Why does the debugger display the wrong function

Sometimes you need to use debugging when solving some problems , This scene is very common , We set a breakpoint somewhere in the code , Then step into a function for single step debugging , But you will find , The debugger will show you in another function . What's the matter ？ Let's take a look at the following code .

 

When you step to call p->GetValue() Local time of , You will find that the debugger shows that it is currently Class1::GetQ in . that , What happened ？
This is because , From the linker's perspective , In the code generation phase , The linker combines all the same functions , As shown in the figure below ：

 

Please note that , At the object code level , These two functions are exactly the same .（ Please note that , Whether the two functions are the same at the object code level depends largely on the compiler version you use and the optimization flag . When you use templates , The same code generation frequency of different functions is very high .）
therefore , Linker says ：” ok , What's the point of having two identical functions ？ I will only keep one copy and use it to represent Class1::GetQ and Class2::GetValue.”

 

Please note that , These two functions have been merged ： The address of the function is the same . Just this code has two names . therefore , When the debugger sees that you have jumped to 0x010010d6 when , It doesn't know which name to use , So it just chose one of them .

That's why it looks like you skipped the wrong function .

That's what's called “ identical COMDAT Fold ”(identical COMDAT folding), You can take /OPT:NOICF Flags are passed to the linker to disable this optimization .

summary

Developing TopomelBox In the process of , One day I had a whim , Want to know the meaning of all compile switches .
Some are easy to understand , For example, various header file directories , But some linker switches are really hard to understand , Today's one ”COMDAT Folding” It's one of them .
Develop this road , It's still a long way to go .

Last

Raymond Chen Of 《The Old New Thing》 It's one of my favorite blogs , There's a lot about Windows Little knowledge , For the vast Windows For platform developers , It's really helpful .
This article is from ：《Why does the debugger show me the wrong function?》