Ever wondered why your "clean" polymorphic design underperforms in benchmarks? Virtual dispatch enables polymorphism, but it comes with hidden overhead: pointer indirection, larger object layouts, and fewer inlining opportunities.
Compilers do their best to devirtualize these calls, but it isn't always possible. On latency-sensitive paths, it's beneficial to manually replace dynamic dispatch with static polymorphism, so calls are resolved at compile time and the abstraction has effectively zero runtime cost.
Virtual dispatch #
Runtime polymorphism occurs when a base interface exposes a virtual method that derived classes override. Calls made through a Base&
are then dispatched to the appropriate override at runtime. Under the hood, a virtual table (vtable
) is created for each class, and a pointer (vptr
) to the vtable
is added to each instance.
On a virtual call, the compiler loads the vptr
, selects the right slot in the vtable
, and performs an indirect call through that function pointer. The drawback is that the extra vptr
increases object size, and the indirection through the vtable
makes the call hard to predict. This prevents inlining, increases branch mispredictions, and reduces cache efficiency.
The best way to observe this phenomenon is by inspecting the assembly 1 code emitted by the compiler for a minimal example
class Base {
public:
auto foo() -> int;
};
auto bar(Base* base) -> int {
return base->foo() + 77;
}
For a non-virtual member function foo
like in the example above, the free function bar
issues a direct call
bar(Base*):
sub rsp, 8
call Base::foo() // Direct call
add rsp, 8
add eax, 77
ret
However, declaring foo
as virtual
changes bar
's assembly into an indirect, vtable-based call
bar(Base*):
sub rsp, 8
mov rax, QWORD PTR [rdi] // vptr (pointer to vtable)
call [QWORD PTR [rax]] // Virtual call
add rsp, 8
add eax, 77
ret
Devirtualization #
Sometimes the compiler can statically deduce which override a virtual call will hit. In those cases, it devirtualizes the call and emits a direct call instead (skipping the vtable
). For example, devirtualization is straightforward 2 when the runtime type is clearly fixed
struct Base {
virtual auto foo() -> int = 0;
};
struct Derived : Base {
auto foo() -> int override { return 77; }
};
auto bar() -> int {
Derived derived;
return derived.foo(); // compiler knows this is Derived::foo
}
The compiler is able to devirtualize even through a base pointer, as long as it can track the allocation and prove there is only one possible concrete type. The problem is that with traditional compilation, object files are created per translation unit (TU)---compiled and optimized in isolation. The linker simply stitches those objects together, so cross-TU optimizations are inherently limited. That's where compiler flags are useful.
-fwhole-program
: tells the compiler "this translation unit is the entire program." If no class derives from Base
in this TU, the compiler is free to assume nothing ever does, and can devirtualize calls on Base
.
-flto
: link-time optimization. Keeps an intermediate representation in the object files and optimizes across all of them at link time, effectively treating multiple source files as a single TU.
On the language side, final
is a lightweight way to give the compiler the same guarantee for specific methods
class Base {
public:
virtual auto foo() -> int;
virtual auto bar() -> int;
};
class Derived : public Base {
public:
auto foo() -> int override; // override
auto bar() -> int final; // final
};
auto test(Derived* derived) -> int {
return derived->foo() + derived->bar();
}
Here, foo()
can still be overridden, so derived->foo()
remains a virtual call. However, bar()
is marked as final
, so the compiler emits a direct call even though it's declared virtual
in the base
test(Derived*):
push rbx
sub rsp, 16
mov rax, QWORD PTR [rdi]
mov QWORD PTR [rsp+8], rdi
call [QWORD PTR [rax]] // Virtual call
mov rdi, QWORD PTR [rsp+8]
mov ebx, eax
call Derived::bar() // Direct call
add rsp, 16
add eax, ebx
pop rbx
ret
Static polymorphism #
When the compiler can't devirtualize, one option is to use static polymorphism instead. The canonical tool for this is the Curiously Recurring Template Pattern 3 (CRTP). With CRTP, the base class is templated on the derived class, and invokes methods on it via
static_cast
---no virtual keyword involved
template <typename Derived>
class Base {
public:
auto foo() -> int {
return 77 + static_cast<Derived*>(this)->bar();
}
};
class Derived : public Base<Derived> {
public:
auto bar() -> int {
return 88;
}
};
auto test() -> int {
Derived derived;
return derived.foo();
}
With -O3
optimization, the compiler inlines everything and constant-folds the result. No vtable
, no vptr
, no indirection. Fully optimized 4 call.
test():
mov eax, 165 // 77 + 88
ret
Deducing this. C++23's deducing this keeps the same static-dispatch model but makes it easier to write. Instead of templating the entire class (and writing Base<Derived>
everywhere), you template only the member function that needs access to the derived type, and let the compiler deduce self
from *this
class Base {
public:
auto foo(this auto&& self) -> int { return 77 + self.bar(); }
};
class Derived : public Base {
public:
auto bar() -> int { return 88; }
};
This yields identical optimized code: foo
is instantiated as foo<Derived>
, and the call to bar
is resolved statically and inlined.
Assembly generated with
gcc
at-O3
on x86-64. Similar results were observed withclang
on the same platform.↩ - The compiler emits a direct call to
Derived::foo
(or inlines it), becausederived
cannot have any other dynamic type.↩ - The curiously recurring template pattern is an idiom where a class X derives from a class template instantiated with X itself as a template argument. More generally, this is known as F-bound polymorphism, a form of F-bounded quantification.
↩ - The trade-off is that each
Base<Derived>
instantiation is a distinct, unrelated type, so there's no common runtime base to upcast to. Any shared functionality that operates across different derived types must itself be templated.↩