使用指针反转 C++ 中的字符串

Question

void reverse(char[] x) {
    char* pStart = x;
    char* pEnd = pStart + sizeof(x) - 2;
    while(pStart < pEnd) {
        char temp = *pStart;
        *pStart = *pEnd;
        *pEnd = temp;
        pStart++;
        pEnd--;
    }
}

int main() {
    char text[] = ['h','e','l','l','o'];
    reverse(text);
    cout << text << endl;
    return 0;
}

I am new to C++ and stack overflow.

I am trying to reverse a string using pointers... I don't quite understand what I did wrong. Please help me out.

Additional question: What is the difference between a string and an array of characters?

Answer 1

sizeof(x) with x being a parameter of type char[] of a function does not give you the number of characters in the string but the size of a char* , probably 8 on a 64 bit system. You need to pass a C-String and use strlen(x) instead. Write char text[] = {'h','e','l','l','o','\\0'} or char text[] = "hello" in main .

Note that sizeof() needs to be evaluatable at compile time; this is not possible on arrays with undetermined size like char[] -typed function arguments. When using sizeof on a variables like your char text[] = {'h','e','l','l','o'} , however, sizeof(text) will result in the actual size of the array.

Answer 2

char x[] is the same as char* x and the sizeof(x) is therefore the size of a pointer. So, because you cannot calculate the size of an array outside of the block it is declared in, I would eliminate that part from your function.

It would be much easier to provide the function with pointers to the first and last characters to be replaced:

void reverse(char* pStart, char* pEnd)
{
    while (pStart < pEnd)
    {
        char temp = *pStart;
        *pStart   = *pEnd;
        *pEnd     = temp;
        pStart++;
        pEnd--;
    }
}

So now it is quite easy to call this function - take the address (using ampersand & ) of the the relevant characters in the array: &text[0] and &text[4] .

To display an array of characters, there is a rule, that such "strings" HAVE to have after the last character a NULL character. A NULL character can be written as 0 or '\\0' . That is why it has to be added to the array here.

int main()
{
    // char text[] = "hello"; // Same like below, also adds 0 at end BUT !!!in read-only memory!!
    char text[] = { 'h', 'e', 'l', 'l', 'o', '\0' };
    reverse(&text[0], &text[4]);

    std::cout << text << std::endl;
    return 0;
}