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I am trying to understand some details of static_cast.

Please have a look at the following code,

struct A { int data = 0; }; void foo(const A* a) { (*static_cast<A**>(static_cast<void*>(&a)))->data = 1; } void bar(const A* a) { const_cast<A*>(a)->data = 1; } int main() { A a; foo(&a); return a.data; }

Is the function foo valid C++ code? Is there any valid usage that gives a different result with foo vs. bar?

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  • The cast in bar is a NOP anyway if I'm not too tired. Commented Nov 4, 2019 at 2:06
  • @Peter-ReinstateMonica I cast away constness to write to data. Commented Nov 4, 2019 at 2:09
  • See, I was tired and missed the const. I assume foo compiles? Interesting. Commented Nov 4, 2019 at 2:11
  • I used -std=c++17 -O3 -Wall -Werror with clang 9.0.0 and the whole thing compiles (even with bar instead of foo). Commented Nov 4, 2019 at 2:12
  • This is undefined behaviour due to strict aliasing rule violation (you alias const A * as A *) Commented Nov 4, 2019 at 3:59

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Both functions are valid C++ and have well-defined behavior (modifying the data member of the A object created in main) in C++11 and later.

You are allowed to obtain a pointer to non-const from a pointer to const object type either directly with const_cast or indirectly with static_cast going through void* as you are doing in foo. That in itself is not a problem:

The cast to void* is possible because &a is a pointer to const A* which is not (top-level) cv-qualified. The cast from void* to A** is possible because void* can be cast to any object pointer type. Dereferencing the result is accessing the const A* object through a pointer to A*, but that is ok because the types are similar. (The last part seems to have been an aliasing rule violation prior to C++11, making this undefined behavior.)

However, modifying a const qualified object through a pointer to non-const obtained in such a way causes undefined behavior.

Since the object A a; that you are passing to the function is not const qualified, there is no problem.

But I think it is obvious why using such functions is dangerous. Changing the declaration A a; to const A a; will still compile because there is no type mismatch, but will have undefined behavior.

Both functions do exactly the same thing in all situations, as far as I can tell.

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2 Comments

Thanks for the answer. Now const_cast seems a bit redundant to me. Maybe there are use-cases I can't see.
@MutableSideEffect The intent of const_cast is to explicitly state what is happening, because as I mentioned in my answer, using it is always dangerous and requires special care to be taken. If you are talking strictly about redundance, then ((A*)a)->data = 1; would also do it and C style cast allows most other casts as well.

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