C++ char * to string

Question

Possible Duplicate:
how to copy char * into a string and vice-versa

I have to pass a value into a method that required a char *

MyMethod(char * parameter)
{
    // code
}

However, the parameter I need to feed it is currently a std::string. How do I convert the std::string to a char *?

std::string myStringParameter = //whatever

char * myCharParameter = //Convert here somehow

MyMethod(myCharParameter);

Duplicate of [how to copy char * into a string and vice-versa](http://stackoverflow.com/questions/2564052/how-to-copy-char-into-a-string-and-vice-versa). — James McNellis, Aug 01 '10 at 02:38
Man how come I can never find these questions when I search for em :-( — Jake Wilson, Aug 01 '10 at 02:42

score 4 · Accepted Answer · edited Aug 28 '13 at 17:38

4

You are looking for the c_str() method:

char * myCharParameter = myStringParameter.c_str();

You might need to const_cast the return to get it into a char * instead of a const char *. Refrain from modifying the string though; it is the internal buffer of the std::string object. If you actually need to modify the string, you should use strncpy to copy it to another buffer:

char * myCharParameter = new char[myStringParameter.length() + 1];
strncpy(myCharParameter, myStringParameter.c_str(), myStringParameter.length() + 1);

// And later...
myStringParameter.assign(myCharParameter);
delete [] myCharParameter;

edited Aug 28 '13 at 17:38

answered Aug 01 '10 at 02:36

jdmichal

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`c_str()` returns a `const char*`, not a `char *`. – James McNellis Aug 01 '10 at 02:39
@James McNellis Already addressed that in the question. – jdmichal Aug 01 '10 at 02:39
1

@jdmichal: If a function has a parameter of type `char*`, it may modify the object pointed to; using `const_cast` to cast away constness is very dangerous and is A Bad Idea. – James McNellis Aug 01 '10 at 02:41
Okay so a char is the same thing as a cstring? I thought cstring was a different type of object? I'm new to C++ so I'm still learning these things. – Jake Wilson Aug 01 '10 at 02:44
`char *` is sometimes called a "cstring". This is because all strings in C are stored that way. – jdmichal Aug 01 '10 at 02:46
@jdmichal + @Jakobud: There is also the `CString` class, which is shipped as part of MFC. – Billy ONeal Aug 01 '10 at 03:43
If MyMethod() takes a non-const string for a good reason, you probably want to persist whatever changes it made to `myCharParameter` back to `myStringParameter.` – JohnMcG Aug 01 '10 at 03:43

score 1 · Answer 2 · answered Aug 01 '10 at 03:41

It depends on whether MyMethod actually changes the string that's passed in.

If the only thing you know about it is the signature, then you have to assume it could change the string, and copy it into a temporary buffer per @jdmichal's answer.

It's also possible that the developer of this API didn't know or care about const correctness. So the function takes in a non-const buffer even though it doesn't change it.

If that's the case you can get by with:

MyMethod(const_cast<char*>(myStringParameter.c_str());

In my career I have come across more than a few API's taking in non-const strings that should have been const.

score 0 · Answer 3 · answered Aug 01 '10 at 04:52

0

If the function is actually going to modify the string, you'd better create a separate copy:

std::string str = // ...
char* cstr = strdup(str.c_str());
MyMethod(cstr);
// If you want to preserve the changes made:
// str = cstr;
free(cstr);

answered Aug 01 '10 at 04:52

Michael Aaron Safyan

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C++ char * to string

3 Answers3