Which function should be use to converting string to long double?

Question

Note that in general, double is different from long double.

strtod converts string to double, but which function should be use to converting string to long double?

Answer 1

In C++03, use boost::lexical_cast, or:

std::stringstream ss(the_string);
long double ld;
if (ss >> ld) {
    // it worked
}

In C99, use strtold.

In C89, use sscanf with %Lg.

In C++11 use stold.

There may be subtle differences as to exactly which formats each one accepts, so check the details first...

Answer 2

You've tagged your question as "C++", so I'm going to give you a C++ answer:

Why not just use streams?

std::stringstream ss(myString);
long double x;
ss >> x;