Cmake tutorial step5 (add system self-test)

CMake Official documents

Refer to the official cmake3.24 Course translation . I'm going to use cmake 3.16 To demonstrate examples .
https://cmake.org/cmake/help/v3.24/guide/tutorial/index.html
https://gitlab.kitware.com/cmake/cmake/-/tree/master/Help/guide/tutorial
step5
https://cmake.org/cmake/help/v3.24/guide/tutorial/Adding%20System%20Introspection.html
My warehouse :
https://github.com/FRBoiling/cmake-tutorial.git

Let's consider adding some code to the project that depends on features that the target platform may not have .
For this example , We will add some that depend on whether the target platform has log and exp Function code . Of course , Almost every platform has these functions , But assume in this tutorial that they are not common .
If the platform has log and exp, Then we will use them in mysqrt Calculate the square root in the function .

We use MathFunctions/CMakeLists.txt Medium CheckCXXSourceCompiles Module to test the usability of these functions .
Calling target_include_directories() after , take log and exp The check of is added to MathFunctions/CMakeLists.txt:

target_include_directories(MathFunctions
          INTERFACE ${CMAKE_CURRENT_SOURCE_DIR}
          )

# does this system provide the log and exp functions?
include(CheckCXXSourceCompiles)
check_cxx_source_compiles("
  #include <cmath>
  int main() {
    std::log(1.0);
    return 0;
  }
" HAVE_LOG)
check_cxx_source_compiles("
  #include <cmath>
  int main() {
    std::exp(1.0);
    return 0;
  }
" HAVE_EXP)

If available , Use target_compile_definitions() take HAVE_LOG and HAVE_EXP Specify private compilation definitions .

if(HAVE_LOG AND HAVE_EXP)
  target_compile_definitions(MathFunctions
                             PRIVATE "HAVE_LOG" "HAVE_EXP")
endif()

If log and exp Available on the system , Then we will use them in mysqrt Calculate the square root in the function . Add the following code to math_functions/mysqrt.cxx Medium mysqrt Function ( Don't forget before returning the result #endif):

#if defined(HAVE_LOG) && defined(HAVE_EXP)
  double result = std::exp(std::log(x) * 0.5);
  std::cout << "Computing sqrt of " << x << " to be " << result
            << " using log and exp" << std::endl;
#else
  double result = x;

We still need to revise it mysqrt.cxx contain cmath The header file .

#include <cmath>

Come here ,mysqrt.cxx The content is long like this ：

#include <cmath>
#include <iostream>

#include "MathFunctions.h"

// a hack square root calculation using simple operations
double mysqrt(double x)
{
    
  if (x <= 0) {
    
    return 0;
  }

  // if we have both log and exp then use them
#if defined(HAVE_LOG) && defined(HAVE_EXP)
  double result = std::exp(std::log(x) * 0.5);
  std::cout << "Computing sqrt of " << x << " to be " << result
            << " using log and exp" << std::endl;
#else
  double result = x;

  // do ten iterations
  for (int i = 0; i < 10; ++i) {
    
    if (result <= 0) {
    
      result = 0.1;
    }
    double delta = x - (result * result);
    result = result + 0.5 * delta / result;
    std::cout << "Computing sqrt of " << x << " to be " << result << std::endl;
  }
#endif
  return result;
}

function cmake Executable or cmake-gui To configure the project , Then build it using the build tool of your choice , And run the tutorial executable .

test

Now which function gives a better result ,sqrt still mysqrt?