const char * to char * in my case

Question

Although I know that converting const char * to char * is almost banned in C/C++ because of many problems, I am caught up in a situation where I think I have to convert const char * to char *.

I get a string from a text file as string by using c_str and I want to modify this string. However the problem is that c_str converts string into const char *. So is it a good choice to use strdup in this case or is there any better idea in doing this?

The best way is to copy, modify copy and then create a new string. Of course this guarantees correctness at a performance cost. You may also do the previous comment if you keep the same length. Shortening is implementation defined, lengthening is undefined. — Paul Stelian, Jul 31 '16 at 11:38
Thanks a lot! and yes, I will keep the same length. @KerrekSB Why is using &s[0] better than c_str? — wannaqc, Jul 31 '16 at 11:44
That depends on how you are going to use the modified string. Do you need the modified string as std::string, or you need it as an old C string? — user31264, Jul 31 '16 at 11:46
@user31264 I actually need std::string. My way of doing this was by extracting char* from the string, modifying it, and then declaring new string with the modified char*. I came up with this idea because I had to modify sparsely located characters which seems to be inefficient if I use replase function. — wannaqc, Jul 31 '16 at 11:52
What is `replase function`? Also, are you allowed to modify the existing string, and not need to create a new one? — PaulMcKenzie, Jul 31 '16 at 12:06
@PaulMcKenzie I actuall meant replace. Sorry for causing confusion — wannaqc, Jul 31 '16 at 12:11
@wannaqc - if you need std::string, don't mess with old C strings. Aconcagua's solution is good, I upvoted it. — user31264, Aug 01 '16 at 01:20

score 2 · Answer 1 · answered Jul 31 '16 at 12:08

Actually, you can modify the std::string object directly, as illustrated in the little program below:

int main()
{
    std::string s("Hello World");
    for(char& c : s)
    {
        c = toupper(c);
    }
    s[0] = 'h';
    s[6] = 'w';
    s.resize(12);
    s[11] = '!';
    std::cout << s;
    return 0;
}

I think, it is self-explaining. Although you mentioned that size does not change, I added an example for that case, too, see resize. There are yet other ways to manipulate the string, such as insert. Have a look at the std::string documentation.

Thank you! I solved the problem using your solution :) – wannaqc Jul 31 '16 at 12:24 — wannaqc, Jul 31 '16 at 12:24

yumetodo · Answer 2 · 2016-07-31T13:17:06.090

Why is using &s[0] better than c_str?

std::basic_string class has data() and c_str(). However, as you know, these function's result type is const CharType*(CharType: char, wchar_t, char16_t, char32_t).

There is two way to get CharType*.

use const_cast. However, this is very dirty and break type system.

use &s[0]. operator[] does not break type system.

#include <string>
#include <type_traits>
int main(){
    std::string s1 = "Hello world";
    static_assert(std::is_same<char&, decltype(s1[0])>::value, "err");
    static_assert(std::is_same<char*, decltype(&s1[0])>::value, "err");
    const std::string s2 = "Hello world";
    static_assert(std::is_same<const char&, decltype(s2[0])>::value, "err");
    static_assert(std::is_same<const char*, decltype(&s2[0])>::value, "err");
}

const char * to char * in my case

2 Answers2