c++ force compiler to opt out some piece of code - c++

Question

I have a piece of code:
// some code, which only do sanity check
expensive checks
// sanity check end
Now how do I tell the compiler to force it to opt out
this piece? Basically it means when I compile with -O2 or
O3, I don't want it to be there...
Thanks!

Answer 1

You can accomplish this with a constant and a single if/def pair. This allows the code to still be compiled and checked for errors but omitted during optimization. This can prevent changes that might break the check code from going undetected.
#if defined(USE_EXPENSIVE_CHECKS) || defined(DEBUG)
#define USE_EXPENSIVE_CHECKS_VALUE true
#else
#define USE_EXPENSIVE_CHECKS_VALUE false
#endif
namespace {
const bool useExpensiveChecks = USE_EXPENSIVE_CHECKS_VALUE;
};
void function()
{
if(useExpensiveChecks == true)
{
// expensive checks
}
}

Answer 2

Instead of relying on the compiler to optimize the code out, you could pass the compiler an additional symbol definition only when you want the code to run:
// some code, which only do sanity check
#ifdef my_symbol
expensive checks
#endif
// sanity check end

Answer 3

Using macros and conditionals in the preprocessor is really the only way to avoid code being generated by the compiler.
So, here's how I would do it:
#ifdef NEED_EXPENSIVE_CHECKS
inline expensive_checking(params...)
{
... do expensive checking here ...
}
#else
inline expensive_checking(params...)
{
}
#endif
Then just call:
some code
expensive_checking(some_parameters...)
some other code
An empty inlined function will result in "no code" in any decent, modern compiler. Use -DNEED_EXPENSIVE_CHECKS in your debug build settings, and don't use that in release build.
I have also been known to use a combination of macro and function, such as this:
#ifdef NEED_EXPENSIVE_CHECKS
#define EXPENSIVE_CHECKS(stuff...) expensive_checks(__FILE__, __LINE__, stuff...)
inline expensive_checks(const char *file, int line, stuff ...)
{
if (some_checking)
{
cerr << "Error, some_checking failed at " << file << ":" << line << endl;
}
}
#else
#define EXPENSIVE_CHECKS(stuff...)
#endif
Now, you get information on which file and what line when something fails, which can be very useful if the checks are made in many places (and you can use __function__ or __pretty_function__ to get the function name as well, if you wish).
Obviously, the assert() macro will essentially do what my macro solution does, except it usually doesn't provide the filename and line-number.

Answer 4

Move your checks into a different function, then import cassert and write assert(expensive_check()). When you want to disable the checks, use #define NDEBUG before the inclusion of cassert.