c++ - What is type punning and what is the purpose of it?

Question

Type punning

A form of pointer aliasing where two pointers and refer to the same location in memory but represent that location as different types. The compiler will treat both "puns" as unrelated pointers. Type punning has the potential to cause dependency problems for any data accessed through both pointers.

What is this article trying to say? What happens if I use it or not use it?

Answer 1

As it says, type punning is when you have two pointers of different type, both pointing at the same location. Example:

// BAD CODE
uint32_t data;
uint32_t* u32 = &data;
uint16_t* u16 = (uint16_t*)&data; // undefined behavior

This code invokes undefined behavior in C++ (and C) since you aren't allowed to access the same memory location through pointers of non-compatible types (with a few special exceptions). This is informally called a "strict aliasing violation" since it violates the strict aliasing rule.

Another way of doing type punning is through unions:

// BAD C++ CODE
typedef union
{
  uint32_t u32;
  uint16_t u16 [2];
} my_type;

my_type mt;
mt.u32 = 1;
std::cout << mt.u16[0]; // access union data through another member, undefined behavior

This is also undefined behavior in C++ (but allowed and perfectly fine in C).