How to convert a char* pointer into a C++ string?

Question

I have a C++ string. I need to pass this string to a function accepting a char* parameter (for example - strchr()).

a) How do I get that pointer?

b) Is there some function equivalent to strschr() that works for C++ strings?

Solution 1

1. To get the C string equivalent of the C++ string object use c_str function.
   
2. To locate the first occurence of a char in a string object use find_first_of function.

Example:

string s = "abc";

// call to strlen expects char *
cout<<strlen(s.c_str());  // prints 3

// on failure find_first_of return string::npos
if(s.find_first_of('a') != string::npos)
    cout<<s<<" has an a"<<endl;
else
    cout<<s<<" has no a"<<endl;

Note: I gave the strlen just an example of a function that takes char*.

Solution 2

You can't get a char* from a string

string does not allow you free access to its internal buffer. The closest you can get is a const char* using .c_str() if you want it null terminated or .data() if it doesn't have to be null terminated.

You can then cast the pointer returned by these functions to char* but you do this on your own risk. That being said this is a relatively safe cast to make as long as you make sure you're not changing the string. If you changed it then the pointer you got from c_str() may no longer be valid.

This code:

string str("Hello World!");
char* sp = (char*)str.c_str();
sp[5] = 'K';

is probably ok
However this:

string str("Hello World!");
char* sp = (char*)str.c_str();
str = "Chaged string";
sp[5] = 'K';

is most definitely not ok.

Comments

[Carl Smotricz]

Are you sure it's c_tsr and not c_str? I'm no C++ expert but I Googled for c_tsr and was corrected to c_str.

[Admin]

Wrong. c_str() returns a const char *

[JRL]

@Neil: it's an example, jeez... but I've edited it for your pleasure ;-)

[Moeb]

@unicornaddict: thanks. now how do I find the other occurrences of that character in the string? [the length of the string is huge, so creating a whole new string is not an option]

[codaddict]

@cambr: There is a overloaded version of find_first_of which accepts the position to start the search from as an argument. I've posted an example here: ideone.com/bESwL

[Justin Ardini]

This isn't a very good idea. There's no guarantee that pw will point to the same data at a later point, even if some_string is still around. It's much safer to call c_str() to each function you need to pass a char*.

[a paid nerd]

+1 for this being the answer to the question I was searching for :)

[a paid nerd]

More discussion here: bytes.com/topic/c/answers/…

[Frerich Raabe]

You can avoid manual memory management by doing std::vector<char> charBuf(yourString.begin(), yourString.end()); charBuf.push_back('\0'); - this will let you use &charBuf[0] to get a char*.