ABI stability
- C++ course
- Expert
- Lesson 23/47
- 5 min
Learn ABI stability and versioning.
ABI stability
ABI stability means a library's new version keeps working with old compiled code. Binary details like class size, member order and the virtual function table must not change.
ABI break points: adding a new member (size changes), adding a virtual function (vtable changes), reordering members (layout changes), changing an inline function's body (implementation is distributed). Libraries like Qt and libstdc++ use PIMPL and d-pointer techniques to keep ABI stable.
Virtual destructor rule
If a class contains virtual functions, its destructor must also be virtual. This ensures that when a derived class is destroyed through a base pointer, the right destructor runs.
Example: class Base { public: virtual ~Base() = default; virtual int value() const { return 42; } };. Here virtual ~Base() = default asks the compiler to make the default destructor virtual. Following this rule makes ABI less likely to break; derived classes are destroyed safely.
Practice Write a class with a virtual destructor
Write class Base. In its public section, include virtual ~Base() = default; and virtual int value() const { return 42; }. In main write Base b; std::cout << b.value();.
Starter code
#include <iostream>
// Define the Base class here
int main() {
// Create a Base instance and print value
return 0;
}Sample solution
#include <iostream>
class Base {
public:
virtual ~Base() = default;
virtual int value() const {
return 42;
}
};
int main() {
Base b;
std::cout << b.value();
return 0;
}Expected output
42
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the PIMPL pointer
class Widget {
struct Impl;
std::unique_ptr<Impl> ____;
};class Widget {
struct Impl;
std::unique_ptr<Impl> pimpl;
};Explanation: The pimpl pointer hides the Impl definition.
2 Match each technique to its role
| Term | Meaning |
|---|---|
PIMPL |
Hidden implementation |
virtual destructor |
Derived class destruction |
#pragma pack |
Struct alignment |
extern C |
Unmangled symbol |
3 Which change breaks the ABI?
class Base { public: virtual ~Base(); int x; };- Adding a private member
- Adding a comment
- Adding a new virtual function Correct answer
- Changing a function's body without renaming
Explanation: A new virtual function changes the vtable; old code calls the wrong function.