编译器优化问题

Question

Most of the functions in <functional> use functors. If I write a struct like this:

struct Test
{
   bool operator()
   {
       //Something
   }
   //No member variables
};

Is there a perf hit? Would an object of Test be created? Or can the compiler optimize the object away?

Answer 1

Yes, the compiler can optimize "object creation" (which is trivial in this case) out if it wants so. However if you really care you should compile your program and inspect the assembly code.

Answer 2

Even if the compiler was having a bad day and somehow couldn't figure out how to optimize this (it's very simple as optimizations go) - with no data members and no constructor the "performance hit" to "create an object" would be at most one instruction (plus maybe a couple more to copy the object, if the compiler also doesn't figure out how to inline the function call that uses the functor) to increment the stack pointer (since every object must have a unique address). "Creating objects" is cheap . What takes time is allocating memory , via new (because the OS has to be petitioned for the memory, and it has to search for a contiguous block that isn't being used by something else). Putting things on the stack is trivial.

Answer 3

GCC at least can optimize the object creation and inline your functor, so you can expect performance as with hand-crafted loop. Of cource you must compile with -O2.

Answer 4

There is no "use" of the structure, so as the code currently stands, it is still just a definition (and takes up no space).

If you create an object of type Test , it will take up non-zero space. If the compiler can deduce that nothing takes its address (or anything similar), it is free to optimize away the space usage.