Step 6: Adding a Custom Command and Generated File

Suppose, for the purpose of this tutorial, we decide that we never want to use the platform log and exp functions and instead would like to generate a table of precomputed values to use in the mysqrt function. In this section, we will create the table as part of the build process, and then compile that table into our application.

First, let's remove the check for the log and exp functions in MathFunctions/CMakeLists.txt. Then remove the check for HAVE_LOG and HAVE_EXP from mysqrt.cxx. At the same time, we can remove #include <cmath>.

In the MathFunctions subdirectory, a new source file named MakeTable.cxx has been provided to generate the table.

After reviewing the file, we can see that the table is produced as valid C++ code and that the output filename is passed in as an argument.

The next step is to add the appropriate commands to the MathFunctions/CMakeLists.txt file to build the MakeTable executable and then run it as part of the build process. A few commands are needed to accomplish this.

First, at the top of MathFunctions/CMakeLists.txt, the executable for MakeTable is added as any other executable would be added.

MathFunctions/CMakeLists.txt

add_executable(MakeTable MakeTable.cxx)

Then we add a custom command that specifies how to produce Table.h by running MakeTable.

MathFunctions/CMakeLists.txt

add_custom_command(
  OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/Table.h
  COMMAND MakeTable ${CMAKE_CURRENT_BINARY_DIR}/Table.h
  DEPENDS MakeTable
  )

Next we have to let CMake know that mysqrt.cxx depends on the generated file Table.h. This is done by adding the generated Table.h to the list of sources for the library MathFunctions.

MathFunctions/CMakeLists.txt

add_library(MathFunctions
            mysqrt.cxx
            ${CMAKE_CURRENT_BINARY_DIR}/Table.h
            )

We also have to add the current binary directory to the list of include directories so that Table.h can be found and included by mysqrt.cxx.

MathFunctions/CMakeLists.txt

target_include_directories(MathFunctions
          INTERFACE ${CMAKE_CURRENT_SOURCE_DIR}
          PRIVATE ${CMAKE_CURRENT_BINARY_DIR}
          )

Now let's use the generated table. First, modify mysqrt.cxx to include Table.h. Next, we can rewrite the mysqrt function to use the table:

MathFunctions/mysqrt.cxx

double mysqrt(double x)
{
  if (x <= 0) {
    return 0;
  }

  // use the table to help find an initial value
  double result = x;
  if (x >= 1 && x < 10) {
    std::cout << "Use the table to help find an initial value " << std::endl;
    result = sqrtTable[static_cast<int>(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;
  }

  return result;
}

Run the cmake executable or the cmake-gui to configure the project and then build it with your chosen build tool.

When this project is built it will first build the MakeTable executable. It will then run MakeTable to produce Table.h. Finally, it will compile mysqrt.cxx which includes Table.h to produce the MathFunctions library.

Run the Tutorial executable and verify that it is using the table.