问题
I have read that there is some overhead to using C++ exceptions for exception handling as opposed to, say, checking return values. I'm only talking about overhead that is incurred when no exception is thrown. I'm also assuming that you would need to implement the code that actually checks the return value and does the appropriate thing, whatever would be the equivalent to what the catch block would have done. And, it's also not fair to compare code that throws exception objects with 45 state variables inside to code that returns a negative integer for every error.
I'm not trying to build a case for or against C++ exceptions solely based on which one might execute faster. I heard someone make the case recently that code using exceptions ought to run just as fast as code based on return codes, once you take into account all the extra bookkeeping code that would be needed to check the return values and handle the errors. What am I missing?
回答1:
There is a cost associated with exception handling on some platforms and with some compilers.
Namely, Visual Studio, when building a 32-bit target, will register a handler in every function that has local variables with non-trivial destructor. Basically, it sets up a try/finally handler.
The other technique, employed by gcc and Visual Studio targeting 64-bits, only incurs overhead when an exception is thrown (the technique involves traversing the call stack and table lookup). In cases where exceptions are rarely thrown, this can actually lead to a more efficient code, as error codes don't have to be processed.
回答2:
Only try/catch and try/except block take a few instructions to set up. The overhead should generally be negligible in every case except the tighest loops. But you wouldn't normally use try/catch/except in an inner loop anyway.
I would advise not to worry about this, and use a profiler instead to optimize your code where needed.
回答3:
It's completely implementation dependent but many recent implementations have very little or no performance overhead when exceptions aren't thrown. In fact you are right. Correctly checking return codes from all functions in code that doesn't use exceptions can be slower then doing nothing for code using exceptions.
Of course, you would need to measure the performance for your particular requirements to be sure.
回答4:
There is some overhead with exceptions (as the other answers pointed out).
But you do not have much of a choice nowadays. Try do disable exceptions in your project, and make sure that ALL dependent code and libraries can compile and run without.
Do they work with exceptions disabled?
Lets assume they do! Then benchmark some cases, but note that you have to set a "disable exceptions" compile switch. Without that switch you still have the overhead - even if the code never throws exceptions.
回答5:
Only overhead is ~6 instructions which add 2 SEH at the start of the function and leave them at the end. No matter how many try/catches you have in a thread it is always the same.
Also what is this about local variables? I hear people always complaining about them when using try/catch. I don't get it, because the deconstructors would eventually be called anyways. Also you shouldn't be letting an exception go up more then 1-3 calls.
回答6:
I took Chip Uni's test code and expanded it a bit. I split the code into two source files (one with exceptions; one without). I made each benchmark run 1000 times, and I used clock_gettime() with CLOCK_REALTIME to record the start and end times of each iteration. Then I computed the mean and variance of the data. I ran this test with 64-bit versions of g++ 5.2.0 and clang++ 3.7.0 on an Intel Core i7 box with 16GB RAM that runs ArchLinux with kernel 4.2.5-1-ARCH. You can find the expanded code and the full results here.
g++
No Exceptions- Average: 30,022,994 nanoseconds
- Standard Deviation: 1.25327e+06 nanoseconds
- Average: 30,025,642 nanoseconds
- Standard Deviation: 1.83422e+06 nanoseconds
clang++
No Exceptions- Average: 20,954,657 nanoseconds
- Standard Deviation: 426,662 nanoseconds
- Average: 23,916,638 nanoseconds
- Standard Deviation: 1.72583e+06 nanoseconds
C++ Exceptions only incur a non-trivial performance penalty with clang++, and even that penalty is only ~14%.
来源：https://stackoverflow.com/questions/1897940/in-what-ways-do-c-exceptions-slow-down-code-when-there-are-no-exceptions-thown