C++ :: Strings And Constant Chars - Stream A Constantly Changing Float Into Coord

So, I'm working with cocos2d-x and there's a specific function used to print true type fonts to the screen. My problem is that I want to feed a string into the function, but it only takes a string of const char (Which I don't even know what that is. AFAIK strings are a string of chars, not const chars).

Here's a definition of the function :

CCLabelTTF::create(const char *string, const char *fontname, float *fontsize);

an example of it being used

CCLabelTTF *pCoord = CCLabelTTF::create("Coordinates", "Arial", 42.0);

What I want to use

std::string coord;
//stream a constantly changing float into coord ...
CCLabelTTF *pCoord = CCLabelTTF::create(coord, "Arial", 42.0);

What happens : Error : no instance of overloaded function "...CCLabelTTF::create" matches the argument list. Argument types are (std::string, const char [6], int)

Basically, What I need to know is how would I feed a constantly updating string into the 1st argument?