convert a char* to std::string

std::string has a constructor for this:

const char *s = "Hello, World!";
std::string str(s);

Note that this construct deep copies the character list at s and s should not be nullptr, or else behavior is undefined.

If you already know size of the char*, use this instead

char* data = ...;
int size = ...;
std::string myString(data, size);

This doesn't use strlen.

EDIT: If string variable already exists, use assign():

std::string myString;
char* data = ...;
int size = ...;
myString.assign(data, size);

Most answers talks about constructing std::string.

If already constructed, just use assignment operator.

std::string oString;
char* pStr;

... // Here allocate and get character string (e.g. using fgets as you mentioned)

oString = pStr; // This is it! It copies contents from pStr to oString