Appropriate to cast string literal to char * in function argument?

Question

I've got a function which receives a char * argument:

Foo::Foo (char * arg0) {
    ....
}

In the original example, a char[] is used to pass this value...

char bar[] = "Bar";
Instance.foo (bar);

...which works fine.

But, I've found that I can pass a string literal, cast as a char * , without any warnings from the compiler.

Instance.Foo ((char *) "Bar");

However, from my reading, it seems that should be avoided - the value of the memory that is pointed to could change.

Is the above statement true ("this should be avoided") or is this appropriate in this situation?

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Edit - further research turned up this article which addresses my question pretty well...

Answer 1

Yes, avoid that. Now, if your function took a const char * there is nothing wrong with calling it with a string literal.

C++ compilers support string-literal to char * for backwards compatibility reasons only, and writing to a string literal results in undefined behavior.

When you do char bar[] = "Bar"; you are doing something fundamentally different (namely initializing an array of 4 characters with the values {'B', 'a', 'r', '\\0'} that you are free to modify) than when you do char bar* = "Bar"; (where you are creating a non- const pointer to a 4 byte string that you may not modify).

In my opinion, you should never turn a string literal directly into a char* , instead put it into a const char* then (if you are communicating with a legacy API) explicitly const_cast<char*> the const ness away, with a comment saying you are talking to a legacy API that has guarantees not to change the char s. The advantage of this is you can search your program for those const_cast s when the API is upgraded, or you want to find where the segmentation fault involving writing to a char* came from.

One could even wrap the legacy API with const char* versions that do the const_cast within them.

The absolute worst situation would be having a bunch of char* s hanging around, some of them writable, others from string literals.

Answer 2

This should be avoided at almost all cost and only be done if you have to interface with broken legacy API's and only if you have had a look at their source code and made sure they are not writing to the string.

Stay on the safe side of things and copy the string with strcpy before you pass it in.

What exactly is evil? Writing to string literals is undefined behavior.