CODEWITHSUNDEEP
c++
Roland
Roland

Reputation: 31

Can I initialize a *char?

On page 113, in The C++ Programming Language (Third Edition and Special Edition), Stroustrup states:

struct address {
char * name ; // "Jim Dandy"
long int number ; // 61
//...
};

void f ()
{
address jd ;
jd.name = "Jim Dandy"; // Is this possible?
jd.number = 61 ;
}

Is this possible since there was not any memory allocated for the char* field of jd?

Update: Thank you all for your answers! Given that it's not safe, I won't use it. It just caught my attention when I saw it in the book.

Upvotes: 1

Views: 146

Answers (2)

Mike Seymour
Mike Seymour

Reputation: 254461

The memory was allocated: enough for a pointer. Now that points to the static array that contains the string.

If you were expecting it to put a copy of the string in the structure, then that's not how C-style strings work; if you want that behaviour, then use the C++ std::string class instead.

I hope the example goes on to explain how dangerous this is. The static array is constant, but a quirk of the language means you're allowed to assign a non-const pointer to point to it. This allows you to write code that attempts to modify a constant object, which gives undefined behaviour at run time:

jd.name[0] = 'T'; // BOOM! Undefined behaviour.

If you're lucky, the compiler might warn you about that mistake. You can prevent it by declaring the pointer const:

char const * name;
...
jd.name[0] = 'T'; // Gives a friendly compile-time error

Upvotes: 2

Fred Foo
Fred Foo

Reputation: 363577

Enough memory is allocated to hold a pointer to char, and the assignment sets the pointer to point to a static buffer holding the string "Jim Dandy", so yes, this is possible. No allocation is needed since the string is not copied.

(However, setting a char* to a string literal is deprecated; use a char const* instead.)

Upvotes: 2

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