C++ has types, and the only way they normally convert between each other is by well-defined conversion operators that you write. In general, that's all you both need and should use to write your programs.

Sometimes, however, you want to reinterpret the bits that represent a type into something else. This is usually used for very low-level operations and is not something you should typically use. For those cases, you can use reinterpret_cast.

It is implementation defined because the C++ standard does not really say much at all about how things should actually be laid out in memory. That is controlled by your specific implementation of C++. Because of this, the behaviour of reinterpret_cast depends upon how your compiler lays structures out in memory and how it implements reinterpret_cast.

C-style casts are quite similar to reinterpret-casts, but they have much less syntax and are not recommended. The thinking goes that casting is inherently an ugly operation and it requires ugly syntax to inform the programmer that something dubious is happening.

An easy example of how it could go wrong:

std::string a;
double* b;
b = reinterpret_cast<double*>(&a);
*b = 3.4;

That programs behaviour is undefined - a compiler could do anything it likes to that. Most probably, you would get a crash when the string's destructor is called, but who knows! It might just corrupt your stack and cause a crash in an unrelated function.

C++ has types, and the only way they normally convert between each other is by well-defined conversion operators that you write. In general, that's all you both need and should use to write your programs.

Sometimes, however, you want to reinterpret the bits that represent a type into something else. This is usually used for very low-level operations and is not something you should typically use. For those cases, you can use reinterpret_cast.

It is implementation defined because the C++ standard does not really say much at all about how things should actually be laid out in memory. That is controlled by your specific implementation of C++. Because of this, the behaviour of reinterpret_cast depends upon how your compiler lays structures out in memory and how it implements reinterpret_cast.

C-style casts are quite similar to reinterpret_casts, but they have much less syntax and are not recommended. The thinking goes that casting is inherently an ugly operation and it requires ugly syntax to inform the programmer that something dubious is happening.

An easy example of how it could go wrong:

std::string a;
double* b;
b = reinterpret_cast<double*>(&a);
*b = 3.4;

That program's behaviour is undefined - a compiler could do anything it likes to that. Most probably, you would get a crash when the string's destructor is called, but who knows! It might just corrupt your stack and cause a crash in an unrelated function.