2014 m. kovo 31 d., pirmadienis

Darkest corners of C++

It is good to know language you are programming in.

Placement-new array offset

It turns out that on some compilers new[] might allocate an integer before an actual array even when placement-new is used:
void *mem = new char[sizeof(A) * n];
A *arr = new(mem) A[n];
Whether mem and arr will point at the same address depends on compiler and code. On GCC pointers I get the same pointers, but on Microsoft compiler, when A has a destructor, arr is mem + sizeof(int) or similar. While this mismatch might look harmless at first sight, it isn't - your array gets outside of allocated memory at the end!
Solution - cast pointer and manually loop over array creating each object individually via placement-new.

Pointer that changes

class Base {int x; };
class Derived : public Base {virtual void foo() {} };

Derived *d = new Derived;
Base *b = d;
Here b and d will not point to the same address. Comparing and casting them does the right thing, but if you cast them to void*, you'll see they're not equal! This is because Base is non-polymorphic (no virtual methods), while Derived is polymorphic. So, Derived object has a pointer to vtable at the beginning of it, followed by Base sub-object and then by it's own additional members.
Things get more funny when there are many classes in the hierarchy and multiple inheritance is involved.
Solution: well, don't cast pointers to objects into void*.

Return void

This code is valid:
void foo() {}
void bar() { return foo(); }
Useful, when writing templates.

Pure-virtual function with implementation

Pure-virtual function means that derived class must override it in order to create objects of it. But it does not mean that such method can not be implemented in the base class. The code bellow compiles and works:
class A
  virtual void foo() = 0;

void A::foo() { std::cout << "A::foo called\n"; }

class B : public A
  virtual void foo() override
    std::cout << "B::foo called\n";
Note that it did not compile for me using GCC, when I tried to provide implementation for A::foo inline.

Function-try block

This is quite a tricky feature. Function-try block basically looks like this:
void foo()
try {
  throw int();
} catch(...) {
  std::cout << "Exception caught\n";
However, in this form there is no particular use for it. It's just a shorter way of wrapping entire function body in a try-catch.
The real use for this feature (which also works differently) is for constructors. First of all, when used for constructor, it does not really catch exception! It catches and rethrows them! The real use for it is to free resources allocated in initializer list:
class A
  A(int x) { throw x; }

class B
  A a;
    : a(5)
    { } catch(...) {
      std::cout << "Exception in initializer merely-caught\n";
In here exception is thrown in an initializer list. There is no way to catch it in a constructor itself. But the initializer list may be long and some initilizer can have resource allocations, like memory allocation. To free such resources, you have to use such function-try block for you constructor and free them in a catch block. Remember, that exceptions are rethrown here.


When defining a struct it is possible to specify variable sizes in bits:
struct Bitfields
  int i:16;
  int j:8;
  bool b:1;
  char c:7;
The size of this structure is 4 bytes (on my machine at least). Each variable in the struct takes as much bits as specified and can hold appropriate value range.

And, since this is about C++, there are definitly more :)

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