Unused variable warning even when explicitly using it inside IF statement - c++

Question

I am trying to create (using C++17) a simple debug header that only executes some lines of code if the flag LOGGER_DEBUG_MODE is enabled. This is how my header is defined (I also tried using { x; } instead of x but the warning persists):
debug.h
#ifndef _HDEBUG
#define _HDEBUG
static bool LOGGER_DEBUG_MODE = true;
#define R_DEBUG(x) if(LOGGER_DEBUG_MODE == true) x
#endif
I included debug.h and at some point of my code I call the macro function R_DEBUG to print some values:
logger_adc.cpp
double anlg_device_t::eval_formula()
{
double result = -9999;
try
{
result = parser.Eval();
}
catch (mu::Parser::exception_type &e)
{
std::cout << e.GetMsg() << std::endl;
}
R_DEBUG(std::cout << "Eval Result: " << result << std::endl);
return result;
}
I expected everything to work properly but when I run the makefile I got this warning:
inc/debug.h:5:14: warning: 'LOGGER_DEBUG_MODE' defined but not used [-Wunused-variable]
static bool LOGGER_DEBUG_MODE = true;
I thought that my definition was messed up but after checking the temporary files created by g++, it appears that the preprocessor did everything as I expected:
logger_adc.ii
double anlg_device_t::eval_formula()
{
double result = -9999;
try
{
result = parser.Eval();
}
catch (mu::Parser::exception_type &e)
{
std::cout << e.GetMsg() << std::endl;
}
if(LOGGER_DEBUG_MODE == true) std::cout << "Eval Result: " << result << std::endl;
return result;
}
Why do I get the warning message even when the variable LOGGER_DEBUG_MODE is clearly being used inside the if statement? Did I mess up something obvious that I'm not picking up? My compile flags for the object files (where the warning occurs) are g++ -Wall -Wextra -O1 -g -std=c++17 -save-temps=obj -Iinc -I/usr/local/include -c plus pkg-config --cflags --libs libmodbus
If needed, this is my main function:
main.cpp
#include "logger_adc.h"
int main()
{
anlg_device_t test (ADC_CHIP_1, 1, 18, 1, 1, true);
test.set_formula("2*x","x", test.get_voltage_ptr());
std::cout << "Test Voltage: " << test.get_voltage() << std::endl << "Test Relative: " << test.get_relative() << std::endl;
std::cout << "Test Formula (2*x): " << test.eval_formula() << std::endl;
return 0;
}
Thanks in advance!

Answer 1

You have a header that defines a static bool LOGGER_DEBUG_MODE =true;. If you include that header in multiple C++ files then each file will gets its own copy of that bool.
In your main.cpp you aren't using R_DEBUG so the copy of that bool (which presumably comes from including logger_adc.h ) is indeed unused in that file.
Possible solutions are:
You should make it so you only have a single copy of that bool (declare it in the header with extern and define it in a single C++ file.
Use build defines instead of runtime checks
etc