cmath functions generating compiler error

问题

I've written a small program that utilizes the Fast Light Toolkit and for some reason a compiler error is generated when trying to access the functions in the cmath header.

Such as error ::acos has not been declared.

This goes on for pretty much every function it tries to use in the header. What could I be missing?

The header files I have included are

Simple_window.h
Graph.h

both of which are part of the FLTK.

The code is this:

    #include "Simple_window.h"  // get access to our windows library
    #include "Graph.h"          // get access to graphics library facilities

    int main()
    {
        using namespace Graph_lib; // our graphics facilities are in Graph_lib

        Point tl(100,100);         // to become top left corner of window

        Simple_window win(tl,600,400,"Canvas"); // make a simple window

        Polygon poly; // make a shape (a polygon)

        poly.add(Point(300,200));     // add a point
        poly.add(Point(350,100));     // add another point
        poly.add(Point(400,200));     // add a third point

        poly.set_color(Color::red);   // adjust properties of poly

        win.attach(poly);             // connect poly to the window

        win.wait_for_button();        // give control to display engine
    }

Edit: Here is example code of when the compiler error is generated. This is inside the cmath header.

namespace std
{
  // Forward declaration of a helper function.  This really should be
  // an `exported' forward declaration.
  template<typename _Tp> _Tp __cmath_power(_Tp, unsigned int);

  inline double
  abs(double __x)
  { return __builtin_fabs(__x); }

  inline float
  abs(float __x)
  { return __builtin_fabsf(__x); }

  inline long double
  abs(long double __x)
  { return __builtin_fabsl(__x); }

  using ::acos;  //ERROR HERE

  inline float
  acos(float __x)
  { return __builtin_acosf(__x); }

  inline long double
  acos(long double __x)
  { return __builtin_acosl(__x); }

  template<typename _Tp>
    inline typename __enable_if<double, __is_integer<_Tp>::_M_type>::_M_type
    acos(_Tp __x)
    {
      return __builtin_acos(__x);
    }

Edit: Code::blocks is saving files as C files....

回答1:

When you include the C++ version (<cXXXX>) of standard C libraries all the symbols are defined within the std namespace. In C++ you do not need to link against the math library (-lm is not required)

#include <cmath>
#include <iostream>

int main()
{
   std::cout << std::fabs( -10.5 ) << std::endl;
}

回答2:

I had this problem - it was driving me crazy but I tracked down the cause, and it was a little different than what I've seen reported on this issue.

In this case, the general cmath header (or math.h - the error and solution occur in C++ or C) had architectural environment switches to include architecture specific math subheaders. The architecture switch (environment variable) hadn't been defined, so it was punting and not actually including the headers that truly defined the math functions.

So there was indeed a single math.h or cmath.h, and it was included, but that wasn't enough to get the math functions. In my case, rather than define the architectural variable, I instead found the location of the correct sub math headers and added them to my compile path. Then the project worked!

This seems to be an issue that comes up a lot when porting Linux projects to OS-X. I'd imagine it might occur anytime a project was moved betwee platforms such that the standard library headers are arranged differently.

Jeff

回答3:

Since your code as shown above does not directly call acos(), there is arguably a bug in one of the headers that you do use. It appears there is some (inline) code in one of the headers that invokes the acos() function without ensuring that the function is properly declared. This might be a macro or an inline function.

The best fix is to ensure that the headers are self-contained - change the headers.

If that is not possible, the hackaround is to include the appropriate header (#include <cmath>, probably) in the source code.

The program is able to access the cmath header, the error is in the cmath header itself.

In that case, you will probably need to provide a global acos() function (declaration at least, possibly definition too) that calls onto std::acos():

double acos(double x) { return std::acos(x); }

Just make sure this is not inside any namespace - not even the anonymous one. (Check compiled with G++ 4.0.1 on MacOS X, with '#include <cmath>' preceding it. Given that you have a problematic <cmath> header, you might need to get fancy:

extern double std::acos(double);
double acos(double x) { return std::acos(x); }
#include <cmath>

This is pretty nasty - are you sure there isn't a bug-fixed version of your compiler?

Is there any chance that you've got '#include <cmath>' inside a namespace?

回答4:

The error is most likely to be in your code and not in cmath... unless you changed something in cmath. Could you copy the errors and tell us what is the application you are using to program?

回答5:

It also happens in Visual C++, in programs that do not sapuse to use cmath.

I found that the problem is that I used main.c file instead of main.cpp file.

来源：https://stackoverflow.com/questions/1216878/cmath-functions-generating-compiler-error

标签

c++

compiler-construction

cmath