When a function is possible to be given a char* or literal string, what is the right way to declare this function?

Question

This question is not a duplicate of question:deprecated-conversion-from-string-literal-to-char

But yes I'm trying to get rid of this annoying Clang warning. In that Answer there are the following ways:

1. const_cast or (char*)
2. foo(char*) -> foo(const char*)

I also found some solution in warning-deprecated-conversion-from-string-constant-to-'char'

3. strcpy
4. -Wno-write-string

My further question is :

1. How to set -Wno-write-string in Xcode? Or is there actually such a flag in Clang just as in gcc/g++? I didn't find it in Clang Flag Ref

2. In my code, I get functions that are defined as foo(char*), they receive string or char* for some time, and receive literal string(e.g. "hello") for other time. For such kind of case, what is the right or proper way to decl/def foo?

Answer 1

You can just make your input parameter of your function a const char* and it will work. A function that has its input parameter as const char* means that the function will not make changes to what the pointer points to! So this means it will work with both string literlas (which are const char* themselves) and with regular char* parameters.

void printString(const char* str)
{
    printf("%s", str);
}

This function can be called like this:

int main()
{
    const char* pointerToLiteral = "string";
    char* str = new char[16];
    std::strcpy(str, pointerToLiteral );

    printString("true literal");
    printString(pointerToLiteral );
    printString(str);

    delete [] str;
}

When you pass a pointer to printString function it makes a copy of that pointer on its stack. And that copy is now of type const char*. Which means that the code of printString cannot change what the pointer points to. There is no reason this would not work if you pass a pointer of type char* to it.

Note that the other way around does not work. If we had printString(char* str), you cannot pass it a const char* parameter, because now the function is saying that it can make changes to what the pointer points too, but the const char* parameter does not allow that!