Cmake tutorial step6 (add custom commands and generate files)

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
step6
https://cmake.org/cmake/help/v3.24/guide/tutorial/Adding%20a%20Custom%20Command%20and%20Generated%20File.html
My warehouse :
https://github.com/FRBoiling/cmake-tutorial.git
hypothesis , For the purposes of this tutorial , We decided never to use the platform log and exp function , Instead, you need to generate a table of pre calculated values , be used for mysqrt function .
In this section , We will create tables as part of the build process , Then compile the table into our application .

initialization

First , Let's delete MathFunctions/CMakeLists.txt in log and exp Function check .

add_library(MathFunctions mysqrt.cxx)

# state that anybody linking to us needs to include the current source dir to find MathFunctions.h, while we don't.
target_include_directories(MathFunctions
INTERFACE ${CMAKE_CURRENT_SOURCE_DIR}
)

# install rules
install(TARGETS MathFunctions DESTINATION lib)
install(FILES MathFunctions.h DESTINATION include)

then , from mysqrt.cxx Delete the right HAVE_LOG and HAVE_EXP The inspection of . meanwhile , We can delete #include .

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

stay MathFunctions Directory , newly added MakeTable.cxx The source file , Used to generate table. We can see table As valid c++ Code generated , And the output file name is passed in as a parameter .

// A simple program that builds a sqrt table
#include <cmath>
#include <fstream>
#include <iostream>

int main(int argc, char* argv[])
{
    
  // make sure we have enough arguments
  if (argc < 2) {
    
    return 1;
  }

  std::ofstream fout(argv[1], std::ios_base::out);
  const bool fileOpen = fout.is_open();
  if (fileOpen) {
    
    fout << "double sqrtTable[] = {" << std::endl;
    for (int i = 0; i < 10; ++i) {
    
      fout << sqrt(static_cast<double>(i)) << "," << std::endl;
    }
    // close the table with a zero
    fout << "0};" << std::endl;
    fout.close();
  }
  return fileOpen ? 0 : 1; // return 0 if wrote the file
}

Custom command generation

The next step is to MathFunctions/CMakeLists.txt Add the appropriate command to the file , In order to build MakeTable Executable file , Then run it as part of the build process . Several commands are required to complete this operation .

First , stay MathFunctions/CMakeLists.txt At the top of the , add to MakeTable The executable of

add_executable(MakeTable MakeTalbe.cxx)

Then add a custom command , Specify how to run MakeTable Generate table.h

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

Next we will let CMake know mysqrt.cxx Depends on the generated file table.h. This is generated by table.h Add to standard library MathFunctions Source list of .

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

We must also add the current binary directory to include Directory list , In order to mysqrt.cxx Be able to find and contain table.h.

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

Use the generated table

First , modify mysqrt.cxx contain table.h. Next , We can rewrite mysqrt Function to use this table :

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;
}

Test practice

function cmake Executable or cmake-gui To configure the project , Then build it using the build tool of your choice .
When the project is built , It will first build MakeTable Executable file . And then run MakeTable Generate table.h. Last , It will compile mysqrt.cxx, These include table.h To generate MathFunctions library .
function Tutorial Executable and verify that it uses tables .