Why is converting from int to char* different from converting from std::string to char*?

Question

I'm learning C++ and reading Andrei Alexandrescu's book on generic programming. He presents a templated class that can be used to convert between types:

template <class To, class From>
To safe_reinterpret_cast(From from)
{
    assert(sizeof(From) <= sizeof(To));
    return reinterpret_cast<To>(from);
}

This works fine for:

int i = 5;
char* p = safe_reinterpret_cast<char*>(i);

but fails for

std::string a("apple");
char* pp = safe_reinterpret_cast<char*>(a);

This is the error failure at compile time:

invalid cast from type 'std::basic_string<char>' to type 'char*'

Why does this cast fail?

Bathsheba · Accepted Answer · 2015-07-24 15:12:47Z

1

Andrei Alexandrescu's infamous example only works for plain-old-data types.

It does not work for pointers. The behaviour of casting unrelated pointer types is undefined.

You can only reinterpret_cast to void*, and reinterpret_cast from void* back to the original pointer type.

answered Jul 24, 2015 at 15:12

Bathsheba

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1 Comment

Aaron McDaid Over a year ago

I think the point here is that std::string is not a pointer, nor a POD.

Lawrence Aiello · Accepted Answer · 2015-07-24 15:10:08Z

1

Because int and char are primitive types, while std::string is not.

answered Jul 24, 2015 at 15:10

Lawrence Aiello

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1 Comment

sunny Over a year ago

so my larger question is can reinterpret_cast never be used for non-primitive types?

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