{{{+1 Solution 1 }}}
This is the most simple way to append simple log strings to a file if DEBUG is defined.
{{{#!gcode C++
#include "stdio.h"
void WriteLogFile(const char* szString)
{
    #ifdef DEBUG

    FILE* pFile = fopen("logFile.txt", "a");
    fprintf(pFile, "%s\n",szString);
    fclose(pFile);

    #endif
}
}}}

{{{+1 Solution 2 }}}
C++ answer for the same thing
{{{#!gcode C++
#include <fstream>
void write_text_to_log_file(const std::string &text)
{
    std::ofstream log_file(
        "log_file.txt", std::ios_base::out | std::ios_base::app );
    log_file << text << std::end;
}
}}}