C++ 虚拟方法引发 EXC_BAD_ACCESS（仅当被 Objective-C++ 调用时）

Question

I have the following things as part of a very large codebase:

class FooObserver {
public:
     virtual void FooObjectChanged() = 0;
};

class MainStuff :  public FooObserver {
public:
    /* ... */
    void FooObjectChanged();
    void doSomething();
};

void MainStuff::doSomething() {    
    this->FooObjectChanged();
    FooObserver *o = this;
    o->FooObjectChanged();
}

void MainStuff::FooObjectChanged() {
    std::cout << "object changed\n";
}

My understanding of the C++ standard is that this is valid code and FooObjectChanged() would be called twice without any error when doSomething() runs.

But my application fails on the second call. (with segfault, or more precisely EXC_BAD_ACCESS on iOS)

Another compiler issue is that, If I remove MainStuff::FooObjectChanged() (both the body and its declaration), I would expect the linker to fail with complaints about an abstract class. But compiler does not fail. It links and then the program crashes on the first call to the virtual function with

libc++abi.dylib: Pure virtual function called!

What is going on? What may cause these problems? FooObserver is not a base class of anything else, only of MainStuff .

Answer 1

1. Check if there is any other pure virtual function inherited from some other base class in MainStuff and you didn't override in MainStuff
2. Check doSomething is called on a valid object that is alive and within scope (not destroyed while MainStuff::doSomething() has been called and in processing)

Answer 2

This turned out to be some problem with XCode. For some reason it stopped recompiling some of the source files. After removing everything from the DerivedData folder and restarting XCode the code compiles, links and runs as I expect.

Answer 3

I just meet similar issue, where I had multiple (2 or more) C++ private-Classes (ie where Class has no Header), and all with exact same name, but defined in different .mm files.

Sometimes when calling one class's virtual method, Clang decided to use another class's vtable (which's header was NOT even ever included), resulting in binary that calls wrong method-address (and that wrong-method accessed fields, which do simply not exist in class, aka SIGSEG or BAD_ACCESS).

I was shocked, because private-class is a common C++ approach, but changing each private Class's name fixed Objective-C++'s (or rather Clang's) issue.