Do polymorphic C-style casts have any overhead?

• Does casting a pointer to an instance of a d开发者_运维百科ervived class to the instances base class have any run-time overhead in C++, or is it resolved at compile-time?

• If it does have any, what exactly has to be computed in the casting operation?

Example:

class Foo;
class Bar : public Foo;

Bar* y = new Bar;
Foo* x = (Foo*) y;

(I know I should use C++-style casts and that the answer is probably the same for them)

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Yes, though it's negligible.

In this case, a C-Style cast is interpreted as a static_cast, which may incur a pointer adjustment.

struct Base {};

struct Derived: Base { virtual ~Derived(); } // note: introduced a virtual method

int main()
{
  Derived derived;
  Base* b = &derived;
  Derived* d = (Derived*) b;

  void* pb = b;
  void* pd = d;

  printf("%p %p", pb, pd);

  return 0;
}

This overhead happens whenever the base subobject is not aligned at the same address that the derived object, which happens when:

• introducing the first virtual method
• using multi-inheritance

Of course, pointer adjustment is usually counted as negligible, and the compiler shall be smart enough to eliminate it if it's unnecessary anyway (an adjustment of 0).

Note: this is implementation dependent, not mandated by the standard

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Foo* x = (Foo*) y;

You don't need to cast Bar* to Foo*. Casting from Derived to Base is implicit:

Foo* x = y ; // is enough!

As for polymorphic cast, C-style cast is not equivalent to dynamic_cast. C++ dynamic_cast is completely a new feature added to the language, and has no match in C-style cast.

Does casting a pointer to an instance of a dervived class to the instances base class have any run-time overhead in C++, or is it resolved at compile-time?

Casting from derived to base is resolved at compile-time, if you do as I did above. So it has not runtime overhead.

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C casts have no runtime overhead whatsoever. The only cast operation which creates runtime overhead is dynamic_cast<>() as it checks runtime type information.

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See the answer to a similar question: regular cast vs static cast vs dynamic cast

Summarized: a C-style cast has no run-time overhead as it tries a bunch of C++ casting methods except dynamic_cast (which incurs a run-time penalty).