is an IDE. Asked for compiler because, the answer will depend on that as you can already see. — P.W, Apr 08 '19 at 08:06
@user463035818, I misremembered. Yes, you'll get a compiler error, which isn't ideal over having the program be portable, but better than UB. (I believe I was mixing it up with multiple [ODR-related](http://eel.is/c++draft/basic.def.odr#12) definitions.) — chris, Apr 08 '19 at 08:23

john · Answer 1 · 2019-04-08T07:43:29.703

3

#define M_PI

should be

#define M_PI 3.14159

(or whatever value you want to give for pi).

You defined M_PI as nothing, which means that this code

  area = M_PI * radius * radius;

becomes this code

  area = * radius * radius;

and your compiler is complaining about the unexpected *.

edited Apr 08 '19 at 07:43

answered Apr 08 '19 at 07:40

john

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2

This is one reason why `constexpr double M_PI` would be better imo. You can't forget to assign a value – perivesta Apr 08 '19 at 07:43
yeah That was the problem, Thanks! – Jonathan Goh Apr 08 '19 at 08:07

score 2 · Answer 2 · answered Apr 08 '19 at 07:46

2

I suggest to use:

#define _USE_MATH_DEFINES
#include <cmath>

and remove this line:

#define M_PI

More info in this answer: M_PI works with math.h but not with cmath in Visual Studio

answered Apr 08 '19 at 07:46

Jaime

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+1 best answer as no one should redefine pi. though it is quite upsetting that it doesn't work in Visual Studio. – Stack Danny Apr 08 '19 at 07:51

CiaPan · Answer 3 · 2019-07-20T16:12:05.410

You used the preprocessor directive #define M_PI which defined M_PI as an empty string. So, after substitution of the empty contents for M_PI, the expression

area = M_PI * radius * radius

became

area = * radius * radius

and the first asterisk became an unary operator, with the whole expression interpreted as

area = (* radius) * radius

That unary asterisk can't reasonably work with a double argument, hence an error message.

invalid type argument of unary '*' (have double)

3 Answers3